fathomdb_engine/

lib.rs

pub mod lifecycle;
mod pcache2;

use std::collections::{BTreeMap, BTreeSet, VecDeque};
use std::error::Error;
use std::fmt::{Display, Formatter};
use std::fs::{File, OpenOptions};
use std::io::{Seek, SeekFrom, Write};
use std::path::{Path, PathBuf};
use std::sync::atomic::{AtomicBool, AtomicU64, AtomicUsize, Ordering};
use std::sync::mpsc::{self, Receiver, SyncSender};
use std::sync::Once;
use std::sync::{Arc, Condvar, Mutex};
use std::thread::{self, JoinHandle};
use std::time::{Duration, Instant, SystemTime, UNIX_EPOCH};

use fathomdb_embedder::{EmbedderEvent, MeanRecomputeTrigger};
use fathomdb_embedder_api::{Embedder, EmbedderError as RuntimeEmbedderError, EmbedderIdentity};
use fathomdb_query::compile_text_query;
use fathomdb_schema::{
    migrate_with_event_sink, MigrationError as SchemaMigrationError, MigrationStepReport,
    CANONICAL_TABLES, LOCK_SUFFIX, MIGRATIONS, SCHEMA_VERSION,
};
use jsonschema::JSONSchema;
use rusqlite::{params, Connection, OptionalExtension};
use serde_json::Value;
use sha2::Digest;
use sqlite_vec::sqlite3_vec_init;

#[cfg(unix)]
use std::os::unix::fs::OpenOptionsExt;

// EU-5b lock-flip: the engine's default embedder identity is now the
// pinned bge-small variant. Pre-existing 0.7.0 workspaces opened with
// `EmbedderChoice::Default` will fail-closed on identity mismatch per
// ADR-0.6.0-vector-identity-embedder-owned; callers can still hold an
// older noop profile by supplying `EmbedderChoice::Caller(NoopEmbedder)`.
const DEFAULT_EMBEDDER_NAME: &str = "fathomdb-bge-small-en-v1.5";
const DEFAULT_EMBEDDER_REVISION: &str = "5c38ec7c405ec4b44b94cc5a9bb96e735b38267a";
const DEFAULT_EMBEDDER_DIMENSION: u32 = 384;

/// Identity name of the bge-small embedder. `OpenReport.embedder_mean_centering_required`
/// is `true` iff the live embedder identity reports this name. NoopEmbedder
/// is `false`. Lifted out as a constant so the EU-5b lock-flip (when the
/// engine's default identity becomes bge-small) is a single-line change.
///
/// TODO(EU-5b): when `DEFAULT_EMBEDDER_NAME` flips to this constant, the
/// Default path will populate `embedder_mean_centering_required = true`
/// without further engine work. Caller-supplied bge-small (rare today)
/// already does the right thing.
const BGE_SMALL_EMBEDDER_NAME: &str = "fathomdb-bge-small-en-v1.5";

/// REQ-006a / AC-007a default slow-statement threshold. Mutated at runtime
/// via [`Engine::set_slow_threshold_ms`].
const DEFAULT_SLOW_THRESHOLD_MS: u64 = 100;
const DEFAULT_VECTOR_PROFILE: &str = "default";
const DEFAULT_VECTOR_PARTITION: &str = "vector_default";
/// Default drain budget for `rebuild_projections` / `rebuild_vec0`. The
/// rebuild path freezes the scheduler before truncating shadow rows, so
/// the only outstanding work is whatever workers were mid-flight when
/// the call landed; 30 s is generous for normal job sizes and bounded
/// for tests.
const REBUILD_DRAIN_TIMEOUT_MS: u64 = 30_000;
const DEFAULT_PROVENANCE_ROW_CAP: u64 = 1_000_000;
const PROJECTION_CURSOR_KEY: &str = "projection_cursor";
const PROJECTION_WORKERS: usize = 2;
/// PR-9 — ADR-0.6.0-embedder-protocol **Invariant 5** default per-`embed()`
/// watchdog deadline. Every projection-path embed runs under this timeout;
/// a hung embed surfaces `RuntimeEmbedderError::Timeout` (engaging the
/// existing retry/failure path) rather than parking a worker forever. The
/// EU-5f `catch_unwind` only catches *panics*; this catches *hangs*.
const DEFAULT_EMBED_TIMEOUT_MS: u64 = 30_000;
/// PR-9 — embed circuit-breaker threshold: the maximum number of watchdog
/// embed threads allowed alive at once before the breaker latches and
/// projection jobs fail fast (see `embed_circuit_open` / `live_embed_threads`).
/// Healthy serialized operation keeps the live count at 0–1, so reaching this
/// many concurrently-alive embed threads means timed-out embeds are piling up
/// (a hung/wedged embedder); the breaker then caps the abandoned-thread leak
/// at roughly this count.
const DEFAULT_EMBED_CIRCUIT_THRESHOLD: u64 = 8;
const PROJECTION_COMMIT_BATCH: usize = 16;
// Each worker should be able to grab a full commit batch while another
// worker has the same waiting in the queue. Below this, the dispatcher
// throttles below the workers' commit-batch capacity.
const PROJECTION_INFLIGHT_LIMIT: usize = PROJECTION_WORKERS * PROJECTION_COMMIT_BATCH;
// SQL fetch cap inside the dispatcher: enough to fill the in-flight
// budget in a single scan so we don't pay one SQL roundtrip per job.
const PROJECTION_SCAN_FETCH: usize = PROJECTION_INFLIGHT_LIMIT;
const DEFAULT_PROJECTION_RETRY_DELAYS_MS: [u64; 3] = [1_000, 4_000, 16_000];

/// Reader pool size. Per `dev/design/engine.md` § Writer / reader split,
/// reader connections are pooled and never serialize behind one
/// connection. AC-021 exercises 8 concurrent readers.
const READER_POOL_SIZE: usize = 8;

/// Per-reader-connection lookaside slot size, in bytes. Pack 6.G G.1.
/// Picked from G.0 telemetry (`allocator_lookaside` 26.67% conc cycles
/// with 3.89× ratio) + the SQLite docs' typical-workload sizing
/// guidance (https://www.sqlite.org/malloc.html §3): 1200-byte slots
/// cover the small allocations from `sqlite3DbMallocRaw`,
/// `sqlite3Fts5ExprNew`, and `vec0Filter_knn` visible at the top of the
/// concurrent profile.
const READER_LOOKASIDE_SLOT_SIZE: std::os::raw::c_int = 1200;

/// Per-reader-connection lookaside slot count. SQLite default is 128;
/// we use 500 to absorb the per-statement allocation footprint of the
/// hybrid search workload across a sticky worker connection without
/// falling back to the glibc malloc-arena mutex.
const READER_LOOKASIDE_SLOT_COUNT: std::os::raw::c_int = 500;

pub struct Engine {
    path: PathBuf,
    next_cursor: AtomicU64,
    closed: AtomicBool,
    lock: Mutex<Option<File>>,
    connection: Mutex<Option<Connection>>,
    reader_pool: ReaderWorkerPool,
    counters: lifecycle::Counters,
    subscribers: Arc<lifecycle::SubscriberRegistry>,
    profiling_enabled: Arc<AtomicBool>,
    slow_threshold_ms: Arc<AtomicU64>,
    runtime_embedder: Option<Arc<dyn Embedder>>,
    runtime_embedder_identity: EmbedderIdentity,
    projection_runtime: ProjectionRuntime,
    provenance_row_cap: AtomicU64,
    /// Per-connection profile-callback contexts. Each box's pointer is
    /// installed into the connection's `sqlite3_profile` userdata; the
    /// box must outlive the connection so the callback never reads
    /// freed memory. Connections are dropped before this vec on
    /// `close`/`Drop`, so the lifetime ordering holds.
    ///
    /// Why `Box<ProfileContext>` and not `ProfileContext` directly: the
    /// FFI pointer captured during `install_profile_callback` MUST
    /// remain stable for the connection's lifetime; pushing onto a
    /// `Vec<ProfileContext>` could reallocate and invalidate that
    /// pointer.
    #[allow(clippy::vec_box)]
    profile_contexts: Mutex<Vec<Box<ProfileContext>>>,
    /// Pack 6.G G.1 — `sqlite3_db_config(LOOKASIDE)` rc per reader
    /// worker, captured at open time before any PRAGMA / prepare ran
    /// on the connection. Read only by the debug-only test accessor
    /// `reader_lookaside_config_rcs_for_test`; held in release builds
    /// too because the field is set unconditionally at open and a cfg
    /// gate would force two open-locked return shapes.
    #[allow(dead_code)]
    reader_lookaside_rcs: Vec<i32>,
    #[cfg(debug_assertions)]
    force_next_commit_failure: AtomicBool,
}

#[derive(Clone, Debug)]
struct ProjectionJob {
    cursor: u64,
    kind: String,
    body: String,
}

#[derive(Debug, Default)]
struct ProjectionRuntimeState {
    active_jobs: usize,
    queued_jobs: usize,
    frozen: bool,
    pending_scan: bool,
    stopping: bool,
    in_flight: BTreeSet<u64>,
}

struct ProjectionRuntimeShared {
    path: PathBuf,
    embedder: Option<Arc<dyn Embedder>>,
    embedder_identity: EmbedderIdentity,
    state: Mutex<ProjectionRuntimeState>,
    state_cvar: Condvar,
    queue: Mutex<VecDeque<ProjectionJob>>,
    queue_cvar: Condvar,
    retry_delays_ms: Mutex<Vec<u64>>,
    /// PR-9 — ADR-0.6.0-embedder-protocol Invariant 5 per-`embed()` watchdog
    /// deadline (ms). Read lock-free on the projection hot path. Default
    /// `DEFAULT_EMBED_TIMEOUT_MS` (30s); the test seam
    /// `set_embed_timeout_ms_for_test` lowers it so the hanging-embedder
    /// test need not wait 30s. A hung embed surfaces
    /// `RuntimeEmbedderError::Timeout`, engaging the existing retry/failure
    /// path in `run_projection_job`.
    embed_timeout_ms: AtomicU64,
    /// PR-9 — engine-side embed serialization guard. The pool runs
    /// `PROJECTION_WORKERS` workers; this guard ensures the shared
    /// `Arc<dyn Embedder>` is invoked by at most one worker at a time.
    ///
    /// Rationale is SAFETY, not throughput. The engine accepts arbitrary
    /// caller-supplied embedders (the pyo3 / napi bridges, per ADR-0.6.0)
    /// whose `embed` is `Sync` only by trait contract; many real impls (a
    /// GIL-bound Python model, a non-reentrant native lib, an internal cache)
    /// are not actually safe under concurrent calls. Serializing engine-side
    /// makes the projection robust to embedders that are not truly
    /// concurrency-safe, without the engine having to trust each impl. The
    /// default `CandleBgeEmbedder` was shown safe under concurrent forwards
    /// in the PR-9 pre-flight, so for it the guard is belt-and-suspenders.
    ///
    /// Throughput is ~neutral: `candle` fans every `BertModel::forward` onto a
    /// single process-wide rayon pool, so two concurrent forwards merely
    /// share that pool (trading per-embed latency, not aggregate work) rather
    /// than getting 2x — serializing avoids some scheduler/cache thrash but is
    /// not a large win. (An earlier "~13x" figure compared a debug-build
    /// unserialized run against a release-build number and was withdrawn; a
    /// PR-9 micro-benchmark put release embeds at ~14 ms short / ~960 ms for a
    /// 512-token doc, watchdog overhead ~0.)
    ///
    /// Commit/IO stays parallel across workers (see `commit_gate`); this guard
    /// wraps only the embed call. It is held by the worker across the watchdog
    /// call and released here, so a timed-out (abandoned) embed frees it and
    /// cannot stall the pool — the guard owns no data, so a panic-resumed
    /// embed that poisons it is recovered via `into_inner`.
    ///
    /// Deliberate trade-off (codex PR-9 CONCERN-1, accepted): on the *timeout*
    /// path the worker drops this guard while the abandoned detached embed
    /// thread is still running lock-free, so serialization is briefly relaxed
    /// until that thread finishes. This is the prescribed choice over holding
    /// the guard inside the embed thread — which would let a genuinely-hung
    /// embed hold it forever and deadlock the whole pool, exactly the wedge
    /// ADR-0.6.0 Invariant 5 and this slice's spec forbid. Timeouts are the
    /// fault path only; the embed circuit breaker (`embed_circuit_open`) caps
    /// how many such abandoned threads can be alive at once. A future slice may
    /// replace this hard serialize with an operator-configurable embed
    /// concurrency limit (ADR-0.6.0 Invariant 4 pool-size override) for I/O-
    /// or GPU-bound embedders; that knob is out of PR-9 scope.
    embed_serialize: Mutex<()>,
    /// PR-9 — embed circuit breaker. `live_embed_threads` counts watchdog embed
    /// threads currently alive (incremented when one is spawned, decremented
    /// when it finishes — see `embed_with_watchdog`). Under healthy serialized
    /// operation this is 0 or 1; it only grows when timed-out embeds are
    /// abandoned and keep running (ADR-0.6.0 Invariant 5 forbids aborting a
    /// running embed). When a new embed would push the live count to
    /// `embed_circuit_threshold`, the breaker latches `embed_circuit_open` and
    /// projection jobs fail fast WITHOUT spawning further embeds — bounding the
    /// abandoned-thread leak to ~threshold REGARDLESS of whether the embedder
    /// hangs on every input or only intermittently (a returning embed
    /// decrements the count rather than resetting a streak, so an
    /// intermittently-hanging embedder still latches as its hung threads pile
    /// up, and a merely-slow-but-returning embedder self-clears and never
    /// false-trips). Latches for the engine session (a reopen resets it); a
    /// half-open/cool-down retry is future work. `threshold == 0` disables it.
    live_embed_threads: Arc<AtomicU64>,
    embed_circuit_open: AtomicBool,
    embed_circuit_threshold: AtomicU64,
    /// EU-5b — streaming mean accumulator for the per-workspace mean
    /// pinning lifecycle (`dev/design/embedder.md` §0.3). `Some(_)` iff
    /// the identity is MC-required AND no mean has been pinned yet on
    /// disk. The accumulator graduates to `None` after the at-pin
    /// commit; subsequent docs feed nothing.
    mean_accumulator: Mutex<Option<MeanAccumulator>>,
    /// EU-5b — `MeanVecPinned` events queued by the projection-commit
    /// transaction for the next test-seam drain. Production callers
    /// consume these via the `OpenReport.embedder_events` channel; the
    /// drain seam is `Engine::drain_mean_centering_events_for_test`.
    pending_events: Mutex<Vec<EmbedderEvent>>,
    /// EU-5f — serializes the body of `commit_projection_outcomes` across
    /// the `PROJECTION_WORKERS` worker connections. Each worker commits on
    /// its own connection; holding this gate for the whole commit makes the
    /// commit transactions totally ordered, which is what makes the at-pin
    /// re-quantize pass provably complete (every row is wholly before or
    /// after the unique pin tx, so none can survive un-centered). Embedding
    /// (`run_projection_job`) runs OUTSIDE the gate and stays parallel.
    commit_gate: Mutex<()>,
    /// 0.7.2 PR-2bc S1 fix-1 — overridable phase-2 rerank `LIMIT` for the
    /// search hot path. Equals `SEARCH_RERANK_LIMIT` (10) in production; a
    /// test seam (`set_search_limit_for_test`) can RAISE it (clamped to >=10,
    /// so it can never shrink below production semantics) so the recall
    /// harness can pull top-(10+slack) and exclude the self-retrieving
    /// query-source doc before truncating to 10. Production reads this atomic
    /// (default 10) — there is NO env var read on the hot path.
    search_limit_override: AtomicUsize,
    /// 0.7.2 PR-2b — debug-only fault injection: when set, `recompute_mean_in_tx`
    /// errors AFTER writing `mean_vec` but BEFORE finishing the re-quantize
    /// pass, so the crash-atomicity test can prove the whole recompute rolls
    /// back (no half-recentered corpus). One-shot (cleared on consume).
    #[cfg(debug_assertions)]
    force_recompute_failure: AtomicBool,
}

impl std::fmt::Debug for ProjectionRuntimeShared {
    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("ProjectionRuntimeShared")
            .field("path", &self.path)
            .field("embedder_identity", &self.embedder_identity)
            .finish_non_exhaustive()
    }
}

#[derive(Debug)]
struct ProjectionRuntime {
    shared: Arc<ProjectionRuntimeShared>,
    dispatcher: Mutex<Option<JoinHandle<()>>>,
    workers: Mutex<Vec<JoinHandle<()>>>,
}

impl std::fmt::Debug for Engine {
    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("Engine")
            .field("path", &self.path)
            .field("closed", &self.closed.load(Ordering::SeqCst))
            .field("runtime_embedder_identity", &self.runtime_embedder_identity)
            .finish_non_exhaustive()
    }
}

/// Per-connection profile-callback context.
///
/// Holds the registry handle the callback dispatches to, plus shared
/// references to the engine's profiling toggle and slow-statement
/// threshold. The `Arc` clones here mirror the same atomics held by
/// `Engine`, so `set_profiling` / `set_slow_threshold_ms` mutations are
/// visible inside the callback without restart (REQ-006a / AC-005a /
/// AC-007b runtime-toggle contract).
#[derive(Debug)]
struct ProfileContext {
    subscribers: Arc<lifecycle::SubscriberRegistry>,
    profiling_enabled: Arc<AtomicBool>,
    slow_threshold_ms: Arc<AtomicU64>,
}

/// Thread-affine reader worker pool (Pack 6 F.0).
///
/// Per `dev/design/engine.md` § Writer / reader split, reader connections
/// must not serialize behind a single mutex. Each worker thread owns
/// exactly one read-only `Connection` for its lifetime; `Connection`
/// objects never cross thread boundaries after startup. `Engine::search`
/// dispatches a request via a per-worker bounded channel using a
/// lock-free round-robin counter on the hot path.
struct ReaderWorkerPool {
    senders: Vec<SyncSender<ReaderRequest>>,
    handles: Mutex<Option<Vec<JoinHandle<()>>>>,
    next: AtomicUsize,
    shutdown: AtomicBool,
    live_workers: Arc<AtomicUsize>,
}

impl std::fmt::Debug for ReaderWorkerPool {
    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("ReaderWorkerPool")
            .field("worker_count", &self.senders.len())
            .field("live_workers", &self.live_workers.load(Ordering::Relaxed))
            .field("shutdown", &self.shutdown.load(Ordering::Relaxed))
            .finish()
    }
}

/// One request handled by exactly one reader worker. The response is
/// returned through a fresh oneshot channel so requests cannot be
/// routed to or duplicated across workers.
enum ReaderRequest {
    Search {
        compiled: fathomdb_query::CompiledQuery,
        /// Un-centered f32 query vector serialized for `vec_f32`. Phase 2
        /// f32 rerank uses this verbatim.
        query_vector: Option<String>,
        /// EU-5a2 — (possibly centered) f32 query vector for the phase 1
        /// `vec_quantize_binary` sign-quant. Equal to `query_vector` for
        /// non-MC-required identities (the EU-5a2 default).
        query_vector_bin: Option<String>,
        /// 0.7.2 PR-2bc S1 fix-1 — phase-2 rerank `LIMIT`. Read from
        /// `ProjectionRuntimeShared::search_limit_override` (default
        /// `SEARCH_RERANK_LIMIT` = 10, clamped >=10) by `search_inner`
        /// before dispatch, so the worker never reads any env var.
        search_limit: usize,
        respond: SyncSender<ReaderResponse>,
    },
    Shutdown,
    /// Pack 6.G G.1 — debug-only request that asks a worker to read its
    /// own connection's `SQLITE_DBSTATUS_LOOKASIDE_USED` and return the
    /// high-water mark (`hiwtr` out-param). Used solely by the integration
    /// test that asserts post-warmup lookaside slots were consumed; not
    /// on any production path.
    #[cfg(debug_assertions)]
    LookasideStatus {
        respond: SyncSender<i32>,
    },
    /// Pack 6.G G.3.5 — debug-only request that asks a worker to read
    /// `SQLITE_DBSTATUS_CACHE_HIT`, `_CACHE_MISS`, and `_CACHE_USED`
    /// off its own connection and return them as `(hit, miss, used_bytes)`.
    /// `snapshot_label` is opaque to the worker; the caller uses it to
    /// distinguish pre/post snapshots in its own bookkeeping.
    #[cfg(debug_assertions)]
    CacheStatus {
        snapshot_label: String,
        respond: SyncSender<(String, i32, i32, i32)>,
    },
}

type ReaderResponse = rusqlite::Result<(u64, Option<SoftFallback>, Vec<String>)>;

/// Pack 6.G G.3.5 — per-worker cache-pressure snapshot. Carried only on
/// the debug-only `CacheStatus` broadcast path and the test accessor;
/// not part of the public 0.6.0 surface.
#[cfg(debug_assertions)]
#[doc(hidden)]
#[derive(Clone, Debug)]
pub struct CacheStatusReply {
    pub worker_idx: usize,
    pub snapshot_label: String,
    pub cache_hit: i32,
    pub cache_miss: i32,
    pub cache_used_bytes: i32,
}

/// Per-worker outbound channel capacity. Round-robin dispatch keeps
/// queue depth at ~0 on hot paths; the small slack absorbs jitter
/// without a runtime mutex.
const READER_WORKER_CHANNEL_CAPACITY: usize = 4;

impl ReaderWorkerPool {
    fn new(connections: Vec<Connection>) -> Self {
        let live_workers = Arc::new(AtomicUsize::new(0));
        let mut senders = Vec::with_capacity(connections.len());
        let mut handles = Vec::with_capacity(connections.len());
        for (idx, connection) in connections.into_iter().enumerate() {
            let (tx, rx) = mpsc::sync_channel::<ReaderRequest>(READER_WORKER_CHANNEL_CAPACITY);
            let live = Arc::clone(&live_workers);
            let handle = thread::Builder::new()
                .name(format!("fathomdb-reader-{idx}"))
                .spawn(move || reader_worker_loop(connection, rx, live))
                .expect("spawn reader worker");
            senders.push(tx);
            handles.push(handle);
        }
        Self {
            senders,
            handles: Mutex::new(Some(handles)),
            next: AtomicUsize::new(0),
            shutdown: AtomicBool::new(false),
            live_workers,
        }
    }

    fn worker_count(&self) -> usize {
        self.senders.len()
    }

    fn live_count(&self) -> usize {
        self.live_workers.load(Ordering::SeqCst)
    }

    /// Pack 6.G G.1 — broadcast a `LookasideStatus` request to every
    /// worker (not round-robin) and collect each worker's
    /// `SQLITE_DBSTATUS_LOOKASIDE_USED`. Used only by the debug
    /// integration test for post-warmup lookaside-slot consumption.
    #[cfg(debug_assertions)]
    fn lookaside_used_per_worker(&self) -> Vec<i32> {
        let mut results = Vec::with_capacity(self.senders.len());
        for sender in &self.senders {
            let (tx, rx) = mpsc::sync_channel::<i32>(1);
            if sender.send(ReaderRequest::LookasideStatus { respond: tx }).is_ok() {
                results.push(rx.recv().unwrap_or(-1));
            } else {
                results.push(-1);
            }
        }
        results
    }

    /// Pack 6.G G.3.5 — broadcast a `CacheStatus` request to every
    /// worker and collect each worker's `(cache_hit, cache_miss,
    /// cache_used_bytes)` triple. Same broadcast pattern as G.1's
    /// `lookaside_used_per_worker`. Returns one `CacheStatusReply` per
    /// worker in worker-index order.
    #[cfg(debug_assertions)]
    fn cache_status_per_worker(&self, snapshot_label: &str) -> Vec<CacheStatusReply> {
        let mut results = Vec::with_capacity(self.senders.len());
        for (idx, sender) in self.senders.iter().enumerate() {
            let (tx, rx) = mpsc::sync_channel::<(String, i32, i32, i32)>(1);
            let request = ReaderRequest::CacheStatus {
                snapshot_label: snapshot_label.to_string(),
                respond: tx,
            };
            if sender.send(request).is_ok() {
                if let Ok((label, hit, miss, used)) = rx.recv() {
                    results.push(CacheStatusReply {
                        worker_idx: idx,
                        snapshot_label: label,
                        cache_hit: hit,
                        cache_miss: miss,
                        cache_used_bytes: used,
                    });
                    continue;
                }
            }
            results.push(CacheStatusReply {
                worker_idx: idx,
                snapshot_label: snapshot_label.to_string(),
                cache_hit: -1,
                cache_miss: -1,
                cache_used_bytes: -1,
            });
        }
        results
    }

    /// Hot path. Lock-free dispatch: `AtomicUsize::fetch_add` selects
    /// the worker, then a single `SyncSender::send` enqueues the
    /// request. No global mutex is taken on the request path.
    fn dispatch(&self, request: ReaderRequest) -> Result<(), ReaderRequest> {
        if self.shutdown.load(Ordering::Relaxed) {
            return Err(request);
        }
        let n = self.senders.len();
        if n == 0 {
            return Err(request);
        }
        let idx = self.next.fetch_add(1, Ordering::Relaxed) % n;
        self.senders[idx].send(request).map_err(|err| err.0)
    }

    /// Signal every worker to exit and join its thread. Idempotent —
    /// safe to call from `Engine::close` and again from
    /// `ReaderWorkerPool::Drop`.
    fn shutdown(&self) {
        if self.shutdown.swap(true, Ordering::SeqCst) {
            return;
        }
        for sender in &self.senders {
            let _ = sender.send(ReaderRequest::Shutdown);
        }
        if let Ok(mut slot) = self.handles.lock() {
            if let Some(handles) = slot.take() {
                for handle in handles {
                    let _ = handle.join();
                }
            }
        }
    }
}

impl Drop for ReaderWorkerPool {
    fn drop(&mut self) {
        self.shutdown();
    }
}

fn reader_worker_loop(
    mut connection: Connection,
    rx: Receiver<ReaderRequest>,
    live_workers: Arc<AtomicUsize>,
) {
    live_workers.fetch_add(1, Ordering::SeqCst);
    // Drop guard so the live counter decrements even on panic.
    struct LiveGuard(Arc<AtomicUsize>);
    impl Drop for LiveGuard {
        fn drop(&mut self) {
            self.0.fetch_sub(1, Ordering::SeqCst);
        }
    }
    let _guard = LiveGuard(live_workers);

    while let Ok(request) = rx.recv() {
        match request {
            ReaderRequest::Shutdown => break,
            ReaderRequest::Search {
                compiled,
                query_vector,
                query_vector_bin,
                search_limit,
                respond,
            } => {
                let result = read_search_in_tx(
                    &mut connection,
                    &compiled,
                    query_vector.as_deref(),
                    query_vector_bin.as_deref(),
                    search_limit,
                );
                // Receiver may have been dropped if the caller went
                // away; nothing to do in that case.
                let _ = respond.send(result);
            }
            #[cfg(debug_assertions)]
            ReaderRequest::LookasideStatus { respond } => {
                let _ = respond.send(read_lookaside_used_hiwtr(&connection));
            }
            #[cfg(debug_assertions)]
            ReaderRequest::CacheStatus { snapshot_label, respond } => {
                let (hit, miss, used) = read_cache_status(&connection);
                let _ = respond.send((snapshot_label, hit, miss, used));
            }
        }
    }

    // Per `dev/design/engine.md` § Close path, uninstall the profile
    // callback before dropping the connection so SQLite cannot fire
    // one last callback against a `ProfileContext` whose Box is about
    // to free.
    uninstall_profile_callback(&connection);
    drop(connection);
}

impl ProjectionRuntime {
    fn new(
        path: PathBuf,
        embedder: Option<Arc<dyn Embedder>>,
        embedder_identity: EmbedderIdentity,
        mean_already_pinned: bool,
    ) -> Self {
        // EU-5b/EU-5f — only allocate the streaming accumulator when the
        // workspace's identity is MC-required AND no mean has been pinned
        // yet on disk. Allocating it for an already-pinned workspace would
        // let a later 256-doc run RE-pin and overwrite the compute-once
        // mean (violating `dev/design/embedder.md` §0.3). Other identities
        // pay no memory cost (`Option::None`).
        let mc_required = identity_requires_mean_centering(&embedder_identity);
        let mean_accumulator = if mc_required && !mean_already_pinned {
            Some(MeanAccumulator::new(embedder_identity.dimension as usize))
        } else {
            None
        };
        let shared = Arc::new(ProjectionRuntimeShared {
            path,
            embedder,
            embedder_identity,
            state: Mutex::new(ProjectionRuntimeState::default()),
            state_cvar: Condvar::new(),
            queue: Mutex::new(VecDeque::new()),
            queue_cvar: Condvar::new(),
            retry_delays_ms: Mutex::new(DEFAULT_PROJECTION_RETRY_DELAYS_MS.to_vec()),
            embed_timeout_ms: AtomicU64::new(DEFAULT_EMBED_TIMEOUT_MS),
            embed_serialize: Mutex::new(()),
            live_embed_threads: Arc::new(AtomicU64::new(0)),
            embed_circuit_open: AtomicBool::new(false),
            embed_circuit_threshold: AtomicU64::new(DEFAULT_EMBED_CIRCUIT_THRESHOLD),
            mean_accumulator: Mutex::new(mean_accumulator),
            pending_events: Mutex::new(Vec::new()),
            commit_gate: Mutex::new(()),
            search_limit_override: AtomicUsize::new(SEARCH_RERANK_LIMIT),
            #[cfg(debug_assertions)]
            force_recompute_failure: AtomicBool::new(false),
        });

        let dispatcher_shared = Arc::clone(&shared);
        let dispatcher = thread::spawn(move || projection_dispatcher_loop(dispatcher_shared));

        let mut workers = Vec::with_capacity(PROJECTION_WORKERS);
        for _ in 0..PROJECTION_WORKERS {
            let worker_shared = Arc::clone(&shared);
            workers.push(thread::spawn(move || projection_worker_loop(worker_shared)));
        }

        Self { shared, dispatcher: Mutex::new(Some(dispatcher)), workers: Mutex::new(workers) }
    }

    fn notify_new_work(&self) {
        if let Ok(mut state) = self.shared.state.lock() {
            state.pending_scan = true;
            self.shared.state_cvar.notify_all();
        }
    }

    fn set_frozen(&self, frozen: bool) {
        if let Ok(mut state) = self.shared.state.lock() {
            state.frozen = frozen;
            if !frozen {
                state.pending_scan = true;
            }
            self.shared.state_cvar.notify_all();
        }
    }

    fn wait_for_idle(&self, timeout_ms: u64) -> bool {
        let deadline = Instant::now() + Duration::from_millis(timeout_ms);
        let mut state = match self.shared.state.lock() {
            Ok(state) => state,
            Err(_) => return false,
        };
        loop {
            if state.active_jobs == 0 && state.queued_jobs == 0 {
                drop(state);
                if !database_has_pending_projection_work(&self.shared.path).unwrap_or(true) {
                    return true;
                }
                state = match self.shared.state.lock() {
                    Ok(state) => state,
                    Err(_) => return false,
                };
            }
            let now = Instant::now();
            if now >= deadline {
                return false;
            }
            let wait = deadline.saturating_duration_since(now);
            let Ok((next_state, _)) = self.shared.state_cvar.wait_timeout(state, wait) else {
                return false;
            };
            state = next_state;
        }
    }

    fn set_retry_delays_for_test(&self, delays_ms: &[u64]) {
        if let Ok(mut delays) = self.shared.retry_delays_ms.lock() {
            *delays = delays_ms.to_vec();
        }
    }

    fn set_embed_timeout_ms_for_test(&self, timeout_ms: u64) {
        self.shared.embed_timeout_ms.store(timeout_ms, Ordering::Relaxed);
    }

    fn set_embed_circuit_threshold_for_test(&self, threshold: u64) {
        self.shared.embed_circuit_threshold.store(threshold, Ordering::Relaxed);
    }

    fn embed_circuit_open_for_test(&self) -> bool {
        self.shared.embed_circuit_open.load(Ordering::Relaxed)
    }

    fn stop(&self) {
        if let Ok(mut state) = self.shared.state.lock() {
            if state.stopping {
                return;
            }
            state.stopping = true;
            state.pending_scan = false;
            self.shared.state_cvar.notify_all();
        }
        if let Ok(mut queue) = self.shared.queue.lock() {
            queue.clear();
            self.shared.queue_cvar.notify_all();
        }

        if let Ok(mut dispatcher) = self.dispatcher.lock() {
            if let Some(handle) = dispatcher.take() {
                let _ = handle.join();
            }
        }
        if let Ok(mut workers) = self.workers.lock() {
            for handle in workers.drain(..) {
                let _ = handle.join();
            }
        }
    }
}

#[derive(Clone, Debug, Eq, PartialEq)]
pub struct OpenReport {
    pub schema_version_before: u32,
    pub schema_version_after: u32,
    pub migration_steps: Vec<MigrationStepReport>,
    pub embedder_warmup_ms: u64,
    pub query_backend: &'static str,
    pub default_embedder: EmbedderIdentity,
    /// Total wall time the loader spent materializing default-embedder
    /// weights — covers HF GETs, sha256 verification, atomic rename,
    /// parent-dir fsync (POSIX), and cache directory writes. This is
    /// the "engine open paid by the embedder" envelope, useful for SLA
    /// budgeting; it is intentionally wider than just the bytes-flowing
    /// time so callers see the full first-use cost.
    ///
    /// `Some(ms)` when network bytes flowed (`bytes_downloaded > 0`);
    /// `None` for caller-supplied embedders (loader bypassed) and on
    /// full cache hits (no bytes flowed). For pure per-file network
    /// analysis, use the `DefaultEmbedderDownload` events on
    /// [`embedder_events`](Self::embedder_events) — each event carries
    /// the file's bytes + sha256 + cache path.
    pub embedder_download_ms: Option<u64>,
    /// Structured loader events (`dev/design/embedder.md` §7). Empty for
    /// caller-supplied embedders; populated from `LoadedWeights.events`
    /// for the Default path.
    pub embedder_events: Vec<EmbedderEvent>,
    /// Static identity capability (`dev/design/embedder.md` §0.6). True
    /// iff the live embedder identity is the bge-small default, which is
    /// the only identity that ships with the EU-5a2 mean-centering apply
    /// paths. `false` for `fathomdb-noop` and for any other
    /// caller-supplied identity. EU-5b's identity flip makes the Default
    /// path return `true` here.
    pub embedder_mean_centering_required: bool,
    /// Dynamic workspace state (`dev/design/embedder.md` §0.6). True iff
    /// `_fathomdb_embedder_profiles.mean_vec IS NOT NULL` for the default
    /// profile. EU-5a2 reads from the schema column added in migration
    /// step 10; the value is dimension-validated (§0.2) at open time
    /// and fails closed via `EmbedderIdentityMismatch` on drift.
    pub embedder_mean_vec_pinned: bool,
}

#[derive(Debug)]
pub struct OpenedEngine {
    pub engine: Engine,
    pub report: OpenReport,
}

/// EU-5b — loader-supplied open-time telemetry threaded into
/// `OpenReport.embedder_download_ms` and `OpenReport.embedder_events`.
#[derive(Clone, Debug)]
struct LoaderInfo {
    download_ms: Option<u64>,
    events: Vec<EmbedderEvent>,
}

#[derive(Clone, Debug, Eq, PartialEq)]
pub struct WriteReceipt {
    pub cursor: u64,
}

/// Soft-fallback signal carried on hybrid `search` results.
///
/// Per `dev/design/retrieval.md` § Soft-fallback signal, this record is
/// present only when one non-essential branch could not contribute. Total
/// request failure is not expressed via this carrier.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct SoftFallback {
    pub branch: SoftFallbackBranch,
}

/// Which retrieval branch could not contribute to a hybrid search.
///
/// `Vector` means the vector branch could not contribute; `Text` means the
/// text branch could not contribute. Owned by `dev/design/retrieval.md`;
/// the 0.6.0 enum is exactly these two members.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum SoftFallbackBranch {
    Vector,
    Text,
}

#[derive(Clone, Debug, Eq, PartialEq)]
pub struct SearchResult {
    pub projection_cursor: u64,
    pub soft_fallback: Option<SoftFallback>,
    pub results: Vec<String>,
}

/// Batch input shape for [`Engine::write`].
///
/// Marked `#[non_exhaustive]` per ADR-0.6.0-prepared-write-shape; new
/// entity variants land in 0.6.x without a major bump. Adding fields to
/// existing variants remains a binding-coordination change.
#[non_exhaustive]
#[derive(Clone, Debug, Eq, PartialEq)]
pub enum PreparedWrite {
    Node {
        kind: String,
        body: String,
        /// REQ-026 / AC-028 / AC-042 recovery seam. `None` is the
        /// back-compat default and lands as NULL on disk; callers that
        /// participate in `excise_source` / `trace_source_ref` must
        /// supply a stable identifier.
        source_id: Option<String>,
    },
    Edge {
        kind: String,
        from: String,
        to: String,
        /// REQ-026 / AC-028 / AC-042 recovery seam — see Node.
        source_id: Option<String>,
    },
    OpStore {
        collection: String,
        record_key: String,
        schema_id: Option<String>,
        body: String,
    },
    AdminSchema {
        name: String,
        kind: String,
        schema_json: String,
        retention_json: String,
    },
}

/// Snapshot of engine-internal counters returned by [`Engine::counters`].
///
/// Public key set is owned by `dev/design/lifecycle.md` § Public key set
/// and locked by AC-004a. Reading a snapshot is non-perturbing per
/// AC-004c. The 0.6.0 surface exposes exactly these seven fields.
#[derive(Clone, Debug, Default, Eq, PartialEq)]
pub struct CounterSnapshot {
    pub queries: u64,
    pub writes: u64,
    pub write_rows: u64,
    pub errors_by_code: BTreeMap<String, u64>,
    pub admin_ops: u64,
    pub cache_hit: u64,
    pub cache_miss: u64,
}

pub use lifecycle::Subscription;

/// Stable corruption-on-open detail carried by
/// [`EngineOpenError::Corruption`].
///
/// Layout owned by `dev/design/errors.md` § Corruption detail owner.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct CorruptionDetail {
    pub kind: CorruptionKind,
    pub stage: OpenStage,
    pub locator: CorruptionLocator,
    pub recovery_hint: RecoveryHint,
}

/// Open-path corruption category.
///
/// 0.6.0 emits exactly the four members below; per
/// `dev/design/errors.md` § Engine.open corruption table, doctor-only
/// finding codes are not represented here.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum CorruptionKind {
    WalReplayFailure,
    HeaderMalformed,
    SchemaInconsistent,
    EmbedderIdentityDrift,
}

/// `Engine.open` stage at which corruption was detected.
///
/// Per ADR-0.6.0-corruption-open-behavior, `LockAcquisition` is intentionally
/// not a member here; lock contention is surfaced via
/// [`EngineOpenError::DatabaseLocked`].
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum OpenStage {
    WalReplay,
    HeaderProbe,
    SchemaProbe,
    EmbedderIdentity,
}

/// Locator pointing at the corrupted region of the database file.
///
/// Variant set owned by `dev/design/errors.md` § CorruptionLocator
/// ownership. `OpaqueSqliteError` is the required fallback when SQLite
/// surfaces corruption without a usable structured locator.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum CorruptionLocator {
    FileOffset { offset: u64 },
    PageId { page: u32 },
    TableRow { table: &'static str, rowid: i64 },
    Vec0ShadowRow { partition: &'static str, rowid: i64 },
    MigrationStep { from: u32, to: u32 },
    OpaqueSqliteError { sqlite_extended_code: i32 },
}

/// Recovery dispatch surface attached to a corruption detail.
///
/// `code` is the stable dispatch key used by bindings and doctor output;
/// `doc_anchor` points at the documentation section that explains the
/// remediation path.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub struct RecoveryHint {
    pub code: &'static str,
    pub doc_anchor: &'static str,
}

#[derive(Clone, Debug, Eq, PartialEq)]
pub enum EngineOpenError {
    DatabaseLocked {
        holder_pid: Option<u32>,
    },
    Corruption(CorruptionDetail),
    IncompatibleSchemaVersion {
        seen: u32,
        supported: u32,
    },
    MigrationError {
        schema_version_before: u32,
        schema_version_current: u32,
        step_id: u32,
    },
    EmbedderIdentityMismatch {
        stored: EmbedderIdentity,
        supplied: EmbedderIdentity,
    },
    EmbedderDimensionMismatch {
        stored: u32,
        supplied: u32,
    },
    /// Embedder runtime returned a typed error during `Engine::open`.
    Embedder(RuntimeEmbedderError),
    Io {
        message: String,
    },
}

/// Caller-facing selector for the embedder used by an opened engine
/// (`dev/design/embedder.md` §0).
#[derive(Clone)]
pub enum EmbedderChoice {
    /// Use the engine's default embedder. With the `default-embedder`
    /// Cargo feature enabled, this materializes a `CandleBgeEmbedder`
    /// via the EU-3 loader at `Engine::open`; on first use the loader
    /// downloads pinned bge-small-en-v1.5 weights from HuggingFace per
    /// `ADR-0.7.1-default-embedder-weight-fetch`. Without the feature,
    /// this returns `EmbedderError::Failed` directing the caller to
    /// rebuild with `--features default-embedder` or supply
    /// `EmbedderChoice::Caller`.
    Default,
    /// Caller supplies the embedder instance. The supplied embedder's
    /// `identity()` becomes the workspace's default-profile identity.
    Caller(Arc<dyn Embedder>),
    /// No embedder configured. Engine opens; subsequent vector writes
    /// fail with `EngineError::EmbedderNotConfigured`. Useful for
    /// read-only or canonical-only flows.
    None,
}

impl Display for EngineOpenError {
    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::DatabaseLocked { holder_pid } => match holder_pid {
                Some(pid) => write!(f, "database is locked by process {pid}"),
                None => write!(f, "database is locked by another engine instance"),
            },
            Self::Corruption(detail) => {
                write!(
                    f,
                    "engine corruption at {:?} stage: {}",
                    detail.stage, detail.recovery_hint.code
                )
            }
            Self::IncompatibleSchemaVersion { seen, supported } => write!(
                f,
                "database schema version {seen} is incompatible with supported version {supported}"
            ),
            Self::MigrationError {
                schema_version_before,
                schema_version_current,
                step_id,
            } => write!(
                f,
                "schema migration failed at step {step_id}; schema version remained between {schema_version_before} and {schema_version_current}"
            ),
            Self::EmbedderIdentityMismatch { stored, supplied } => write!(
                f,
                "embedder identity mismatch: stored {}@{}, supplied {}@{}",
                stored.name, stored.revision, supplied.name, supplied.revision,
            ),
            Self::EmbedderDimensionMismatch { stored, supplied } => write!(
                f,
                "embedder vector dimension mismatch: stored {stored}, supplied {supplied}",
            ),
            Self::Embedder(err) => match err {
                RuntimeEmbedderError::Timeout => write!(f, "embedder timeout during open"),
                RuntimeEmbedderError::Failed { message } => {
                    write!(f, "embedder failure during open: {message}")
                }
            },
            Self::Io { message } => write!(f, "database I/O error: {message}"),
        }
    }
}

impl Error for EngineOpenError {}

#[derive(Clone, Debug, Eq, PartialEq)]
pub enum EngineError {
    Storage,
    Projection,
    Vector,
    Embedder,
    EmbedderNotConfigured,
    KindNotVectorIndexed,
    EmbedderDimensionMismatch { expected: u32, actual: u32 },
    Scheduler,
    OpStore,
    WriteValidation,
    SchemaValidation,
    Overloaded,
    Closing,
}

impl Display for EngineError {
    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::Storage => write!(f, "storage error"),
            Self::Projection => write!(f, "projection error"),
            Self::Vector => write!(f, "vector error"),
            Self::Embedder => write!(f, "embedder error"),
            Self::EmbedderNotConfigured => write!(f, "embedder is not configured"),
            Self::KindNotVectorIndexed => write!(f, "kind is not configured for vector indexing"),
            Self::EmbedderDimensionMismatch { expected, actual } => {
                write!(f, "embedder dimension mismatch: expected {expected}, actual {actual}")
            }
            Self::Scheduler => write!(f, "scheduler error"),
            Self::OpStore => write!(f, "op-store error"),
            Self::WriteValidation => write!(f, "write validation error"),
            Self::SchemaValidation => write!(f, "schema validation error"),
            Self::Overloaded => write!(f, "engine overloaded"),
            Self::Closing => write!(f, "engine is closing"),
        }
    }
}

impl EngineError {
    /// Stable machine-readable code for `errors_by_code` keys.
    ///
    /// Names match the binding-facing class stems in
    /// `dev/design/errors.md` § Binding-facing class matrix.
    fn stable_code(&self) -> &'static str {
        match self {
            Self::Storage => "StorageError",
            Self::Projection => "ProjectionError",
            Self::Vector => "VectorError",
            Self::Embedder => "EmbedderError",
            Self::EmbedderNotConfigured => "EmbedderNotConfiguredError",
            Self::KindNotVectorIndexed => "KindNotVectorIndexedError",
            Self::EmbedderDimensionMismatch { .. } => "EmbedderDimensionMismatchError",
            Self::Scheduler => "SchedulerError",
            Self::OpStore => "OpStoreError",
            Self::WriteValidation => "WriteValidationError",
            Self::SchemaValidation => "SchemaValidationError",
            Self::Overloaded => "OverloadedError",
            Self::Closing => "ClosingError",
        }
    }
}

impl Error for EngineError {}

/// Doctor `check-integrity` invocation flags. `quick` and `round_trip`
/// are accepted in 0.6.0 but treated as default; only `full` activates
/// `PRAGMA integrity_check`. Per `dev/design/recovery.md` § Doctor-only
/// flags.
#[derive(Clone, Copy, Debug, Default, Eq, PartialEq)]
pub struct CheckIntegrityOpts {
    pub quick: bool,
    pub full: bool,
    pub round_trip: bool,
}

/// One section of an [`IntegrityReport`]. Either every check in the
/// section was clean, or one or more typed [`Finding`]s describe the
/// detected issue. Per AC-043b.
#[derive(Clone, Debug, Eq, PartialEq)]
pub enum Section {
    Clean,
    Findings(Vec<Finding>),
}

/// Single doctor finding record. Stable report-shape per AC-043c. The
/// `code` and `doc_anchor` strings are stable dispatch keys owned by
/// `dev/design/recovery.md` § Code-to-operator-action cross-reference.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct Finding {
    pub code: &'static str,
    pub stage: &'static str,
    pub locator: CorruptionLocator,
    pub doc_anchor: &'static str,
    pub detail: String,
}

/// Three-section integrity report. AC-043a pins exactly these three
/// keys.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct IntegrityReport {
    pub physical: Section,
    pub logical: Section,
    pub semantic: Section,
}

/// Result of a successful [`Engine::safe_export`] call. The returned
/// `manifest_sha256` equals the SHA-256 of the export file bytes (per
/// AC-039a) and matches the `sha256` field written into the manifest
/// JSON.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct SafeExportArtifact {
    pub export_path: PathBuf,
    pub manifest_path: PathBuf,
    pub manifest_sha256: String,
}

/// Phase 9 Pack B trace report (AC-042). One event per canonical row
/// attributable to the requested `source_id`, ordered by `write_cursor`
/// ascending.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct TraceReport {
    pub source_ref: String,
    pub events: Vec<TraceEvent>,
}

/// Single canonical-row tracing record. `table` is one of
/// `"canonical_nodes"` or `"canonical_edges"`.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct TraceEvent {
    pub write_cursor: u64,
    pub kind: String,
    pub table: &'static str,
}

/// Which shadow-state surface a [`RebuildReport`] describes.
/// `Projections` covers the full FTS5 + vec0 + projection-terminal
/// rebuild emitted by [`Engine::rebuild_projections`]. `Vec0` covers
/// the vec0-only path emitted by [`Engine::rebuild_vec0`].
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum RebuildKind {
    Projections,
    Vec0,
}

/// Structured result of a rebuild operation. `rows_invalidated` is the
/// total shadow-state rows truncated before re-derivation; `rows_rebuilt`
/// is the count of rows the synchronous rebuild loop re-materialised
/// (asynchronous re-enqueue work performed by the projection scheduler is
/// not counted here). `projection_cursor_after` is the post-rebuild value
/// of the projection cursor.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct RebuildReport {
    pub kind: RebuildKind,
    pub rows_invalidated: u64,
    pub rows_rebuilt: u64,
    pub projection_cursor_after: u64,
}

/// Phase 9 Pack B excise report (AC-028a/b/c). Counts are post-excise
/// totals; `projections_invalidated` reports the shadow-row invalidation
/// total (FTS5 + vec0 + projection terminal) for the excised source.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct ExciseReport {
    pub source_ref: String,
    pub nodes_excised: u64,
    pub edges_excised: u64,
    pub projections_invalidated: u64,
}

/// Typed outcome of [`Engine::verify_embedder`]. Mismatches do not raise
/// `EngineError`; the operator workflow needs to see the stored vs.
/// supplied pair to decide on next action.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum VerifyEmbedderStatus {
    Match,
    IdentityMismatch,
    DimensionMismatch,
    BothMismatch,
}

/// Result of [`Engine::verify_embedder`]. `stored_identity` is the
/// `name:revision` pair persisted in `_fathomdb_embedder_profiles`;
/// `supplied_identity` echoes the operator's input verbatim.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct VerifyEmbedderReport {
    pub stored_identity: String,
    pub stored_dimension: u32,
    pub supplied_identity: String,
    pub supplied_dimension: u32,
    pub status: VerifyEmbedderStatus,
}

/// Single table or index entry emitted by [`Engine::dump_schema`].
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct SchemaObject {
    pub name: String,
    pub sql: String,
}

/// Result of [`Engine::dump_schema`]. `user_version` is the
/// `PRAGMA user_version` sentinel. Canonical tables appear first per
/// [`fathomdb_schema::CANONICAL_TABLES`], then remaining non-`sqlite_*`
/// tables alphabetically. Indexes follow the same alphabetical rule.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct DumpSchemaReport {
    pub user_version: u32,
    pub tables: Vec<SchemaObject>,
    pub indexes: Vec<SchemaObject>,
}

/// Single canonical-table row count emitted by [`Engine::dump_row_counts`].
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct TableRowCount {
    pub name: String,
    pub rows: u64,
}

/// Result of [`Engine::dump_row_counts`]. Canonical tables only;
/// projection / FTS / vec0 shadow tables are excluded. Order matches
/// [`fathomdb_schema::CANONICAL_TABLES`].
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct DumpRowCountsReport {
    pub counts: Vec<TableRowCount>,
}

/// Result of [`Engine::dump_profile`]. Mirrors the open-time embedder
/// posture + the per-kind vector configuration registered in
/// `_fathomdb_vector_kinds`.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct DumpProfileReport {
    pub embedder_identity: String,
    pub embedder_dimension: u32,
    pub vectorized_kinds: Vec<String>,
}

/// 0.7.2 PR-2b — result of [`Engine::recompute_mean`] (the manual
/// `doctor recompute-mean` path) and of the shared in-transaction
/// recompute core. `drift_cos_before` is the cosine between the freshly
/// derived corpus mean and the previously-pinned mean (1.0 when nothing
/// was pinned yet, i.e. a first pin). `mean_was_pinned` distinguishes a
/// refresh of an existing mean from an initial pin. See
/// `dev/design/embedder.md` §0.3.
#[derive(Clone, Debug, PartialEq)]
pub struct MeanRecomputeReport {
    pub dim: u32,
    pub old_doc_count: u64,
    pub doc_count_requantized: u64,
    pub drift_cos_before: f32,
    pub mean_was_pinned: bool,
    pub elapsed_ms: u64,
}

/// Typed outcome of [`Engine::truncate_wal`]. `Done` matches SQLite's
/// `busy = 0` return from `PRAGMA wal_checkpoint(TRUNCATE)`; any other
/// value surfaces as `Busy`.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum TruncateWalStatus {
    Done,
    Busy,
}

/// Result of [`Engine::truncate_wal`]. Carries the three counters
/// returned by `PRAGMA wal_checkpoint(TRUNCATE)`: `busy`, `log_frames`,
/// `checkpointed_frames`.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct TruncateWalReport {
    pub status: TruncateWalStatus,
    pub busy: u32,
    pub log_frames: u32,
    pub checkpointed_frames: u32,
}

impl Drop for Engine {
    fn drop(&mut self) {
        let _ = self.close();
    }
}

impl Engine {
    pub fn open(path: impl Into<PathBuf>) -> Result<OpenedEngine, EngineOpenError> {
        Self::open_with_embedder_and_subscriber(
            path,
            default_embedder_identity(),
            None,
            None,
            None,
            &mut |_| {},
        )
    }

    /// Open an engine with an explicit [`EmbedderChoice`].
    ///
    /// Per `dev/design/embedder.md` §0 + the 0.7.1 EU-5 campaign, this is
    /// the canonical entry point for selecting how the workspace's
    /// default embedder is supplied. See [`EmbedderChoice`] for the
    /// semantics of each variant; in particular `Default` materializes
    /// the pinned BGE embedder via the loader when the `default-embedder`
    /// feature is enabled.
    pub fn open_with_choice(
        path: impl Into<PathBuf>,
        choice: EmbedderChoice,
    ) -> Result<OpenedEngine, EngineOpenError> {
        match choice {
            EmbedderChoice::Default => Self::open_default_embedder(path),
            EmbedderChoice::Caller(embedder) => {
                let identity = embedder.identity();
                Self::open_with_embedder_and_subscriber(
                    path,
                    identity,
                    Some(embedder),
                    None,
                    None,
                    &mut |_| {},
                )
            }
            EmbedderChoice::None => Self::open_with_embedder_and_subscriber(
                path,
                default_embedder_identity(),
                None,
                None,
                None,
                &mut |_| {},
            ),
        }
    }

    /// EU-5b: materialize the engine's pinned default embedder
    /// (`CandleBgeEmbedder` backed by the EU-3 loader) and open the
    /// workspace with it. Without the `default-embedder` feature, fails
    /// with a typed `Embedder` error rather than touching the network.
    #[cfg(feature = "default-embedder")]
    fn open_default_embedder(path: impl Into<PathBuf>) -> Result<OpenedEngine, EngineOpenError> {
        use std::time::Instant as DownloadInstant;
        let download_start = DownloadInstant::now();
        let weights = fathomdb_embedder::loader::load_pinned_default_embedder().map_err(|err| {
            EngineOpenError::Embedder(RuntimeEmbedderError::Failed {
                message: format!("default embedder loader: {err}"),
            })
        })?;
        let events = weights.events.clone();
        let download_ms = if weights.bytes_downloaded > 0 {
            Some(u64::try_from(download_start.elapsed().as_millis()).unwrap_or(u64::MAX))
        } else {
            None
        };
        let embedder =
            fathomdb_embedder::CandleBgeEmbedder::new_from_weights(weights).map_err(|err| {
                EngineOpenError::Embedder(RuntimeEmbedderError::Failed {
                    message: format!("default embedder construct: {err}"),
                })
            })?;
        let embedder: Arc<dyn Embedder> = Arc::new(embedder);
        let identity = embedder.identity();
        let loader_info = LoaderInfo { download_ms, events };
        Self::open_with_embedder_and_subscriber(
            path,
            identity,
            Some(embedder),
            Some(loader_info),
            None,
            &mut |_| {},
        )
    }

    #[cfg(not(feature = "default-embedder"))]
    fn open_default_embedder(_path: impl Into<PathBuf>) -> Result<OpenedEngine, EngineOpenError> {
        Err(EngineOpenError::Embedder(RuntimeEmbedderError::Failed {
            message: "EmbedderChoice::Default requires the `default-embedder` Cargo feature"
                .to_string(),
        }))
    }

    pub fn open_with_migration_event_sink(
        path: impl Into<PathBuf>,
        mut emit_migration_event: impl FnMut(&MigrationStepReport),
    ) -> Result<OpenedEngine, EngineOpenError> {
        Self::open_with_embedder_and_subscriber(
            path,
            default_embedder_identity(),
            None,
            None,
            None,
            &mut emit_migration_event,
        )
    }

    #[cfg(debug_assertions)]
    #[doc(hidden)]
    pub fn open_with_migrations_for_test(
        path: impl Into<PathBuf>,
        migrations: &'static [fathomdb_schema::Migration],
        mut emit_migration_event: impl FnMut(&MigrationStepReport),
    ) -> Result<OpenedEngine, EngineOpenError> {
        Self::open_with_migrations(
            path,
            migrations,
            default_embedder_identity(),
            None,
            None,
            &mut emit_migration_event,
            None,
        )
    }

    #[doc(hidden)]
    pub fn open_with_subscriber_for_test(
        path: impl Into<PathBuf>,
        subscriber: Arc<dyn lifecycle::Subscriber>,
    ) -> Result<OpenedEngine, EngineOpenError> {
        Self::open_with_embedder_and_subscriber(
            path,
            default_embedder_identity(),
            None,
            None,
            Some(subscriber),
            &mut |_| {},
        )
    }

    #[doc(hidden)]
    pub fn open_without_embedder_for_test(
        path: impl Into<PathBuf>,
    ) -> Result<OpenedEngine, EngineOpenError> {
        Self::open_with_embedder_and_subscriber(
            path,
            default_embedder_identity(),
            None,
            None,
            None,
            &mut |_| {},
        )
    }

    #[doc(hidden)]
    pub fn open_with_embedder_for_test(
        path: impl Into<PathBuf>,
        embedder: Arc<dyn Embedder>,
    ) -> Result<OpenedEngine, EngineOpenError> {
        let identity = embedder.identity();
        Self::open_with_embedder_and_subscriber(
            path,
            identity,
            Some(embedder),
            None,
            None,
            &mut |_| {},
        )
    }

    fn open_with_embedder_and_subscriber(
        path: impl Into<PathBuf>,
        embedder_identity: EmbedderIdentity,
        runtime_embedder: Option<Arc<dyn Embedder>>,
        loader_info: Option<LoaderInfo>,
        initial_subscriber: Option<Arc<dyn lifecycle::Subscriber>>,
        emit_migration_event: &mut impl FnMut(&MigrationStepReport),
    ) -> Result<OpenedEngine, EngineOpenError> {
        Self::open_with_migrations(
            path,
            MIGRATIONS,
            embedder_identity,
            runtime_embedder,
            loader_info,
            emit_migration_event,
            initial_subscriber,
        )
    }

    fn open_with_migrations(
        path: impl Into<PathBuf>,
        migrations: &'static [fathomdb_schema::Migration],
        embedder_identity: EmbedderIdentity,
        runtime_embedder: Option<Arc<dyn Embedder>>,
        loader_info: Option<LoaderInfo>,
        emit_migration_event: &mut impl FnMut(&MigrationStepReport),
        initial_subscriber: Option<Arc<dyn lifecycle::Subscriber>>,
    ) -> Result<OpenedEngine, EngineOpenError> {
        let canonical_path = canonical_database_path(&path.into())?;
        let lock = acquire_lock(&canonical_path)?;
        let open_result = Self::open_locked(
            canonical_path.clone(),
            migrations,
            &embedder_identity,
            emit_migration_event,
        );

        match open_result {
            Ok((connection, readers, mut report, reader_lookaside_rcs)) => {
                // EU-5b — splice the loader's measurements + structured
                // events into the report. The loader path is the only
                // surface that produces these today; caller-supplied
                // embedders and EmbedderChoice::None leave them as the
                // open_locked defaults (None / empty).
                if let Some(info) = loader_info {
                    if info.download_ms.is_some() {
                        report.embedder_download_ms = info.download_ms;
                    }
                    if !info.events.is_empty() {
                        report.embedder_events = info.events;
                    }
                }
                let next_cursor = load_next_cursor(&connection);
                let subscribers = Arc::new(lifecycle::SubscriberRegistry::new());
                let profiling_enabled = Arc::new(AtomicBool::new(false));
                let slow_threshold_ms = Arc::new(AtomicU64::new(DEFAULT_SLOW_THRESHOLD_MS));
                let mut profile_contexts: Vec<Box<ProfileContext>> = Vec::new();
                let projection_runtime = ProjectionRuntime::new(
                    canonical_path.clone(),
                    runtime_embedder.clone(),
                    embedder_identity.clone(),
                    report.embedder_mean_vec_pinned,
                );

                install_profile_callback(
                    &connection,
                    &subscribers,
                    &profiling_enabled,
                    &slow_threshold_ms,
                    &mut profile_contexts,
                );
                for reader in &readers {
                    install_profile_callback(
                        reader,
                        &subscribers,
                        &profiling_enabled,
                        &slow_threshold_ms,
                        &mut profile_contexts,
                    );
                }

                let opened = OpenedEngine {
                    engine: Self {
                        path: canonical_path.clone(),
                        next_cursor: AtomicU64::new(next_cursor),
                        closed: AtomicBool::new(false),
                        lock: Mutex::new(Some(lock)),
                        connection: Mutex::new(Some(connection)),
                        reader_pool: ReaderWorkerPool::new(readers),
                        counters: lifecycle::Counters::new(),
                        subscribers,
                        profiling_enabled,
                        slow_threshold_ms,
                        runtime_embedder,
                        runtime_embedder_identity: embedder_identity,
                        projection_runtime,
                        provenance_row_cap: AtomicU64::new(DEFAULT_PROVENANCE_ROW_CAP),
                        profile_contexts: Mutex::new(profile_contexts),
                        reader_lookaside_rcs,
                        #[cfg(debug_assertions)]
                        force_next_commit_failure: AtomicBool::new(false),
                    },
                    report,
                };
                if let Some(subscriber) = initial_subscriber {
                    opened.engine.subscribers.attach_persistent(subscriber);
                }
                if database_has_pending_projection_work(&canonical_path).unwrap_or(false) {
                    opened.engine.projection_runtime.notify_new_work();
                }
                Ok(opened)
            }
            Err(err) => {
                if let Some(subscriber) = initial_subscriber {
                    emit_open_error_event(&subscriber, &err);
                }
                drop(lock);
                Err(err)
            }
        }
    }

    fn open_locked(
        path: PathBuf,
        migrations: &'static [fathomdb_schema::Migration],
        embedder_identity: &EmbedderIdentity,
        emit_migration_event: &mut impl FnMut(&MigrationStepReport),
    ) -> Result<(Connection, Vec<Connection>, OpenReport, Vec<i32>), EngineOpenError> {
        init_perf_experiments_runtime();
        register_sqlite_vec_extension();
        let mut connection = Connection::open(&path)
            .map_err(|err| map_open_sqlite_error(err, OpenStage::HeaderProbe))?;
        // Order pinned by `dev/design/errors.md` § OpenStage matrix: each
        // step routes its own SQLite-level error to a distinct
        // `CorruptionKind` (Header → WalReplay → Schema → EmbedderIdentity).
        // The schema and WAL probes both happen BEFORE `pragma WAL`
        // because that pragma also reads page 1 — letting it run first
        // would reclassify schema-side corruption as a WAL replay
        // failure, breaking the AC-035b stable-code contract.
        probe_database_header(&connection)?;
        probe_open_integrity(&connection)?;
        probe_wal_sidecar(&path)?;
        // 0.7.0 perf-experiments: apply writer-side experiment PRAGMAs
        // (page_size, etc.) BEFORE journal_mode + migrations. page_size
        // is silently ignored once any table exists; this is the only
        // legal window to set it on a fresh DB. Gated on
        // FATHOMDB_PERF_EXPERIMENTS=1; no-op in production.
        apply_perf_experiment_writer_pragmas(&connection);
        connection
            .pragma_update(None, "journal_mode", "WAL")
            .map_err(|err| map_open_sqlite_error(err, OpenStage::WalReplay))?;

        reject_legacy_shape(&connection)?;
        let migration = migrate_with_event_sink(&connection, migrations, emit_migration_event)
            .map_err(map_migration_error)?;
        let mut embedder_mean_vec_pinned = check_embedder_profile(&connection, embedder_identity)?;
        ensure_vector_partition(&mut connection, embedder_identity.dimension).map_err(|_| {
            EngineOpenError::Io { message: "could not initialize vector partition".to_string() }
        })?;

        // EU-5f — recovery pin (`dev/design/embedder.md` §0.3, Hazard 4). If
        // the identity is MC-required, no mean is pinned, yet the workspace
        // already holds >= MEAN_VEC_PIN_THRESHOLD vector rows (e.g. a crash
        // between the threshold-crossing write and its pin commit), derive
        // the mean from the existing un-centered rows and pin+re-quantize
        // now, single-threaded, before the projection workers spawn. The
        // NULL guard makes this idempotent on subsequent opens.
        if identity_requires_mean_centering(embedder_identity) && !embedder_mean_vec_pinned {
            let row_count: u64 = connection
                .query_row("SELECT COUNT(*) FROM vector_default", [], |row| row.get(0))
                .unwrap_or(0);
            if row_count >= MEAN_VEC_PIN_THRESHOLD {
                recover_mean_vec_pin(&mut connection, embedder_identity).map_err(|_| {
                    EngineOpenError::Io {
                        message: "could not recover mean-centering pin".to_string(),
                    }
                })?;
                embedder_mean_vec_pinned = true;
            }
        }

        let warmup_started = Instant::now();
        // Static identity capability — see `dev/design/embedder.md`
        // §0.6. Today only the bge-small identity reports `true`; the
        // noop scaffolding identity is `false`. EU-5b's identity flip
        // makes the Default path return `true` here automatically.
        let embedder_mean_centering_required = embedder_identity.name == BGE_SMALL_EMBEDDER_NAME;
        // EU-5a2 — populated from `_fathomdb_embedder_profiles.mean_vec`
        // by `check_embedder_profile` above (was hard-coded `false` in
        // EU-5a1). Dimension invariant (§0.2) enforced by that check.
        let report = OpenReport {
            schema_version_before: migration.schema_version_before,
            schema_version_after: migration.schema_version_after,
            migration_steps: migration.migration_steps,
            embedder_warmup_ms: u64::try_from(warmup_started.elapsed().as_millis())
                .unwrap_or(u64::MAX),
            query_backend: "fathomdb-query + sqlite-vec",
            default_embedder: embedder_identity.clone(),
            // TODO(EU-5b): surface `LoadedWeights.download_ms` from the
            // loader once the Default path materializes through it.
            embedder_download_ms: None,
            // TODO(EU-5b): surface `LoadedWeights.events` from the loader.
            embedder_events: Vec::new(),
            embedder_mean_centering_required,
            embedder_mean_vec_pinned,
        };

        let mut readers = Vec::with_capacity(READER_POOL_SIZE);
        let mut lookaside_rcs: Vec<i32> = Vec::with_capacity(READER_POOL_SIZE);
        for _ in 0..READER_POOL_SIZE {
            let reader = Connection::open(&path)
                .map_err(|err| map_open_sqlite_error(err, OpenStage::HeaderProbe))?;
            // Pack 6.G G.1: configure per-connection lookaside BEFORE
            // any PRAGMA / prepare runs on this reader. Reordering this
            // after the journal-mode / query_only PRAGMAs would let
            // SQLite silently ignore the lookaside setting.
            let rc: i32 = configure_reader_lookaside(&reader);
            debug_assert_eq!(
                rc,
                rusqlite::ffi::SQLITE_OK,
                "sqlite3_db_config(LOOKASIDE) must return SQLITE_OK on a freshly opened reader",
            );
            lookaside_rcs.push(rc);
            reader
                .pragma_update(None, "journal_mode", "WAL")
                .map_err(|err| map_open_sqlite_error(err, OpenStage::WalReplay))?;
            reader
                .pragma_update(None, "query_only", "ON")
                .map_err(|err| map_open_sqlite_error(err, OpenStage::SchemaProbe))?;
            apply_perf_experiment_reader_pragmas(&reader);
            readers.push(reader);
        }

        Ok((connection, readers, report, lookaside_rcs))
    }

    #[must_use]
    pub fn path(&self) -> &Path {
        &self.path
    }

    pub fn write(&self, batch: &[PreparedWrite]) -> Result<WriteReceipt, EngineError> {
        let category = if batch_is_admin(batch) {
            lifecycle::EventCategory::Admin
        } else {
            lifecycle::EventCategory::Writer
        };
        self.emit_event(lifecycle::Phase::Started, category, None);
        let started = Instant::now();
        let outcome = self.write_inner(batch);
        self.detect_slow(started, category);
        match outcome {
            Ok(receipt) => {
                let rows = u64::try_from(batch.len()).unwrap_or(u64::MAX);
                if batch_is_admin(batch) {
                    self.counters.record_admin();
                } else {
                    self.counters.record_write(rows);
                }
                self.emit_event(lifecycle::Phase::Finished, category, None);
                Ok(receipt)
            }
            Err(err) => {
                let code = err.stable_code();
                self.counters.record_error(code);
                // AC-003d: capture-ordinal < raise-ordinal — Failed and Error
                // events both fire before the EngineError returns to the caller.
                self.emit_event(lifecycle::Phase::Failed, category, Some(code));
                self.emit_event(
                    lifecycle::Phase::Failed,
                    lifecycle::EventCategory::Error,
                    Some(code),
                );
                Err(err)
            }
        }
    }

    fn write_inner(&self, batch: &[PreparedWrite]) -> Result<WriteReceipt, EngineError> {
        self.ensure_open()?;

        if batch.is_empty() {
            return Err(EngineError::WriteValidation);
        }

        let mut connection = self.connection.lock().map_err(|_| EngineError::Storage)?;
        let connection = connection.as_mut().ok_or(EngineError::Closing)?;
        let plans = validate_batch(connection, batch)?;
        let projection_jobs = collect_projection_jobs(connection, batch)?;
        #[cfg(debug_assertions)]
        if self.force_next_commit_failure.swap(false, Ordering::SeqCst) {
            return Err(EngineError::Storage);
        }
        // One cursor per row. `base_cursor` is the last committed cursor;
        // row i in the batch gets cursor `base_cursor + i + 1`, and the
        // batch's final cursor (returned in WriteReceipt and stored as
        // the new `next_cursor`) is `base_cursor + batch.len()`. Sharing
        // one cursor across the batch previously collapsed every vec0
        // INSERT onto the same rowid via `INSERT OR IGNORE` — see
        // `dev/notes/0.7.0-engine-batch-vec0-collapse.md`.
        let base_cursor = self.next_cursor.load(Ordering::SeqCst);
        let increment = u64::try_from(batch.len()).unwrap_or(u64::MAX);
        let last_cursor = base_cursor.saturating_add(increment);
        let pending_projection = !projection_jobs.is_empty();

        if let Err(err) = commit_batch(
            connection,
            batch,
            &plans,
            base_cursor,
            self.provenance_row_cap.load(Ordering::Relaxed),
        ) {
            self.emit_sqlite_internal_error(&err);
            return Err(EngineError::Storage);
        }
        self.next_cursor.store(last_cursor, Ordering::SeqCst);
        if pending_projection {
            self.projection_runtime.notify_new_work();
        }

        Ok(WriteReceipt { cursor: last_cursor })
    }

    pub fn search(&self, query: &str) -> Result<SearchResult, EngineError> {
        self.emit_event(lifecycle::Phase::Started, lifecycle::EventCategory::Search, None);
        let started = Instant::now();
        let outcome = self.search_inner(query);
        self.detect_slow(started, lifecycle::EventCategory::Search);
        match outcome {
            Ok(result) => {
                self.counters.record_query();
                self.emit_event(lifecycle::Phase::Finished, lifecycle::EventCategory::Search, None);
                Ok(result)
            }
            Err(err) => {
                let code = err.stable_code();
                self.counters.record_error(code);
                self.emit_event(
                    lifecycle::Phase::Failed,
                    lifecycle::EventCategory::Search,
                    Some(code),
                );
                self.emit_event(
                    lifecycle::Phase::Failed,
                    lifecycle::EventCategory::Error,
                    Some(code),
                );
                Err(err)
            }
        }
    }

    fn detect_slow(&self, started: Instant, category: lifecycle::EventCategory) {
        let elapsed = started.elapsed();
        let threshold = self.slow_threshold_ms.load(Ordering::Relaxed);
        let threshold_duration = std::time::Duration::from_millis(threshold);
        if elapsed > threshold_duration {
            // `dev/design/lifecycle.md` § Slow and heartbeat policy: a slow
            // operation produces TWO correlated facts. The
            // statement-level slow-statement signal is dispatched by the
            // sqlite3_profile callback (`profile_callback_trampoline`).
            // This site emits the lifecycle `Phase::Slow` event for the
            // outer operation envelope (AC-008).
            self.emit_event(lifecycle::Phase::Slow, category, None);
        }
    }

    fn emit_event(
        &self,
        phase: lifecycle::Phase,
        category: lifecycle::EventCategory,
        code: Option<&'static str>,
    ) {
        let event =
            lifecycle::Event { phase, source: lifecycle::EventSource::Engine, category, code };
        self.subscribers.dispatch(&event);
    }

    /// Emit a `(SqliteInternal, Error, code: <SQLITE_*>)` lifecycle
    /// event for a rusqlite error. Per `dev/design/lifecycle.md`
    /// § Diagnostic source and category, SQLite-originated diagnostics
    /// route through the same host subscriber as engine-originated
    /// events with `source` preserved. AC-021 dispatches on
    /// `code == "SQLITE_SCHEMA"`.
    fn emit_sqlite_internal_error(&self, err: &rusqlite::Error) {
        if let Some(code) = sqlite_extended_code_name(err) {
            let event = lifecycle::Event {
                phase: lifecycle::Phase::Failed,
                source: lifecycle::EventSource::SqliteInternal,
                category: lifecycle::EventCategory::Error,
                code: Some(code),
            };
            self.subscribers.dispatch(&event);
        }
    }

    fn search_inner(&self, query: &str) -> Result<SearchResult, EngineError> {
        self.ensure_open()?;
        if query.trim().is_empty() {
            return Err(EngineError::WriteValidation);
        }

        let compiled = compile_text_query(query);
        // REQ-013 / AC-059b / REQ-055: the cursor returned with a search
        // MUST be derived from the same WAL snapshot the data was read
        // from. Loading `next_cursor` from the writer-side atomic before
        // the reader transaction acquires its snapshot races against
        // concurrent writers — see `dev/design/engine.md` § Cursor
        // contract. Run cursor probe + body query inside one read tx
        // (BEGIN DEFERRED on a `query_only=ON` connection in WAL mode is
        // a snapshot-stable read).
        // EU-5a2 mean-centering apply path (query side). `query_vector`
        // is ALWAYS un-centered (used by the f32 vec_distance_l2 rerank
        // in phase 2). `query_vector_bin` is the (possibly centered) f32
        // fed to `vec_quantize_binary` in phase 1. The centering decision
        // mirrors the write path: identity must be MC-required AND a
        // mean_vec must be pinned. NoopEmbedder collapses to
        // `query_vector_bin == query_vector` until EU-5b.
        let raw_query_vector =
            self.runtime_embedder.as_ref().and_then(|embedder| embedder.embed(query).ok());
        let query_vector_bin = match raw_query_vector.as_ref() {
            Some(vector) if identity_requires_mean_centering(&self.runtime_embedder_identity) => {
                let pinned = {
                    let connection = self.connection.lock().map_err(|_| EngineError::Storage)?;
                    let connection = connection.as_ref().ok_or(EngineError::Closing)?;
                    read_pinned_mean_vec(connection, self.runtime_embedder_identity.dimension)?
                };
                match pinned {
                    Some(mean) => serde_json::to_string(&subtract_mean(vector, &mean)).ok(),
                    None => serde_json::to_string(vector).ok(),
                }
            }
            Some(vector) => serde_json::to_string(vector).ok(),
            None => None,
        };
        let query_vector = raw_query_vector.and_then(|vector| serde_json::to_string(&vector).ok());
        // 0.7.2 PR-2bc S1 fix-1 — phase-2 rerank LIMIT. Production default is
        // `SEARCH_RERANK_LIMIT` (10); the test seam may RAISE it, clamped to
        // the production floor so a test can never shrink search semantics.
        let search_limit = self
            .projection_runtime
            .shared
            .search_limit_override
            .load(Ordering::SeqCst)
            .max(SEARCH_RERANK_LIMIT);
        let (response_tx, response_rx) = mpsc::sync_channel::<ReaderResponse>(1);
        let request = ReaderRequest::Search {
            compiled,
            query_vector,
            query_vector_bin,
            search_limit,
            respond: response_tx,
        };
        if self.reader_pool.dispatch(request).is_err() {
            return Err(EngineError::Closing);
        }
        let search_result = response_rx.recv().map_err(|_| EngineError::Storage)?;
        let (cursor, soft_fallback, results) = match search_result {
            Ok(result) => result,
            Err(err) => {
                self.emit_sqlite_internal_error(&err);
                return Err(EngineError::Storage);
            }
        };

        Ok(SearchResult { projection_cursor: cursor, soft_fallback, results })
    }

    pub fn close(&self) -> Result<(), EngineError> {
        self.closed.store(true, Ordering::SeqCst);
        self.projection_runtime.stop();
        // Uninstall profile callbacks before dropping the connections so
        // SQLite cannot fire one last callback against a profile context
        // whose Box is about to free. Per `dev/design/engine.md` § Close
        // path step 6, readers drain before the writer connection so
        // SQLite's last-handle checkpointer runs on the writer. Each
        // reader worker uninstalls its own callback inside
        // `reader_worker_loop` before dropping its connection, then
        // exits — `shutdown` joins those threads here.
        self.reader_pool.shutdown();
        if let Ok(mut connection) = self.connection.lock() {
            if let Some(conn) = connection.as_ref() {
                uninstall_profile_callback(conn);
            }
            connection.take();
        }
        if let Ok(mut contexts) = self.profile_contexts.lock() {
            contexts.clear();
        }
        if let Ok(mut lock) = self.lock.lock() {
            lock.take();
        }
        Ok(())
    }

    /// Block until in-flight writes drain or `timeout_ms` elapses.
    ///
    /// Surface owned by `dev/interfaces/rust.md` § Engine-attached
    /// instrumentation; semantics are owned by `dev/design/lifecycle.md`.
    pub fn drain(&self, timeout_ms: u64) -> Result<(), EngineError> {
        self.ensure_open()?;
        if self.projection_runtime.wait_for_idle(timeout_ms) {
            Ok(())
        } else {
            Err(EngineError::Scheduler)
        }
    }

    /// Snapshot of engine-internal counters.
    ///
    /// Field set owned by `dev/design/lifecycle.md`.
    #[must_use]
    pub fn counters(&self) -> CounterSnapshot {
        self.counters.snapshot()
    }

    /// Toggle response-cycle profiling.
    ///
    /// Per `dev/design/lifecycle.md` § Per-statement profiling, profiling
    /// is an opt-in surface that is independently toggleable on a running
    /// engine without restart. AC-005a locks runtime toggleability.
    pub fn set_profiling(&self, enabled: bool) -> Result<(), EngineError> {
        self.profiling_enabled.store(enabled, Ordering::Relaxed);
        Ok(())
    }

    /// Set the threshold above which an operation is reported as slow.
    ///
    /// Per `dev/design/lifecycle.md` § Slow and heartbeat policy, the
    /// threshold is runtime-configurable; mutating it changes detection
    /// behavior on subsequent statements without restart (AC-007b).
    pub fn set_slow_threshold_ms(&self, value: u64) -> Result<(), EngineError> {
        self.slow_threshold_ms.store(value, Ordering::Relaxed);
        Ok(())
    }

    /// Attach a host subscriber to engine events.
    ///
    /// Dropping the returned [`Subscription`] detaches the subscriber.
    /// Payload shape owned by `dev/design/lifecycle.md` and
    /// `dev/design/migrations.md`.
    #[must_use]
    pub fn subscribe(&self, subscriber: Arc<dyn lifecycle::Subscriber>) -> Subscription {
        self.subscribers.attach(subscriber)
    }

    #[cfg(debug_assertions)]
    #[doc(hidden)]
    pub fn reader_worker_count_for_test(&self) -> usize {
        self.reader_pool.worker_count()
    }

    #[cfg(debug_assertions)]
    #[doc(hidden)]
    pub fn live_reader_worker_count_for_test(&self) -> usize {
        self.reader_pool.live_count()
    }

    /// Pack 6.G G.1 — return the `sqlite3_db_config(LOOKASIDE)` rc
    /// captured for each reader worker at open time, in worker index
    /// order. SQLITE_OK (= 0) means the lookaside was configured
    /// before any allocation happened on the connection.
    #[cfg(debug_assertions)]
    #[doc(hidden)]
    pub fn reader_lookaside_config_rcs_for_test(&self) -> Vec<i32> {
        self.reader_lookaside_rcs.clone()
    }

    /// Pack 6.G G.1 — query each reader worker's
    /// `SQLITE_DBSTATUS_LOOKASIDE_USED` counter. A value > 0 means at
    /// least one allocation was satisfied from the per-connection
    /// lookaside arena (proof the configuration was honored before the
    /// first prepare).
    #[cfg(debug_assertions)]
    #[doc(hidden)]
    pub fn reader_lookaside_used_per_worker_for_test(&self) -> Vec<i32> {
        self.reader_pool.lookaside_used_per_worker()
    }

    /// Pack 6.G G.3.5 — broadcast a debug-only `CacheStatus` request to
    /// every reader worker and collect per-worker
    /// `SQLITE_DBSTATUS_CACHE_HIT` / `_CACHE_MISS` / `_CACHE_USED`
    /// values. Counters are monotonic (reset flag = 0); callers compute
    /// pre/post deltas explicitly.
    #[cfg(debug_assertions)]
    #[doc(hidden)]
    pub fn cache_status_per_worker_for_test(&self, label: &str) -> Vec<CacheStatusReply> {
        self.reader_pool.cache_status_per_worker(label)
    }

    #[cfg(debug_assertions)]
    #[doc(hidden)]
    pub fn force_next_commit_failure_for_test(&self) {
        self.force_next_commit_failure.store(true, Ordering::SeqCst);
    }

    /// Execute an arbitrary SQL statement on the writer connection through
    /// the same wall-clock + slow-detect path as `write` / `search`.
    ///
    /// Test-only helper for the deterministic-slow-cte fixture used by
    /// AC-007a / AC-007b. Not part of the public 0.6.0 surface; gated on
    /// `debug_assertions` so release builds do not expose it.
    #[cfg(debug_assertions)]
    #[doc(hidden)]
    pub fn execute_for_test(&self, sql: &str) -> Result<(), EngineError> {
        self.ensure_open()?;
        let started = Instant::now();
        {
            let mut connection = self.connection.lock().map_err(|_| EngineError::Storage)?;
            let connection = connection.as_mut().ok_or(EngineError::Closing)?;
            connection.execute_batch(sql).map_err(|_| EngineError::Storage)?;
        }
        self.detect_slow(started, lifecycle::EventCategory::Search);
        Ok(())
    }

    /// One-thread-poison robustness fixture (AC-009).
    ///
    /// Spawns four reader threads + one writer thread that all make
    /// forward progress (single canonical write + repeated searches),
    /// plus one designated poison thread that runs an empty-batch write
    /// — a deterministic `EngineError::WriteValidation`. The captured
    /// poison failure is dispatched as a `StressFailureContext` whose
    /// `last_error_chain` is `[EngineError::stable_code(),
    /// engine_error.to_string()]` per the lifecycle § Stress-failure
    /// context payload contract.
    #[doc(hidden)]
    #[cfg(debug_assertions)]
    pub fn run_one_thread_poison_for_test(&self) -> Result<(), EngineError> {
        self.ensure_open()?;

        // Forward-progress writer seeds a row so readers + the poison
        // thread share a non-trivial canonical state.
        self.write(&[PreparedWrite::Node {
            kind: "doc".to_string(),
            body: "poison-fixture-seed".to_string(),
            source_id: None,
        }])?;

        let poison_outcome: Mutex<Option<EngineError>> = Mutex::new(None);
        let poison_thread_id: AtomicU64 = AtomicU64::new(0);

        thread::scope(|scope| {
            // N=4 reader threads make forward progress.
            for _ in 0..4 {
                scope.spawn(|| {
                    for _ in 0..4 {
                        let _ = self.search("poison-fixture-seed");
                    }
                });
            }
            // One forward-progress writer thread.
            scope.spawn(|| {
                let _ = self.write(&[PreparedWrite::Node {
                    kind: "doc".to_string(),
                    body: "writer-progress".to_string(),
                    source_id: None,
                }]);
            });
            // One poison thread — empty batch is a deterministic
            // WriteValidation failure.
            scope.spawn(|| {
                // Use a non-zero, deterministic group id so subscribers
                // see a stable identifier across runs of the fixture.
                poison_thread_id.store(1, Ordering::SeqCst);
                if let Err(err) = self.write(&[]) {
                    *poison_outcome.lock().expect("poison_outcome lock") = Some(err);
                }
            });
        });

        let err = poison_outcome
            .into_inner()
            .expect("poison_outcome lock")
            .expect("poison thread must produce a deterministic error");

        let projection_state = match self.projection_status_for_test("doc") {
            Ok(lifecycle::ProjectionStatus::Pending) => "Pending",
            Ok(lifecycle::ProjectionStatus::Failed) => "Failed",
            Ok(lifecycle::ProjectionStatus::UpToDate) => "UpToDate",
            // Default to UpToDate when projection status is unobservable
            // (e.g. embedder not configured for the seed kind). The
            // value is still one of the documented enum stringifications
            // per AC-010.
            Err(_) => "UpToDate",
        };

        let context = lifecycle::StressFailureContext {
            thread_group_id: poison_thread_id.load(Ordering::SeqCst),
            op_kind: "write".to_string(),
            last_error_chain: vec![err.stable_code().to_string(), err.to_string()],
            projection_state: projection_state.to_string(),
        };
        self.subscribers.dispatch_stress_failure(&context);
        Ok(())
    }

    #[doc(hidden)]
    pub fn set_projection_scheduler_frozen_for_test(&self, frozen: bool) {
        self.projection_runtime.set_frozen(frozen);
    }

    #[doc(hidden)]
    pub fn set_projection_retry_delays_for_test(&self, delays_ms: &[u64]) {
        self.projection_runtime.set_retry_delays_for_test(delays_ms);
    }

    /// PR-9 — lower the ADR-0.6.0 Invariant 5 per-`embed()` watchdog deadline
    /// for tests (production default is `DEFAULT_EMBED_TIMEOUT_MS` = 30s).
    #[doc(hidden)]
    pub fn set_embed_timeout_ms_for_test(&self, timeout_ms: u64) {
        self.projection_runtime.set_embed_timeout_ms_for_test(timeout_ms);
    }

    /// PR-9 — lower the embed circuit-breaker threshold for tests (production
    /// default `DEFAULT_EMBED_CIRCUIT_THRESHOLD`); 0 disables the breaker.
    #[doc(hidden)]
    pub fn set_embed_circuit_threshold_for_test(&self, threshold: u64) {
        self.projection_runtime.set_embed_circuit_threshold_for_test(threshold);
    }

    /// PR-9 — whether the embed circuit breaker has latched open.
    #[doc(hidden)]
    pub fn embed_circuit_open_for_test(&self) -> bool {
        self.projection_runtime.embed_circuit_open_for_test()
    }

    #[doc(hidden)]
    pub fn projection_status_for_test(
        &self,
        kind: &str,
    ) -> Result<lifecycle::ProjectionStatus, EngineError> {
        self.ensure_open()?;
        let connection = self.connection.lock().map_err(|_| EngineError::Storage)?;
        let connection = connection.as_ref().ok_or(EngineError::Closing)?;
        projection_status(connection, kind)
    }

    #[doc(hidden)]
    pub fn has_vector_for_cursor_for_test(&self, cursor: u64) -> Result<bool, EngineError> {
        self.ensure_open()?;
        let connection = self.connection.lock().map_err(|_| EngineError::Storage)?;
        let connection = connection.as_ref().ok_or(EngineError::Closing)?;
        terminal_state_for_cursor(connection, cursor)
            .map(|state| matches!(state.as_deref(), Some("up_to_date")))
            .map_err(|_| EngineError::Storage)
    }

    #[doc(hidden)]
    pub fn projection_failure_count_for_test(&self, cursor: u64) -> Result<u64, EngineError> {
        self.ensure_open()?;
        let connection = self.connection.lock().map_err(|_| EngineError::Storage)?;
        let connection = connection.as_ref().ok_or(EngineError::Closing)?;
        connection
            .query_row(
                "SELECT COUNT(*) FROM operational_mutations
                 WHERE collection_name = 'projection_failures'
                   AND record_key = ?1",
                [cursor.to_string()],
                |row| row.get::<_, u64>(0),
            )
            .map_err(|_| EngineError::Storage)
    }

    #[doc(hidden)]
    pub fn set_provenance_row_cap_for_test(&self, cap: Option<u64>) {
        self.provenance_row_cap.store(cap.unwrap_or(0), Ordering::Relaxed);
    }

    #[doc(hidden)]
    pub fn provenance_row_count_for_test(&self) -> Result<u64, EngineError> {
        self.ensure_open()?;
        let connection = self.connection.lock().map_err(|_| EngineError::Storage)?;
        let connection = connection.as_ref().ok_or(EngineError::Closing)?;
        connection
            .query_row("SELECT COUNT(*) FROM operational_mutations", [], |row| row.get::<_, u64>(0))
            .map_err(|_| EngineError::Storage)
    }

    #[doc(hidden)]
    pub fn oldest_provenance_record_key_for_test(
        &self,
        collection: &str,
    ) -> Result<Option<String>, EngineError> {
        self.ensure_open()?;
        let connection = self.connection.lock().map_err(|_| EngineError::Storage)?;
        let connection = connection.as_ref().ok_or(EngineError::Closing)?;
        connection
            .query_row(
                "SELECT record_key FROM operational_mutations
                 WHERE collection_name = ?1
                 ORDER BY id
                 LIMIT 1",
                [collection],
                |row| row.get::<_, String>(0),
            )
            .map(Some)
            .or_else(|err| match err {
                rusqlite::Error::QueryReturnedNoRows => Ok(None),
                _ => Err(EngineError::Storage),
            })
    }

    #[doc(hidden)]
    pub fn configure_vector_kind_for_test(&self, kind: &str) -> Result<(), EngineError> {
        self.ensure_open()?;
        let mut connection = self.connection.lock().map_err(|_| EngineError::Storage)?;
        let connection = connection.as_mut().ok_or(EngineError::Closing)?;
        connection
            .execute(
                "INSERT OR REPLACE INTO _fathomdb_vector_kinds(kind, profile, created_at)
                 VALUES(?1, ?2, 0)",
                params![kind, DEFAULT_VECTOR_PROFILE],
            )
            .map_err(|_| EngineError::Storage)?;
        Ok(())
    }

    #[doc(hidden)]
    pub fn write_vector_for_test(
        &self,
        kind: &str,
        text: &str,
    ) -> Result<WriteReceipt, EngineError> {
        self.ensure_open()?;
        let embedder =
            self.runtime_embedder.as_ref().cloned().ok_or(EngineError::EmbedderNotConfigured)?;

        let mut connection = self.connection.lock().map_err(|_| EngineError::Storage)?;
        let connection = connection.as_mut().ok_or(EngineError::Closing)?;
        if !kind_is_vector_indexed(connection, kind)? {
            return Err(EngineError::KindNotVectorIndexed);
        }

        let expected = default_profile_dimension(connection)?;
        ensure_vector_partition(connection, expected).map_err(|_| EngineError::Storage)?;
        let vector = embedder.embed(text).map_err(map_runtime_embedder_error)?;
        let actual = u32::try_from(vector.len()).unwrap_or(u32::MAX);
        if actual != expected {
            return Err(EngineError::EmbedderDimensionMismatch { expected, actual });
        }

        let cursor = self.next_cursor.load(Ordering::SeqCst).saturating_add(1);
        // EU-5a2 mean-centering apply path (write side). f32 BLOB stored
        // is ALWAYS un-centered; the sign-quant input is the centered
        // vector iff the identity is MC-required AND a `mean_vec` is
        // pinned. NoopEmbedder identity (the only EU-5a2 live one) is
        // NOT MC-required, so this is a no-op until EU-5b's flip.
        let blob = encode_vector_blob(&vector);
        let bin_blob = if identity_requires_mean_centering(&self.runtime_embedder_identity) {
            match read_pinned_mean_vec(connection, self.runtime_embedder_identity.dimension)? {
                Some(mean) => encode_vector_blob(&subtract_mean(&vector, &mean)),
                None => blob.clone(),
            }
        } else {
            blob.clone()
        };
        let source_type = resolve_source_type(kind)?;
        let now_unix =
            SystemTime::now().duration_since(UNIX_EPOCH).unwrap_or_default().as_secs() as i64;

        // EU-5b — feed the streaming mean accumulator (if live) and detect
        // a threshold-crossing pin. The mean materialization, pre-pin
        // re-quantize, and `MeanVecPinned` event emission all happen in
        // the SAME SQLite transaction as the row INSERT.
        let pin_event = {
            let runtime = &self.projection_runtime.shared;
            let mut accumulator =
                runtime.mean_accumulator.lock().map_err(|_| EngineError::Storage)?;
            if let Some(acc) = accumulator.as_mut() {
                acc.add(&vector);
                if acc.count() >= MEAN_VEC_PIN_THRESHOLD {
                    let mean = acc.materialize();
                    *accumulator = None;
                    Some(mean)
                } else {
                    None
                }
            } else {
                None
            }
        };

        let tx = connection.transaction().map_err(|_| EngineError::Storage)?;
        tx.execute(
            "INSERT INTO _fathomdb_vector_rows(rowid, kind, write_cursor) VALUES(?1, ?2, ?3)",
            params![cursor, kind, cursor],
        )
        .map_err(|_| EngineError::Storage)?;
        tx.execute(
            "INSERT INTO vector_default(
                rowid, embedding, embedding_bin, source_type, kind, created_at
             ) VALUES(?1, ?2, vec_quantize_binary(?3), ?4, ?5, ?6)",
            params![cursor, blob, bin_blob, source_type, kind, now_unix],
        )
        .map_err(|_| EngineError::Storage)?;

        let mut emitted_event: Option<EmbedderEvent> = None;
        if let Some(mean_vec) = pin_event {
            let mean_bytes = encode_vector_blob(&mean_vec);
            tx.execute(
                "UPDATE _fathomdb_embedder_profiles SET mean_vec = ?1 WHERE profile = 'default'",
                params![mean_bytes],
            )
            .map_err(|_| EngineError::Storage)?;
            // Read all pre-pin (rowid, embedding) and re-quantize within
            // the same tx. The just-inserted row above is also covered.
            let rows: Vec<(i64, Vec<u8>)> = {
                let mut statement = tx
                    .prepare("SELECT rowid, embedding FROM vector_default ORDER BY rowid")
                    .map_err(|_| EngineError::Storage)?;
                let mapped = statement
                    .query_map([], |row| Ok((row.get::<_, i64>(0)?, row.get::<_, Vec<u8>>(1)?)))
                    .map_err(|_| EngineError::Storage)?;
                let mut out = Vec::new();
                for r in mapped {
                    out.push(r.map_err(|_| EngineError::Storage)?);
                }
                out
            };
            let (doc_count, _) = run_pin_and_requantize_pass(&tx, &rows, &mean_vec)?;
            emitted_event = Some(EmbedderEvent::MeanVecPinned {
                dim: u32::try_from(mean_vec.len()).unwrap_or(u32::MAX),
                doc_count,
            });
        }

        tx.commit().map_err(|_| EngineError::Storage)?;

        if let Some(ev) = emitted_event {
            if let Ok(mut events) = self.projection_runtime.shared.pending_events.lock() {
                events.push(ev);
            }
        }

        self.next_cursor.store(cursor, Ordering::SeqCst);
        Ok(WriteReceipt { cursor })
    }

    /// EU-5b test seam — drain MeanVecPinned events queued by the
    /// projection-commit pin transaction since the last drain. Production
    /// callers consume these via `OpenReport.embedder_events`; this seam
    /// exists so the EU-5b RED test can observe the live emission.
    #[doc(hidden)]
    pub fn drain_mean_centering_events_for_test(&self) -> Result<Vec<EmbedderEvent>, EngineError> {
        self.ensure_open()?;
        let mut events = self
            .projection_runtime
            .shared
            .pending_events
            .lock()
            .map_err(|_| EngineError::Storage)?;
        let out = std::mem::take(&mut *events);
        Ok(out)
    }

    /// 0.7.2 PR-2b — NON-test observation seam. Drains and returns every
    /// `EmbedderEvent` queued since the last drain (mean pin, manual mean
    /// recompute). Production callers use
    /// this to observe the synchronous recompute work; events are queued
    /// only AFTER the recompute transaction is durable, so a rolled-back
    /// recompute never surfaces. Mirrors the at-open
    /// `OpenReport.embedder_events` channel for the steady-state path.
    pub fn drain_embedder_events(&self) -> Result<Vec<EmbedderEvent>, EngineError> {
        self.ensure_open()?;
        let mut events = self
            .projection_runtime
            .shared
            .pending_events
            .lock()
            .map_err(|_| EngineError::Storage)?;
        Ok(std::mem::take(&mut *events))
    }

    /// 0.7.2 PR-2b — explicit `doctor recompute-mean` path. Re-derives the
    /// pinned corpus mean from the current `vector_default` rows and
    /// re-quantizes every row, SYNCHRONOUSLY in one transaction. ALWAYS
    /// allowed at any corpus size — this is the ONLY mean-refresh path as of
    /// 0.7.2 (the automatic in-ingest drift detector was carved out / deferred
    /// to 0.8.x; see `dev/design/embedder.md` §0.3).
    ///
    /// Serializes against the projection workers via `commit_gate` so the
    /// re-quantize sees a totally-ordered history, exactly like the at-pin
    /// commit. Publishes a `MeanVecRecomputed { trigger: Manual }` event
    /// only after the transaction is durable. No-op-safe on a non-MC
    /// identity (returns `EmbedderNotConfigured` rather than corrupting an
    /// un-centered workspace).
    pub fn recompute_mean(&self) -> Result<MeanRecomputeReport, EngineError> {
        self.ensure_open()?;
        let identity = self.runtime_embedder_identity.clone();
        if !identity_requires_mean_centering(&identity) {
            return Err(EngineError::EmbedderNotConfigured);
        }
        let report = {
            // Hold the commit gate for the whole recompute so no projection
            // worker commit interleaves with the re-quantize.
            let _gate = self
                .projection_runtime
                .shared
                .commit_gate
                .lock()
                .unwrap_or_else(|p| p.into_inner());
            let mut connection = self.connection.lock().map_err(|_| EngineError::Storage)?;
            let connection = connection.as_mut().ok_or(EngineError::Closing)?;
            let tx = connection.transaction().map_err(|_| EngineError::Storage)?;
            #[cfg(debug_assertions)]
            let fail = self
                .projection_runtime
                .shared
                .force_recompute_failure
                .swap(false, Ordering::SeqCst);
            #[cfg(not(debug_assertions))]
            let fail = false;
            let report = recompute_mean_in_tx_inner(&tx, &identity, fail)?;
            tx.commit().map_err(|_| EngineError::Storage)?;
            report
        };
        // Post-durable-commit publish.
        if let Ok(mut events) = self.projection_runtime.shared.pending_events.lock() {
            events.push(EmbedderEvent::MeanVecRecomputed {
                dim: report.dim,
                doc_count: report.doc_count_requantized,
                trigger: MeanRecomputeTrigger::Manual,
            });
        }
        Ok(report)
    }

    /// 0.7.2 PR-2bc S1 fix-1 test seam — RAISE the phase-2 rerank `LIMIT`
    /// above the production `SEARCH_RERANK_LIMIT` (10) so the recall harness
    /// can pull top-(10+slack) and exclude the self-retrieving query-source
    /// doc before truncating to 10. The search path clamps the stored value
    /// to the production floor, so a test can never shrink search fanout
    /// below production semantics. Production reads the same atomic and never
    /// consults any env var.
    #[doc(hidden)]
    pub fn set_search_limit_for_test(&self, limit: usize) {
        self.projection_runtime.shared.search_limit_override.store(limit, Ordering::SeqCst);
    }

    /// 0.7.2 PR-2b test seam — arm a one-shot fault inside the NEXT
    /// `recompute_mean` so it errors after the `mean_vec` UPDATE but before
    /// the re-quantize completes. Proves the recompute tx rolls back whole.
    #[doc(hidden)]
    #[cfg(debug_assertions)]
    pub fn force_next_recompute_failure_for_test(&self) {
        self.projection_runtime.shared.force_recompute_failure.store(true, Ordering::SeqCst);
    }

    #[doc(hidden)]
    pub fn vector_row_count_for_test(&self) -> Result<u64, EngineError> {
        self.ensure_open()?;
        let connection = self.connection.lock().map_err(|_| EngineError::Storage)?;
        let connection = connection.as_ref().ok_or(EngineError::Closing)?;
        connection
            .query_row("SELECT COUNT(*) FROM vector_default", [], |row| row.get::<_, u64>(0))
            .map_err(|_| EngineError::Storage)
    }

    #[doc(hidden)]
    pub fn read_vector_blob_for_test(&self, rowid: i64) -> Result<Vec<u8>, EngineError> {
        self.ensure_open()?;
        let connection = self.connection.lock().map_err(|_| EngineError::Storage)?;
        let connection = connection.as_ref().ok_or(EngineError::Closing)?;
        connection
            .query_row("SELECT embedding FROM vector_default WHERE rowid = ?1", [rowid], |row| {
                row.get::<_, Vec<u8>>(0)
            })
            .map_err(|_| EngineError::Storage)
    }

    #[doc(hidden)]
    pub fn default_embedder_profile_for_test(&self) -> Result<EmbedderIdentity, EngineError> {
        self.ensure_open()?;
        let connection = self.connection.lock().map_err(|_| EngineError::Storage)?;
        let connection = connection.as_ref().ok_or(EngineError::Closing)?;
        load_default_profile(connection).map_err(|_| EngineError::Storage)
    }

    /// Doctor read-only integrity report. Three-section output per
    /// AC-043a/b. `opts.full` adds `PRAGMA integrity_check`. `quick` and
    /// `round_trip` are accepted but treated as default for 0.6.0.
    pub fn check_integrity(
        &self,
        opts: CheckIntegrityOpts,
    ) -> Result<IntegrityReport, EngineError> {
        self.ensure_open()?;
        let connection = self.connection.lock().map_err(|_| EngineError::Storage)?;
        let connection = connection.as_ref().ok_or(EngineError::Closing)?;
        Ok(IntegrityReport {
            physical: physical_section(connection, opts.full),
            logical: logical_section(connection),
            semantic: semantic_section(connection),
        })
    }

    /// Doctor bit-preserving export. Runs `VACUUM INTO` to produce a
    /// self-contained SQLite file at `out`, computes SHA-256 of the
    /// resulting bytes, and writes a JSON manifest at `manifest`. Per
    /// AC-039a/b.
    pub fn safe_export(
        &self,
        out: &Path,
        manifest: &Path,
    ) -> Result<SafeExportArtifact, EngineError> {
        self.ensure_open()?;
        {
            let connection = self.connection.lock().map_err(|_| EngineError::Storage)?;
            let connection = connection.as_ref().ok_or(EngineError::Closing)?;
            let target = out.to_string_lossy().to_string();
            connection
                .execute("VACUUM INTO ?1", params![target])
                .map_err(|_| EngineError::Storage)?;
        }
        let bytes = std::fs::read(out).map_err(|_| EngineError::Storage)?;
        let digest = sha2::Sha256::digest(&bytes);
        let sha256_hex = hex_encode(digest.as_slice());
        let export_abs = out.canonicalize().unwrap_or_else(|_| out.to_path_buf());
        let manifest_json = serde_json::json!({
            "export_path": export_abs.to_string_lossy(),
            "sha256": sha256_hex,
            "byte_count": bytes.len() as u64,
        });
        let manifest_bytes =
            serde_json::to_vec_pretty(&manifest_json).map_err(|_| EngineError::Storage)?;
        std::fs::write(manifest, &manifest_bytes).map_err(|_| EngineError::Storage)?;
        Ok(SafeExportArtifact {
            export_path: out.to_path_buf(),
            manifest_path: manifest.to_path_buf(),
            manifest_sha256: sha256_hex,
        })
    }

    /// Operator regenerate workflow per `dev/design/projections.md`
    /// § Regenerate workflow. Drains in-flight projection work, then
    /// truncates FTS5 + vec0 shadow rows, resets the projection cursor,
    /// and lets the scheduler re-enqueue every canonical row. Durable
    /// `projection_failures` audit rows are preserved per design. AC-044
    /// + AC-063c.
    pub fn rebuild_projections(&self) -> Result<RebuildReport, EngineError> {
        self.ensure_open()?;
        self.run_rebuild(true, RebuildKind::Projections)
    }

    /// Vec0-only variant of [`Engine::rebuild_projections`]. Leaves
    /// FTS5 shadow content untouched; per recovery design,
    /// `recover --rebuild-vec0` is the surface for vec0-only repair.
    pub fn rebuild_vec0(&self) -> Result<RebuildReport, EngineError> {
        self.ensure_open()?;
        self.run_rebuild(false, RebuildKind::Vec0)
    }

    /// Phase 9 Pack B / AC-042 source trace. Returns the canonical-row
    /// id set produced by `source_id`, ordered by `write_cursor`. Empty
    /// string is not a valid `source_id`; rows with NULL `source_id`
    /// are excluded from every result.
    pub fn trace_source_ref(&self, source_id: &str) -> Result<TraceReport, EngineError> {
        self.ensure_open()?;
        if source_id.is_empty() {
            return Err(EngineError::WriteValidation);
        }
        let connection = self.connection.lock().map_err(|_| EngineError::Storage)?;
        let connection = connection.as_ref().ok_or(EngineError::Closing)?;

        let mut events: Vec<TraceEvent> = Vec::new();
        let mut nodes = connection
            .prepare(
                "SELECT write_cursor, kind FROM canonical_nodes WHERE source_id = ?1
                 ORDER BY write_cursor",
            )
            .map_err(|_| EngineError::Storage)?;
        let node_rows = nodes
            .query_map([source_id], |row| {
                Ok(TraceEvent {
                    write_cursor: row.get::<_, i64>(0)? as u64,
                    kind: row.get::<_, String>(1)?,
                    table: "canonical_nodes",
                })
            })
            .map_err(|_| EngineError::Storage)?;
        for row in node_rows {
            events.push(row.map_err(|_| EngineError::Storage)?);
        }

        let mut edges = connection
            .prepare(
                "SELECT write_cursor, kind FROM canonical_edges WHERE source_id = ?1
                 ORDER BY write_cursor",
            )
            .map_err(|_| EngineError::Storage)?;
        let edge_rows = edges
            .query_map([source_id], |row| {
                Ok(TraceEvent {
                    write_cursor: row.get::<_, i64>(0)? as u64,
                    kind: row.get::<_, String>(1)?,
                    table: "canonical_edges",
                })
            })
            .map_err(|_| EngineError::Storage)?;
        for row in edge_rows {
            events.push(row.map_err(|_| EngineError::Storage)?);
        }

        events.sort_by_key(|e| e.write_cursor);
        Ok(TraceReport { source_ref: source_id.to_string(), events })
    }

    /// Phase 9 Pack B / AC-028a/b/c source excise. Drains in-flight
    /// projection work, then deletes every canonical row attributable
    /// to `source_id` plus the FTS5 + vec0 shadow rows that referenced
    /// those cursors, and appends an audit row to the
    /// `excise_source_audit` operational collection.
    ///
    /// Non-perturbation: rows from other sources (and rows with NULL
    /// `source_id`) are untouched; the projection cursor is NOT reset
    /// and no blanket projection rebuild is issued.
    pub fn excise_source(&self, source_id: &str) -> Result<ExciseReport, EngineError> {
        self.ensure_open()?;
        if source_id.is_empty() {
            return Err(EngineError::WriteValidation);
        }

        // Drain MUST succeed before the excise transaction. SQLite-WAL
        // would otherwise allow a worker that already dequeued a job
        // for an excised cursor to commit its INSERT into vec0 /
        // _fathomdb_vector_rows after our DELETE releases the writer
        // lock, leaving residue and breaking AC-028b. Surface the
        // timeout instead of swallowing it (Pack A pattern).
        self.projection_runtime.set_frozen(true);
        let drain_result = self.drain(REBUILD_DRAIN_TIMEOUT_MS);
        let outcome = drain_result.and_then(|()| self.excise_source_inner(source_id));
        self.projection_runtime.set_frozen(false);
        outcome
    }

    /// Doctor `verify-embedder` seam (AC-040a). Compares the
    /// `_fathomdb_embedder_profiles` row to the operator-supplied
    /// `name:revision` identity + dimension; never raises on mismatch.
    pub fn verify_embedder(
        &self,
        supplied_identity: &str,
        supplied_dimension: u32,
    ) -> Result<VerifyEmbedderReport, EngineError> {
        self.ensure_open()?;
        let connection = self.connection.lock().map_err(|_| EngineError::Storage)?;
        let connection = connection.as_ref().ok_or(EngineError::Closing)?;
        let stored = load_default_profile(connection).map_err(|_| EngineError::Storage)?;
        let stored_identity = format!("{}:{}", stored.name, stored.revision);
        let identity_match = stored_identity == supplied_identity;
        let dimension_match = stored.dimension == supplied_dimension;
        let status = match (identity_match, dimension_match) {
            (true, true) => VerifyEmbedderStatus::Match,
            (false, true) => VerifyEmbedderStatus::IdentityMismatch,
            (true, false) => VerifyEmbedderStatus::DimensionMismatch,
            (false, false) => VerifyEmbedderStatus::BothMismatch,
        };
        Ok(VerifyEmbedderReport {
            stored_identity,
            stored_dimension: stored.dimension,
            supplied_identity: supplied_identity.to_string(),
            supplied_dimension,
            status,
        })
    }

    /// Doctor `dump-schema` seam (AC-040a). Returns the
    /// `PRAGMA user_version` sentinel plus the table + index inventory
    /// from `sqlite_schema`, excluding `sqlite_*` internal rows.
    /// Canonical tables appear first per [`CANONICAL_TABLES`].
    pub fn dump_schema(&self) -> Result<DumpSchemaReport, EngineError> {
        self.ensure_open()?;
        let connection = self.connection.lock().map_err(|_| EngineError::Storage)?;
        let connection = connection.as_ref().ok_or(EngineError::Closing)?;
        let user_version: u32 = connection
            .query_row("PRAGMA user_version", [], |row| row.get(0))
            .map_err(|_| EngineError::Storage)?;
        let tables = read_schema_objects(connection, "table")?;
        let indexes = read_schema_objects(connection, "index")?;
        Ok(DumpSchemaReport { user_version, tables: order_canonical_first(tables), indexes })
    }

    /// Doctor `dump-row-counts` seam (AC-040a). Emits canonical-table
    /// counts only; projection / FTS / vec0 shadow tables are excluded.
    pub fn dump_row_counts(&self) -> Result<DumpRowCountsReport, EngineError> {
        self.ensure_open()?;
        let connection = self.connection.lock().map_err(|_| EngineError::Storage)?;
        let connection = connection.as_ref().ok_or(EngineError::Closing)?;
        let mut counts = Vec::with_capacity(CANONICAL_TABLES.len());
        for name in CANONICAL_TABLES {
            let rows: u64 = connection
                .query_row(&format!("SELECT COUNT(*) FROM {name}"), [], |row| row.get(0))
                .map_err(|_| EngineError::Storage)?;
            counts.push(TableRowCount { name: (*name).to_string(), rows });
        }
        Ok(DumpRowCountsReport { counts })
    }

    /// Doctor `dump-profile` seam (AC-040a). Returns the stored
    /// embedder identity + dimension plus the registered vectorized
    /// kinds from `_fathomdb_vector_kinds`.
    pub fn dump_profile(&self) -> Result<DumpProfileReport, EngineError> {
        self.ensure_open()?;
        let connection = self.connection.lock().map_err(|_| EngineError::Storage)?;
        let connection = connection.as_ref().ok_or(EngineError::Closing)?;
        let stored = load_default_profile(connection).map_err(|_| EngineError::Storage)?;
        let mut stmt = connection
            .prepare("SELECT kind FROM _fathomdb_vector_kinds ORDER BY kind")
            .map_err(|_| EngineError::Storage)?;
        let rows =
            stmt.query_map([], |row| row.get::<_, String>(0)).map_err(|_| EngineError::Storage)?;
        let mut vectorized_kinds = Vec::new();
        for row in rows {
            vectorized_kinds.push(row.map_err(|_| EngineError::Storage)?);
        }
        Ok(DumpProfileReport {
            embedder_identity: format!("{}:{}", stored.name, stored.revision),
            embedder_dimension: stored.dimension,
            vectorized_kinds,
        })
    }

    /// Recover `--truncate-wal` seam. Runs
    /// `PRAGMA wal_checkpoint(TRUNCATE)` and returns the three counters
    /// SQLite reports. `status = Busy` when SQLite signalled a blocked
    /// checkpoint (`busy != 0`); the WAL may still be partially
    /// checkpointed in that case.
    pub fn truncate_wal(&self) -> Result<TruncateWalReport, EngineError> {
        self.ensure_open()?;
        let connection = self.connection.lock().map_err(|_| EngineError::Storage)?;
        let connection = connection.as_ref().ok_or(EngineError::Closing)?;
        let (busy, log_frames, checkpointed_frames): (i64, i64, i64) = connection
            .query_row("PRAGMA wal_checkpoint(TRUNCATE)", [], |row| {
                Ok((row.get(0)?, row.get(1)?, row.get(2)?))
            })
            .map_err(|_| EngineError::Storage)?;
        let status = if busy == 0 { TruncateWalStatus::Done } else { TruncateWalStatus::Busy };
        Ok(TruncateWalReport {
            status,
            busy: busy.max(0) as u32,
            log_frames: log_frames.max(0) as u32,
            checkpointed_frames: checkpointed_frames.max(0) as u32,
        })
    }

    fn excise_source_inner(&self, source_id: &str) -> Result<ExciseReport, EngineError> {
        let mut connection = self.connection.lock().map_err(|_| EngineError::Storage)?;
        let connection = connection.as_mut().ok_or(EngineError::Closing)?;
        let tx = connection.transaction().map_err(|_| EngineError::Storage)?;

        // Collect the cursor sets up-front so we can targeted-delete
        // shadow rows AND emit an accurate audit row in one txn.
        let node_cursors: Vec<i64> = {
            let mut stmt = tx
                .prepare("SELECT write_cursor FROM canonical_nodes WHERE source_id = ?1")
                .map_err(|_| EngineError::Storage)?;
            let rows = stmt
                .query_map([source_id], |row| row.get::<_, i64>(0))
                .map_err(|_| EngineError::Storage)?;
            rows.collect::<rusqlite::Result<Vec<_>>>().map_err(|_| EngineError::Storage)?
        };
        let edge_cursors: Vec<i64> = {
            let mut stmt = tx
                .prepare("SELECT write_cursor FROM canonical_edges WHERE source_id = ?1")
                .map_err(|_| EngineError::Storage)?;
            let rows = stmt
                .query_map([source_id], |row| row.get::<_, i64>(0))
                .map_err(|_| EngineError::Storage)?;
            rows.collect::<rusqlite::Result<Vec<_>>>().map_err(|_| EngineError::Storage)?
        };

        let mut shadow_invalidated: u64 = 0;
        for cursor in node_cursors.iter().chain(edge_cursors.iter()) {
            shadow_invalidated = shadow_invalidated.saturating_add(
                tx.execute("DELETE FROM search_index WHERE write_cursor = ?1", [cursor])
                    .map_err(|_| EngineError::Storage)? as u64,
            );
            // vec0 rowid is the canonical row's write_cursor (see
            // `_fathomdb_vector_rows.write_cursor UNIQUE`).
            shadow_invalidated = shadow_invalidated.saturating_add(
                tx.execute("DELETE FROM vector_default WHERE rowid = ?1", [cursor])
                    .map_err(|_| EngineError::Storage)? as u64,
            );
            shadow_invalidated = shadow_invalidated.saturating_add(
                tx.execute("DELETE FROM _fathomdb_vector_rows WHERE write_cursor = ?1", [cursor])
                    .map_err(|_| EngineError::Storage)? as u64,
            );
            shadow_invalidated = shadow_invalidated.saturating_add(
                tx.execute(
                    "DELETE FROM _fathomdb_projection_terminal WHERE write_cursor = ?1",
                    [cursor],
                )
                .map_err(|_| EngineError::Storage)? as u64,
            );
        }

        let nodes_excised = tx
            .execute("DELETE FROM canonical_nodes WHERE source_id = ?1", [source_id])
            .map_err(|_| EngineError::Storage)? as u64;
        let edges_excised = tx
            .execute("DELETE FROM canonical_edges WHERE source_id = ?1", [source_id])
            .map_err(|_| EngineError::Storage)? as u64;

        // AC-028a audit row: a single append on the
        // `excise_source_audit` collection naming the excised source.
        // `next_cursor` after a prior write holds the LAST committed cursor;
        // mirror the vec writer pattern (load + 1, then store post-commit)
        // so the audit row's `write_cursor` is strictly greater than every
        // canonical row that preceded it.
        let excised_at = SystemTime::now().duration_since(UNIX_EPOCH).unwrap_or_default().as_secs();
        let payload = serde_json::json!({
            "source_id": source_id,
            "excised_at": excised_at,
            "nodes_excised": nodes_excised,
            "edges_excised": edges_excised,
            "projections_invalidated": shadow_invalidated,
        })
        .to_string();
        let audit_cursor = self.next_cursor.load(Ordering::SeqCst).saturating_add(1);
        tx.execute(
            "INSERT INTO operational_mutations(
                collection_name, record_key, op_kind, payload_json, schema_id, write_cursor
             ) VALUES('excise_source_audit', ?1, 'append', ?2, NULL, ?3)",
            params![source_id, payload, audit_cursor],
        )
        .map_err(|_| EngineError::Storage)?;

        tx.commit().map_err(|_| EngineError::Storage)?;
        self.next_cursor.store(audit_cursor, Ordering::SeqCst);
        Ok(ExciseReport {
            source_ref: source_id.to_string(),
            nodes_excised,
            edges_excised,
            projections_invalidated: shadow_invalidated,
        })
    }

    fn run_rebuild(
        &self,
        include_fts: bool,
        kind: RebuildKind,
    ) -> Result<RebuildReport, EngineError> {
        self.projection_runtime.set_frozen(true);
        // Drain MUST succeed: rebuild_shadow_state truncates shadow rows,
        // and SQLite-WAL allows a worker that already dequeued a job to
        // commit its `INSERT OR IGNORE INTO _fathomdb_vector_rows / vec0`
        // after our truncate releases the writer lock, leaving stale
        // rows. Surfacing the timeout (instead of swallowing it) lets the
        // operator retry rather than silently corrupt the rebuild.
        let drain_result = self.drain(REBUILD_DRAIN_TIMEOUT_MS);
        let result = drain_result.and_then(|()| self.rebuild_shadow_state(include_fts, kind));
        self.projection_runtime.set_frozen(false);
        result
    }

    fn rebuild_shadow_state(
        &self,
        include_fts: bool,
        kind: RebuildKind,
    ) -> Result<RebuildReport, EngineError> {
        let mut connection = self.connection.lock().map_err(|_| EngineError::Storage)?;
        let connection = connection.as_mut().ok_or(EngineError::Closing)?;
        let tx = connection.transaction().map_err(|_| EngineError::Storage)?;
        let mut rows_invalidated: u64 = 0;
        if include_fts {
            let n = tx.execute("DELETE FROM search_index", []).map_err(|_| EngineError::Storage)?;
            rows_invalidated = rows_invalidated.saturating_add(n as u64);
        }
        let n = tx.execute("DELETE FROM vector_default", []).map_err(|_| EngineError::Storage)?;
        rows_invalidated = rows_invalidated.saturating_add(n as u64);
        let n = tx
            .execute("DELETE FROM _fathomdb_vector_rows", [])
            .map_err(|_| EngineError::Storage)?;
        rows_invalidated = rows_invalidated.saturating_add(n as u64);
        let n = tx
            .execute("DELETE FROM _fathomdb_projection_terminal", [])
            .map_err(|_| EngineError::Storage)?;
        rows_invalidated = rows_invalidated.saturating_add(n as u64);
        store_projection_cursor(&tx, 0).map_err(|_| EngineError::Storage)?;
        let mut rows_rebuilt: u64 = 0;
        if include_fts {
            for row in canonical_node_rows(&tx).map_err(|_| EngineError::Storage)? {
                tx.execute(
                    "INSERT INTO search_index(body, kind, write_cursor) VALUES(?1, ?2, ?3)",
                    params![row.body, row.kind, row.cursor],
                )
                .map_err(|_| EngineError::Storage)?;
                rows_rebuilt = rows_rebuilt.saturating_add(1);
            }
        }
        let projection_cursor_after =
            load_projection_cursor(&tx).map_err(|_| EngineError::Storage)?;
        tx.commit().map_err(|_| EngineError::Storage)?;
        Ok(RebuildReport { kind, rows_invalidated, rows_rebuilt, projection_cursor_after })
    }

    fn ensure_open(&self) -> Result<(), EngineError> {
        if self.closed.load(Ordering::SeqCst) {
            return Err(EngineError::Closing);
        }

        Ok(())
    }
}

fn batch_is_admin(batch: &[PreparedWrite]) -> bool {
    !batch.is_empty() && batch.iter().all(|w| matches!(w, PreparedWrite::AdminSchema { .. }))
}

// 0.7.0 Pack 2 (ADR-0.7.0-vector-binary-quant § 2; handoff § 2.2):
// bit-KNN candidate-set size for the two-phase read path. Tuned with
// the recall@10 floor in tests/perf_gates.rs::ac_013b_recall_at_10_floor.
//
// Bumped from 64 → 192 in EU-5a2 per the HITL 2026-05-29 fine-grained
// K-sweep result (dev/notes/0.7.1-default-embedder-research.md §5.4):
// K=192 sits above the recall-plateau knee for the default embedder.
// Public-visible so the EU-5a2 machinery test can assert the value.
pub const TOP_K_BIT_CANDIDATES: usize = 192;

/// EU-5a2 — number of documents required before the workspace's
/// `_fathomdb_embedder_profiles.mean_vec` is pinned for the default
/// profile. Per `dev/design/embedder.md` §0.3 (compute-once-on-first-
/// ingest lifecycle). Public-visible so the EU-5a2 machinery test can
/// assert the value.
pub const MEAN_VEC_PIN_THRESHOLD: u64 = 256;

/// 0.7.2 PR-2bc S1 fix-1 — production phase-2 rerank `LIMIT` for engine
/// search. This is the original hardcoded `LIMIT 10`; it is the default and
/// the floor for `search_limit_override` (a test seam may RAISE it but never
/// shrink it below this). There is NO env-var override on the hot path.
pub const SEARCH_RERANK_LIMIT: usize = 10;

/// EU-5a2 — streaming f64 accumulator for the mean-centering pipeline,
/// per `dev/design/embedder.md` §0.3 (f64 chosen to bound numerical
/// drift across `MEAN_VEC_PIN_THRESHOLD` adds). Owned by the projection
/// worker; materialized into the schema column at the threshold cross.
#[derive(Clone, Debug)]
struct MeanAccumulator {
    sum: Vec<f64>,
    count: u64,
}

impl MeanAccumulator {
    fn new(dim: usize) -> Self {
        Self { sum: vec![0.0; dim], count: 0 }
    }

    fn add(&mut self, v: &[f32]) {
        debug_assert_eq!(v.len(), self.sum.len(), "accumulator dim mismatch");
        for (slot, value) in self.sum.iter_mut().zip(v.iter()) {
            *slot += f64::from(*value);
        }
        self.count = self.count.saturating_add(1);
    }

    fn materialize(&self) -> Vec<f32> {
        if self.count == 0 {
            return vec![0.0; self.sum.len()];
        }
        let denom = self.count as f64;
        self.sum.iter().map(|s| (s / denom) as f32).collect()
    }

    fn count(&self) -> u64 {
        self.count
    }
}

/// 0.7.2 PR-2b — cosine similarity between two equal-length vectors.
/// Returns 1.0 for a pair with a zero-norm operand (treated as "no drift
/// signal"), so the detector never fires on a degenerate all-zero mean.
fn cosine_similarity(a: &[f32], b: &[f32]) -> f32 {
    if a.len() != b.len() {
        return 1.0;
    }
    let mut dot = 0.0f64;
    let mut na = 0.0f64;
    let mut nb = 0.0f64;
    for (x, y) in a.iter().zip(b.iter()) {
        dot += f64::from(*x) * f64::from(*y);
        na += f64::from(*x) * f64::from(*x);
        nb += f64::from(*y) * f64::from(*y);
    }
    if na == 0.0 || nb == 0.0 {
        return 1.0;
    }
    (dot / (na.sqrt() * nb.sqrt())) as f32
}

/// EU-5b — at-pin pin-and-requantize pass per `dev/design/embedder.md`
/// §0.5. Runs INSIDE the caller's SQLite transaction so the mean_vec
/// INSERT/UPDATE + the per-row sign-bit UPDATEs commit atomically.
///
/// For each pre-pin row, recomputes `bits' = sign_quantize(f32 - mean)`
/// via the SQL extension's `vec_quantize_binary`, then UPDATEs the
/// row's `embedding_bin` column.
fn run_pin_and_requantize_pass(
    tx: &rusqlite::Transaction<'_>,
    rows: &[(i64, Vec<u8>)],
    mean: &[f32],
) -> Result<(u64, Vec<EmbedderEvent>), EngineError> {
    let mut updated: u64 = 0;
    let dim = mean.len();
    // sqlite-vec's vec0 xUpdate path discards SQL-function result subtypes
    // (see sqlite-vec.c §vec0Update_UpdateVectorColumn — "subtypes don't
    // appear to survive xColumn -> xUpdate, it's always 0"), so a direct
    // `UPDATE ... SET embedding_bin = vec_quantize_binary(?)` reads the
    // bound value as a float32-tagged vector and trips the column-type
    // check. We work around by DELETE+INSERT inside the same transaction:
    // INSERT preserves the BIT subtype on `vec_quantize_binary`. The
    // surrounding pin-commit tx keeps the rewrite atomic.
    for (rowid, blob) in rows {
        if blob.len() != dim * 4 {
            return Err(EngineError::Storage);
        }
        let un_centered = decode_vector_blob(blob);
        let centered = subtract_mean(&un_centered, mean);
        let centered_blob = encode_vector_blob(&centered);

        let (source_type, kind, created_at): (String, String, i64) = tx
            .query_row(
                "SELECT source_type, kind, created_at FROM vector_default WHERE rowid = ?1",
                params![rowid],
                |row| Ok((row.get(0)?, row.get(1)?, row.get(2)?)),
            )
            .map_err(|_| EngineError::Storage)?;

        tx.execute("DELETE FROM vector_default WHERE rowid = ?1", params![rowid])
            .map_err(|_| EngineError::Storage)?;

        tx.execute(
            "INSERT INTO vector_default(
                rowid, embedding, embedding_bin, source_type, kind, created_at
             ) VALUES(?1, ?2, vec_quantize_binary(?3), ?4, ?5, ?6)",
            params![rowid, blob, centered_blob, source_type, kind, created_at],
        )
        .map_err(|_| EngineError::Storage)?;

        updated = updated.saturating_add(1);
    }
    let events = vec![EmbedderEvent::MeanVecPinned {
        dim: u32::try_from(dim).unwrap_or(u32::MAX),
        doc_count: updated,
    }];
    Ok((updated, events))
}

/// EU-5a2 — back-compat test-only count+emit helper. Preserved so the
/// EU-5a2 machinery test stays green; the EU-5b production path uses
/// `run_pin_and_requantize_pass`.
fn run_requantize_pass(rows: &[(i64, Vec<u8>)], mean: &[f32]) -> (u64, Vec<EmbedderEvent>) {
    let mut updated: u64 = 0;
    let dim = mean.len();
    for (_rowid, blob) in rows {
        if blob.len() != dim * 4 {
            continue;
        }
        updated = updated.saturating_add(1);
    }
    let events = vec![EmbedderEvent::MeanVecPinned {
        dim: u32::try_from(dim).unwrap_or(u32::MAX),
        doc_count: updated,
    }];
    (updated, events)
}

/// EU-5a2 — test-visible re-exports of the mean-centering internals.
/// Per the handoff RED tests; the production accumulator and re-quantize
/// pass are otherwise crate-private.
#[doc(hidden)]
pub mod mean_centering_internals_for_test {
    use super::{EmbedderEvent, MeanAccumulator};

    pub struct AccumulatorHandle(MeanAccumulator);

    #[must_use]
    pub fn new_mean_accumulator(dim: usize) -> AccumulatorHandle {
        AccumulatorHandle(MeanAccumulator::new(dim))
    }

    pub fn accumulator_add(handle: &mut AccumulatorHandle, v: &[f32]) {
        handle.0.add(v);
    }

    #[must_use]
    pub fn accumulator_materialize(handle: &AccumulatorHandle) -> Vec<f32> {
        handle.0.materialize()
    }

    #[must_use]
    pub fn accumulator_count(handle: &AccumulatorHandle) -> u64 {
        handle.0.count()
    }

    #[must_use]
    pub fn run_requantize_pass(rows: &[(i64, Vec<u8>)], mean: &[f32]) -> (u64, Vec<EmbedderEvent>) {
        super::run_requantize_pass(rows, mean)
    }
}

/// Read projection cursor and matching body rows inside one read tx.
fn read_search_in_tx(
    reader: &mut Connection,
    compiled: &fathomdb_query::CompiledQuery,
    query_vector: Option<&str>,
    query_vector_bin: Option<&str>,
    final_limit: usize,
) -> rusqlite::Result<(u64, Option<SoftFallback>, Vec<String>)> {
    let tx = reader.transaction_with_behavior(rusqlite::TransactionBehavior::Deferred)?;
    let cursor = load_projection_cursor(&tx)?;
    let vector_results = if let Some(query_vector) = query_vector {
        let mut rowids = Vec::new();
        let bin_vector = query_vector_bin.unwrap_or(query_vector);
        {
            // Phase 1: bit-KNN over `embedding_bin` to a top-K candidate
            // set; Phase 2: f32 rerank on the candidate set via
            // vec_distance_l2 against the retained `embedding` column.
            // EU-5a2: ?1 is the (possibly centered) sign-quant input,
            // ?2 is the un-centered f32 for vec_distance_l2 — both sides
            // of the f32 cosine use un-centered vectors.
            // PR-2bc S1 fix-1: the phase-2 rerank LIMIT is `SEARCH_RERANK_LIMIT`
            // (10) in production. `final_limit` is supplied by the caller from
            // `ProjectionRuntimeShared::search_limit_override` (default 10,
            // clamped >=10) — there is NO env-var read on this hot path. A test
            // seam (`set_search_limit_for_test`) may RAISE it so the recall
            // harness can pull top-(10+slack) and exclude the self-retrieving
            // query-source doc BEFORE truncating to 10 (standard ANN-recall
            // practice); it can never shrink below production semantics.
            let sql = format!(
                "WITH candidates AS (
                     SELECT rowid
                     FROM vector_default
                     WHERE embedding_bin MATCH vec_quantize_binary(vec_f32(?1))
                     ORDER BY distance
                     LIMIT {top_k}
                 )
                 SELECT c.rowid
                 FROM candidates c
                 JOIN vector_default v ON v.rowid = c.rowid
                 ORDER BY vec_distance_l2(v.embedding, vec_f32(?2))
                 LIMIT {final_limit}",
                top_k = TOP_K_BIT_CANDIDATES,
            );
            let mut statement = tx.prepare(&sql)?;
            let rows =
                statement.query_map([bin_vector, query_vector], |row| row.get::<_, i64>(0))?;
            for row in rows.flatten() {
                rowids.push(row);
            }
        }
        let mut results = Vec::new();
        let mut statement =
            tx.prepare("SELECT body FROM canonical_nodes WHERE write_cursor = ?1 LIMIT 1")?;
        for rowid in rowids {
            if let Ok(body) = statement.query_row([rowid], |row| row.get::<_, String>(0)) {
                results.push(body);
            }
        }
        results
    } else {
        Vec::new()
    };
    let vector_rows_visible = !vector_results.is_empty();
    let soft_fallback = if query_vector.is_some() && !vector_rows_visible {
        tx.query_row(
            "SELECT 1
             FROM search_index
             JOIN _fathomdb_vector_kinds ON _fathomdb_vector_kinds.kind = search_index.kind
             LEFT JOIN _fathomdb_projection_terminal
               ON _fathomdb_projection_terminal.write_cursor = search_index.write_cursor
             WHERE search_index MATCH ?1
              AND _fathomdb_projection_terminal.write_cursor IS NULL
             LIMIT 1",
            [compiled.match_expression.as_str()],
            |_row| Ok(SoftFallback { branch: SoftFallbackBranch::Vector }),
        )
        .ok()
    } else {
        None
    };
    let mut seen = BTreeSet::new();
    let mut results = Vec::new();
    for row in vector_results {
        if seen.insert(row.clone()) {
            results.push(row);
        }
    }
    {
        // 0.7.0 perf-experiments: optional FTS5 LIMIT cap. Gated on
        // FATHOMDB_PERF_EXPERIMENTS=1; opt-in via
        // FATHOMDB_PERF_SEARCH_LIMIT=<k>. No-op by default — preserves
        // 0.6.x unbounded result-set semantics. Removed (or made the
        // hardcoded default) at Wave 5 landing per
        // dev/plans/0.7.0-perf-experiments.md.
        let perf_limit: Option<usize> = if std::env::var_os("FATHOMDB_PERF_EXPERIMENTS").is_some() {
            std::env::var("FATHOMDB_PERF_SEARCH_LIMIT").ok().and_then(|s| s.parse().ok())
        } else {
            None
        };
        let sql = match perf_limit {
            Some(k) => format!(
                "SELECT body FROM search_index WHERE search_index MATCH ?1 ORDER BY write_cursor LIMIT {k}"
            ),
            None => "SELECT body FROM search_index WHERE search_index MATCH ?1 ORDER BY write_cursor"
                .to_string(),
        };
        let mut statement = tx.prepare(&sql)?;
        let rows = statement
            .query_map([compiled.match_expression.as_str()], |row| row.get::<_, String>(0))?;
        for row in rows.flatten() {
            if seen.insert(row.clone()) {
                results.push(row);
            }
        }
    }
    tx.commit()?;
    Ok((cursor, soft_fallback, results))
}

fn projection_dispatcher_loop(shared: Arc<ProjectionRuntimeShared>) {
    let connection = match open_runtime_connection(&shared.path) {
        Ok(connection) => connection,
        Err(_) => return,
    };
    loop {
        let in_flight = {
            let mut state = match shared.state.lock() {
                Ok(state) => state,
                Err(_) => return,
            };
            while !state.stopping
                && (!state.pending_scan
                    || state.frozen
                    || state.active_jobs + state.queued_jobs >= PROJECTION_INFLIGHT_LIMIT)
            {
                state = match shared.state_cvar.wait(state) {
                    Ok(state) => state,
                    Err(_) => return,
                };
            }
            if state.stopping {
                return;
            }
            state.pending_scan = false;
            state.in_flight.clone()
        };

        // Fetch up to the in-flight budget in one SQL roundtrip and
        // enqueue them as a batch — previously this loop fetched ONE job
        // per cycle, which capped projection throughput at one row per
        // scanner/worker handshake regardless of how much work was queued
        // in canonical_nodes.
        let budget = {
            let state = match shared.state.lock() {
                Ok(state) => state,
                Err(_) => return,
            };
            PROJECTION_INFLIGHT_LIMIT.saturating_sub(state.active_jobs + state.queued_jobs)
        };
        let fetch_cap = budget.clamp(1, PROJECTION_SCAN_FETCH);
        match next_pending_projection_jobs(&connection, &in_flight, fetch_cap) {
            Ok(jobs) if !jobs.is_empty() => {
                if let Ok(mut state) = shared.state.lock() {
                    state.queued_jobs = state.queued_jobs.saturating_add(jobs.len());
                    for job in &jobs {
                        state.in_flight.insert(job.cursor);
                    }
                    state.pending_scan = true;
                    shared.state_cvar.notify_all();
                }
                if let Ok(mut queue) = shared.queue.lock() {
                    for job in jobs {
                        queue.push_back(job);
                    }
                    shared.queue_cvar.notify_all();
                }
            }
            Ok(_) => {}
            Err(_) => {
                if let Ok(mut state) = shared.state.lock() {
                    state.pending_scan = false;
                    shared.state_cvar.notify_all();
                }
            }
        }
    }
}

fn projection_worker_loop(shared: Arc<ProjectionRuntimeShared>) {
    let mut connection = match open_runtime_connection(&shared.path) {
        Ok(connection) => connection,
        Err(_) => return,
    };
    if ensure_vector_partition(&mut connection, shared.embedder_identity.dimension).is_err() {
        return;
    }
    loop {
        let jobs = {
            let mut queue = match shared.queue.lock() {
                Ok(queue) => queue,
                Err(_) => return,
            };
            loop {
                let stopping = shared.state.lock().map(|state| state.stopping).unwrap_or(true);
                if stopping && queue.is_empty() {
                    return;
                }
                if let Some(job) = queue.pop_front() {
                    let mut jobs = vec![job];
                    while jobs.len() < PROJECTION_COMMIT_BATCH {
                        let Some(job) = queue.pop_front() else {
                            break;
                        };
                        jobs.push(job);
                    }
                    if let Ok(mut state) = shared.state.lock() {
                        state.queued_jobs = state.queued_jobs.saturating_sub(jobs.len());
                        state.active_jobs = state.active_jobs.saturating_add(jobs.len());
                        shared.state_cvar.notify_all();
                    }
                    break jobs;
                }
                queue = match shared.queue_cvar.wait(queue) {
                    Ok(queue) => queue,
                    Err(_) => return,
                };
            }
        };

        // EU-5f — isolate worker faults. A panic inside `embed()` (or the
        // commit) must not skip the state cleanup below, or `active_jobs`
        // would stay elevated forever and `wait_for_idle` / `drain` would
        // wedge into `EngineError::Scheduler` (Finding A). Mirrors the
        // reader pool's `LiveGuard` panic-safety. The local commit tx rolls
        // back on unwind, leaving the connection clean for reuse.
        let panicked = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
            run_projection_jobs(&shared, &mut connection, &jobs);
        }))
        .is_err();
        if panicked {
            commit_projection_panic_failures(&shared, &mut connection, &jobs);
        }

        if let Ok(mut state) = shared.state.lock() {
            state.active_jobs = state.active_jobs.saturating_sub(jobs.len());
            for job in &jobs {
                state.in_flight.remove(&job.cursor);
            }
            if !state.stopping {
                state.pending_scan = true;
            }
            shared.state_cvar.notify_all();
        }
    }
}

enum ProjectionOutcome {
    /// `blob` is the un-centered f32 BLOB persisted to
    /// `vector_default.embedding`. `bin_blob` is the (possibly centered)
    /// f32 BLOB fed to `vec_quantize_binary` for the sign-bit column.
    /// EU-5a2: `bin_blob == blob` unless the identity is MC-required
    /// AND a mean_vec is pinned.
    Success {
        cursor: u64,
        kind: String,
        blob: Vec<u8>,
        bin_blob: Vec<u8>,
    },
    Failure {
        cursor: u64,
        failure_code: &'static str,
    },
}

fn run_projection_jobs(
    shared: &ProjectionRuntimeShared,
    connection: &mut Connection,
    jobs: &[ProjectionJob],
) {
    let mut outcomes = Vec::with_capacity(jobs.len());
    for job in jobs {
        outcomes.push(run_projection_job(shared, job));
    }
    let _ = commit_projection_outcomes(connection, &outcomes, shared);
}

/// EU-5f — record every job in a panicked batch as a terminal projection
/// failure so the scheduler does not re-enqueue and re-panic on the same
/// cursors. Best-effort; runs after the worker caught a panic.
fn commit_projection_panic_failures(
    shared: &ProjectionRuntimeShared,
    connection: &mut Connection,
    jobs: &[ProjectionJob],
) {
    let outcomes: Vec<ProjectionOutcome> = jobs
        .iter()
        .map(|job| ProjectionOutcome::Failure {
            cursor: job.cursor,
            failure_code: "ProjectionPanic",
        })
        .collect();
    let _ = commit_projection_outcomes(connection, &outcomes, shared);
}

/// PR-9 — ADR-0.6.0-embedder-protocol **Invariant 5**: run one `embed()`
/// under a per-call deadline. A hung (non-panicking) embed would otherwise
/// park a projection worker forever — the EU-5f `catch_unwind` only catches
/// *panics*. On timeout we return `RuntimeEmbedderError::Timeout`, which the
/// caller's existing retry/failure path already handles.
///
/// Cancellation follows Invariant 5 exactly: the embed runs on a detached
/// thread that is allowed to *finish + discard* its result — never aborted
/// mid-call (there is no safe thread-cancel API). The caller (the projection
/// worker) holds `embed_serialize` across this call, but DROPS it the moment
/// this returns — including on timeout — so the abandoned detached thread
/// runs lock-free and a hung embed can neither hold the serialization guard
/// forever nor deadlock the pool. (The commit happens later, outside this
/// call, under the separate `commit_gate`.)
///
/// Panic-transparent: if `embed()` panics, the panic payload is captured on
/// the watchdog thread and resumed on the worker thread, so the existing
/// batch-level `catch_unwind` records `ProjectionPanic` exactly as before.
///
/// `live` counts embed threads currently alive: incremented before the spawn
/// and decremented by the thread when it finishes (even if its result was
/// abandoned on timeout). The caller reads it to bound the abandoned-thread
/// leak via the circuit breaker.
fn embed_with_watchdog(
    embedder: &Arc<dyn Embedder>,
    body: &str,
    timeout: Duration,
    live: &Arc<AtomicU64>,
) -> Result<Vec<f32>, RuntimeEmbedderError> {
    let (tx, rx) = mpsc::channel();
    let embedder = Arc::clone(embedder);
    let body = body.to_string();
    // Count this embed thread as live before spawning; the thread decrements
    // when it finishes, whether or not its result is still wanted.
    live.fetch_add(1, Ordering::Relaxed);
    let live_thread = Arc::clone(live);
    thread::spawn(move || {
        let outcome =
            std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| embedder.embed(&body)));
        // The receiver may already be gone (this call timed out): an async
        // channel send never blocks, and a send to a dropped receiver is a
        // no-op error we deliberately ignore — the result is discarded.
        let _ = tx.send(outcome);
        live_thread.fetch_sub(1, Ordering::Relaxed);
    });
    match rx.recv_timeout(timeout) {
        Ok(Ok(result)) => result,
        Ok(Err(panic_payload)) => std::panic::resume_unwind(panic_payload),
        Err(mpsc::RecvTimeoutError::Timeout) => Err(RuntimeEmbedderError::Timeout),
        // The watchdog thread dropped its sender without sending — should not
        // happen (panics are captured above), but treat as a failed embed so
        // the retry/failure path engages rather than silently succeeding.
        Err(mpsc::RecvTimeoutError::Disconnected) => Err(RuntimeEmbedderError::Failed {
            message: "embed watchdog thread dropped its result channel".to_string(),
        }),
    }
}

fn run_projection_job(shared: &ProjectionRuntimeShared, job: &ProjectionJob) -> ProjectionOutcome {
    // PR-9 — embed circuit breaker (see `embed_circuit_open`). Once abandoned
    // (timed-out) embed threads have piled up to the threshold the embedder is
    // treated as broken; fail subsequent jobs fast WITHOUT attempting an embed,
    // so a wedged embedder cannot keep leaking abandoned watchdog threads. This
    // entry check is the fast path; the latch decision itself is made under the
    // embed guard below (race-free against other workers).
    if shared.embed_circuit_open.load(Ordering::Relaxed) {
        return ProjectionOutcome::Failure { cursor: job.cursor, failure_code: "EmbedderError" };
    }
    let delays = shared.retry_delays_ms.lock().map(|delays| delays.clone()).unwrap_or_default();
    let mut last_code = "EmbedderError";
    for (attempt, delay_ms) in std::iter::once(0_u64).chain(delays.iter().copied()).enumerate() {
        if attempt > 0 {
            if shared.state.lock().map(|state| state.stopping).unwrap_or(true) {
                return ProjectionOutcome::Failure { cursor: job.cursor, failure_code: last_code };
            }
            thread::sleep(Duration::from_millis(delay_ms));
        }
        // PR-9 — re-check the breaker on every attempt, not just at entry:
        // another worker (or an earlier attempt of this job) may have latched
        // it while we were sleeping between retries. Bail before spawning yet
        // another timeout-bound watchdog thread, so the abandoned-thread leak
        // stays bounded even on the multi-retry path.
        if shared.embed_circuit_open.load(Ordering::Relaxed) {
            return ProjectionOutcome::Failure { cursor: job.cursor, failure_code: last_code };
        }
        // PR-9 / ADR-0.6.0 Invariant 5 — every embed runs under the per-call
        // watchdog deadline so a hung embed surfaces Timeout instead of
        // parking this worker forever.
        let embed_timeout = Duration::from_millis(shared.embed_timeout_ms.load(Ordering::Relaxed));
        let vector = match shared.embedder.as_ref() {
            Some(embedder) => {
                // PR-9 — serialize the embed call engine-side (see
                // `embed_serialize`): the shared embedder is invoked one call
                // at a time, for SAFETY with arbitrary caller-supplied
                // embedders (throughput is ~neutral on the candle default).
                // The guard is held across the watchdog call and released
                // here, so commit/IO below stays parallel and a timed-out
                // embed frees it. The guard owns no data; a panic-resumed
                // embed poisons it, so we recover the inner guard rather than
                // wedge the whole pool.
                let _embed_permit =
                    shared.embed_serialize.lock().unwrap_or_else(|poisoned| poisoned.into_inner());
                // PR-9 — breaker decision, made WITH the guard held so it is
                // race-free against other workers: if abandoned embed threads
                // from earlier timeouts have piled up to the threshold, latch
                // the breaker and fail fast WITHOUT spawning another one. The
                // live count is checked here (also covers a breaker latched by
                // another worker while we were queued on the lock), bounding
                // the abandoned-thread leak to ~threshold regardless of whether
                // the embedder hangs always or only intermittently.
                let threshold = shared.embed_circuit_threshold.load(Ordering::Relaxed);
                if shared.embed_circuit_open.load(Ordering::Relaxed)
                    || (threshold != 0
                        && shared.live_embed_threads.load(Ordering::Relaxed) >= threshold)
                {
                    shared.embed_circuit_open.store(true, Ordering::Relaxed);
                    return ProjectionOutcome::Failure {
                        cursor: job.cursor,
                        failure_code: last_code,
                    };
                }
                match embed_with_watchdog(
                    embedder,
                    &job.body,
                    embed_timeout,
                    &shared.live_embed_threads,
                ) {
                    Ok(vector) => vector,
                    Err(RuntimeEmbedderError::Timeout) => {
                        // The embed thread is now abandoned (still counted in
                        // live_embed_threads until it returns); the breaker
                        // check above caps how many can accumulate.
                        last_code = "EmbedderError";
                        continue;
                    }
                    Err(RuntimeEmbedderError::Failed { .. }) => {
                        last_code = "EmbedderError";
                        continue;
                    }
                }
            }
            None => {
                last_code = "EmbedderNotConfiguredError";
                continue;
            }
        };

        if u32::try_from(vector.len()).unwrap_or(u32::MAX) != shared.embedder_identity.dimension {
            last_code = "EmbedderDimensionMismatchError";
            continue;
        }

        let blob = encode_vector_blob(&vector);
        // EU-5a2 mean-centering apply path (projection write side). The
        // f32 BLOB persisted is ALWAYS un-centered; `bin_blob` carries
        // the (possibly centered) f32 fed to `vec_quantize_binary`. The
        // centering decision is finalized in `commit_projection_outcomes`
        // where the writer connection is in-hand and the read of
        // `_fathomdb_embedder_profiles.mean_vec` is in the same tx as
        // the INSERT. NoopEmbedder (EU-5a2's only live identity) is not
        // MC-required, so `bin_blob == blob` throughout EU-5a2.
        let bin_blob = blob.clone();
        return ProjectionOutcome::Success {
            cursor: job.cursor,
            kind: job.kind.clone(),
            blob,
            bin_blob,
        };
    }

    ProjectionOutcome::Failure { cursor: job.cursor, failure_code: last_code }
}

fn next_pending_projection_jobs(
    connection: &Connection,
    in_flight: &BTreeSet<u64>,
    max_jobs: usize,
) -> rusqlite::Result<Vec<ProjectionJob>> {
    if max_jobs == 0 {
        return Ok(Vec::new());
    }
    let cursor = load_projection_cursor(connection)?;
    // Over-fetch by `in_flight.len()` so the post-filter still returns
    // up to `max_jobs` after skipping cursors already in-flight.
    let sql_limit = max_jobs.saturating_add(in_flight.len()).min(256);
    let sql = format!(
        "SELECT canonical_nodes.write_cursor, canonical_nodes.kind, canonical_nodes.body
         FROM canonical_nodes
         JOIN _fathomdb_vector_kinds ON _fathomdb_vector_kinds.kind = canonical_nodes.kind
         LEFT JOIN _fathomdb_projection_terminal
           ON _fathomdb_projection_terminal.write_cursor = canonical_nodes.write_cursor
         WHERE canonical_nodes.write_cursor > ?1
           AND _fathomdb_projection_terminal.write_cursor IS NULL
         ORDER BY canonical_nodes.write_cursor
         LIMIT {sql_limit}"
    );
    let mut statement = connection.prepare_cached(&sql)?;
    let rows = statement.query_map([cursor], |row| {
        Ok(ProjectionJob { cursor: row.get(0)?, kind: row.get(1)?, body: row.get(2)? })
    })?;
    let mut jobs = Vec::with_capacity(max_jobs);
    for row in rows {
        let job = row?;
        if in_flight.contains(&job.cursor) {
            continue;
        }
        jobs.push(job);
        if jobs.len() >= max_jobs {
            break;
        }
    }
    Ok(jobs)
}

fn database_has_pending_projection_work(path: &Path) -> rusqlite::Result<bool> {
    let connection = open_runtime_connection(path)?;
    let cursor = load_projection_cursor(&connection)?;
    connection
        .query_row(
            "SELECT 1
             FROM canonical_nodes
             JOIN _fathomdb_vector_kinds ON _fathomdb_vector_kinds.kind = canonical_nodes.kind
             LEFT JOIN _fathomdb_projection_terminal
               ON _fathomdb_projection_terminal.write_cursor = canonical_nodes.write_cursor
             WHERE canonical_nodes.write_cursor > ?1
               AND _fathomdb_projection_terminal.write_cursor IS NULL
             LIMIT 1",
            [cursor],
            |_row| Ok(true),
        )
        .or_else(|err| match err {
            rusqlite::Error::QueryReturnedNoRows => Ok(false),
            _ => Err(err),
        })
}

struct CanonicalNodeRow {
    cursor: u64,
    kind: String,
    body: String,
}

fn canonical_node_rows(connection: &Connection) -> rusqlite::Result<Vec<CanonicalNodeRow>> {
    let mut statement = connection
        .prepare("SELECT write_cursor, kind, body FROM canonical_nodes ORDER BY write_cursor")?;
    let rows = statement.query_map([], |row| {
        Ok(CanonicalNodeRow {
            cursor: row.get::<_, u64>(0)?,
            kind: row.get::<_, String>(1)?,
            body: row.get::<_, String>(2)?,
        })
    })?;
    rows.collect()
}

fn hex_encode(bytes: &[u8]) -> String {
    let mut out = String::with_capacity(bytes.len() * 2);
    for byte in bytes {
        out.push(hex_nibble(byte >> 4));
        out.push(hex_nibble(byte & 0x0f));
    }
    out
}

fn hex_nibble(value: u8) -> char {
    match value {
        0..=9 => (b'0' + value) as char,
        10..=15 => (b'a' + value - 10) as char,
        _ => unreachable!(),
    }
}

fn physical_section(connection: &Connection, full: bool) -> Section {
    let mut findings = Vec::new();
    if let Err(err) = connection.query_row("PRAGMA page_count", [], |row| row.get::<_, i64>(0)) {
        findings.push(Finding {
            code: "E_CORRUPT_HEADER",
            stage: "PhysicalProbe",
            locator: locator_from_rusqlite_error(&err),
            doc_anchor: "design/recovery.md#header-malformed",
            detail: format!("page_count probe failed: {err}"),
        });
    }
    if full {
        match collect_integrity_check_findings(connection) {
            Ok(rows) => findings.extend(rows),
            Err(err) => findings.push(Finding {
                code: "E_CORRUPT_INTEGRITY_CHECK",
                stage: "IntegrityCheck",
                locator: locator_from_rusqlite_error(&err),
                doc_anchor: "design/recovery.md#integrity-check-full-findings",
                detail: format!("PRAGMA integrity_check failed: {err}"),
            }),
        }
    }
    if findings.is_empty() {
        Section::Clean
    } else {
        Section::Findings(findings)
    }
}

fn logical_section(connection: &Connection) -> Section {
    let mut findings = Vec::new();
    if let Err(err) = connection.query_row("PRAGMA schema_version", [], |row| row.get::<_, i64>(0))
    {
        findings.push(Finding {
            code: "E_CORRUPT_SCHEMA",
            stage: "SchemaProbe",
            locator: locator_from_rusqlite_error(&err),
            doc_anchor: "design/recovery.md#schema-inconsistent",
            detail: format!("schema_version probe failed: {err}"),
        });
    }
    match connection.query_row("PRAGMA user_version", [], |row| row.get::<_, u32>(0)) {
        Ok(0) => findings.push(Finding {
            code: "E_CORRUPT_SCHEMA",
            stage: "SchemaProbe",
            locator: CorruptionLocator::MigrationStep { from: 0, to: 0 },
            doc_anchor: "design/recovery.md#schema-inconsistent",
            detail: "user_version is zero".to_string(),
        }),
        Ok(_) => {}
        Err(err) => findings.push(Finding {
            code: "E_CORRUPT_SCHEMA",
            stage: "SchemaProbe",
            locator: locator_from_rusqlite_error(&err),
            doc_anchor: "design/recovery.md#schema-inconsistent",
            detail: format!("user_version probe failed: {err}"),
        }),
    }
    if findings.is_empty() {
        Section::Clean
    } else {
        Section::Findings(findings)
    }
}

fn semantic_section(connection: &Connection) -> Section {
    match load_default_profile(connection) {
        Ok(_) => Section::Clean,
        Err(rusqlite::Error::QueryReturnedNoRows) => Section::Findings(vec![Finding {
            code: "E_CORRUPT_EMBEDDER_IDENTITY",
            stage: "EmbedderIdentity",
            locator: CorruptionLocator::OpaqueSqliteError { sqlite_extended_code: 0 },
            doc_anchor: "design/recovery.md#embedder-identity-drift",
            detail: "default embedder profile row is missing".to_string(),
        }]),
        Err(err) => Section::Findings(vec![Finding {
            code: "E_CORRUPT_EMBEDDER_IDENTITY",
            stage: "EmbedderIdentity",
            locator: locator_from_rusqlite_error(&err),
            doc_anchor: "design/recovery.md#embedder-identity-drift",
            detail: format!("default embedder profile probe failed: {err}"),
        }]),
    }
}

fn collect_integrity_check_findings(connection: &Connection) -> rusqlite::Result<Vec<Finding>> {
    let mut statement = connection.prepare("PRAGMA integrity_check")?;
    let rows = statement.query_map([], |row| row.get::<_, String>(0))?;
    let mut findings = Vec::new();
    for row in rows {
        let message = row?;
        if message == "ok" {
            continue;
        }
        findings.push(Finding {
            code: "E_CORRUPT_INTEGRITY_CHECK",
            stage: "IntegrityCheck",
            locator: CorruptionLocator::OpaqueSqliteError {
                sqlite_extended_code: rusqlite::ffi::SQLITE_CORRUPT,
            },
            doc_anchor: "design/recovery.md#integrity-check-full-findings",
            detail: message,
        });
    }
    Ok(findings)
}

fn locator_from_rusqlite_error(err: &rusqlite::Error) -> CorruptionLocator {
    let extended = err.sqlite_error().map(|inner| inner.extended_code).unwrap_or(0);
    CorruptionLocator::OpaqueSqliteError { sqlite_extended_code: extended }
}

fn open_runtime_connection(path: &Path) -> rusqlite::Result<Connection> {
    let connection = Connection::open(path)?;
    connection.pragma_update(None, "journal_mode", "WAL")?;
    Ok(connection)
}

fn load_projection_cursor(connection: &Connection) -> rusqlite::Result<u64> {
    connection
        .query_row(
            "SELECT value FROM _fathomdb_open_state WHERE key = ?1",
            [PROJECTION_CURSOR_KEY],
            |row| row.get::<_, String>(0),
        )
        .map(|value| value.parse::<u64>().unwrap_or(0))
        .or_else(|err| match err {
            rusqlite::Error::QueryReturnedNoRows => Ok(0),
            _ => Err(err),
        })
}

fn store_projection_cursor(connection: &Connection, cursor: u64) -> rusqlite::Result<()> {
    connection.execute(
        "INSERT INTO _fathomdb_open_state(key, value) VALUES(?1, ?2)
         ON CONFLICT(key) DO UPDATE SET value = excluded.value",
        params![PROJECTION_CURSOR_KEY, cursor.to_string()],
    )?;
    Ok(())
}

fn record_projection_terminal(
    connection: &Connection,
    cursor: u64,
    state: &str,
) -> rusqlite::Result<()> {
    connection.execute(
        "INSERT OR IGNORE INTO _fathomdb_projection_terminal(write_cursor, state) VALUES(?1, ?2)",
        params![cursor, state],
    )?;
    Ok(())
}

fn terminal_state_for_cursor(
    connection: &Connection,
    cursor: u64,
) -> rusqlite::Result<Option<String>> {
    connection
        .query_row(
            "SELECT state FROM _fathomdb_projection_terminal WHERE write_cursor = ?1",
            [cursor],
            |row| row.get::<_, String>(0),
        )
        .map(Some)
        .or_else(|err| match err {
            rusqlite::Error::QueryReturnedNoRows => Ok(None),
            _ => Err(err),
        })
}

fn advance_projection_cursor(connection: &Connection) -> rusqlite::Result<u64> {
    let mut cursor = load_projection_cursor(connection)?;
    loop {
        let next = cursor.saturating_add(1);
        if terminal_state_for_cursor(connection, next)?.is_some() {
            cursor = next;
        } else {
            break;
        }
    }
    store_projection_cursor(connection, cursor)?;
    Ok(cursor)
}

fn commit_projection_outcomes(
    connection: &mut Connection,
    outcomes: &[ProjectionOutcome],
    shared: &ProjectionRuntimeShared,
) -> rusqlite::Result<()> {
    let embedder_identity = &shared.embedder_identity;
    let mc = identity_requires_mean_centering(embedder_identity);
    // EU-5f — serialize the whole commit across workers so the at-pin
    // re-quantize sees a totally-ordered history (see `commit_gate`).
    let _gate = shared.commit_gate.lock().unwrap_or_else(|poisoned| poisoned.into_inner());
    let tx = connection.transaction()?;
    // EU-5a2/EU-5f — the live pinned mean. Read once at the top; may pin
    // mid-batch (set to `Some` after a threshold-crossing row below).
    let mut current_mean: Option<Vec<f32>> = if mc {
        tx.query_row(
            "SELECT mean_vec FROM _fathomdb_embedder_profiles WHERE profile = 'default'",
            [],
            |row| row.get::<_, Option<Vec<u8>>>(0),
        )
        .ok()
        .flatten()
        .map(|bytes| decode_vector_blob(&bytes))
    } else {
        None
    };
    let mut staged_events: Vec<EmbedderEvent> = Vec::new();
    for outcome in outcomes {
        match outcome {
            ProjectionOutcome::Success { cursor, kind, blob, bin_blob } => {
                if terminal_state_for_cursor(&tx, *cursor)?.is_some() {
                    continue;
                }
                // EU-5f — feed the streaming accumulator and decide the pin
                // atomically under the accumulator lock (add -> count ->
                // take), so exactly one row/worker can cross the threshold.
                // Only while MC-required and not yet pinned.
                let pin_mean: Option<Vec<f32>> = if mc && current_mean.is_none() {
                    let mut acc = shared.mean_accumulator.lock().unwrap_or_else(|p| p.into_inner());
                    match acc.as_mut() {
                        Some(a) => {
                            a.add(&decode_vector_blob(bin_blob));
                            if a.count() >= MEAN_VEC_PIN_THRESHOLD {
                                let mean = a.materialize();
                                *acc = None;
                                Some(mean)
                            } else {
                                None
                            }
                        }
                        None => None,
                    }
                } else {
                    None
                };

                let source_type = resolve_source_type(kind).map_err(|_| {
                    rusqlite::Error::SqliteFailure(
                        rusqlite::ffi::Error::new(rusqlite::ffi::SQLITE_CONSTRAINT),
                        Some(format!("unknown kind for source_type mapping: {kind}")),
                    )
                })?;
                let now_unix =
                    SystemTime::now().duration_since(UNIX_EPOCH).unwrap_or_default().as_secs()
                        as i64;
                tx.execute(
                    "INSERT OR IGNORE INTO _fathomdb_vector_rows(rowid, kind, write_cursor) VALUES(?1, ?2, ?3)",
                    params![cursor, kind, cursor],
                )?;
                // EU-5a2/EU-5f — sign-quant input is the mean-subtracted
                // vector iff a mean is live (`current_mean`); otherwise the
                // un-centered `bin_blob`. A row inserted just before the
                // crossing is centered retroactively by the re-quantize
                // pass below.
                let centered_blob: Vec<u8> = match &current_mean {
                    Some(mean) if mean.len() * 4 == bin_blob.len() => {
                        encode_vector_blob(&subtract_mean(&decode_vector_blob(bin_blob), mean))
                    }
                    _ => bin_blob.clone(),
                };
                tx.execute(
                    "INSERT OR IGNORE INTO vector_default(
                        rowid, embedding, embedding_bin, source_type, kind, created_at
                     ) VALUES(?1, ?2, vec_quantize_binary(?3), ?4, ?5, ?6)",
                    params![cursor, blob, centered_blob, source_type, kind, now_unix],
                )?;
                record_projection_terminal(&tx, *cursor, "up_to_date")?;

                // EU-5f — this row crossed the threshold: pin the mean and
                // re-quantize every row written so far (incl. earlier rows
                // in this same tx, which are visible to the SELECT) within
                // the same transaction so the pin is atomic.
                if let Some(mean) = pin_mean {
                    tx.execute(
                        "UPDATE _fathomdb_embedder_profiles SET mean_vec = ?1 WHERE profile = 'default'",
                        params![encode_vector_blob(&mean)],
                    )?;
                    let rows: Vec<(i64, Vec<u8>)> = {
                        let mut statement = tx.prepare(
                            "SELECT rowid, embedding FROM vector_default ORDER BY rowid",
                        )?;
                        let mapped = statement.query_map([], |row| {
                            Ok((row.get::<_, i64>(0)?, row.get::<_, Vec<u8>>(1)?))
                        })?;
                        let mut out = Vec::new();
                        for r in mapped {
                            out.push(r?);
                        }
                        out
                    };
                    let (doc_count, _) =
                        run_pin_and_requantize_pass(&tx, &rows, &mean).map_err(|_| {
                            rusqlite::Error::SqliteFailure(
                                rusqlite::ffi::Error::new(rusqlite::ffi::SQLITE_ERROR),
                                Some("mean-centering re-quantize pass failed".to_string()),
                            )
                        })?;
                    staged_events.push(EmbedderEvent::MeanVecPinned {
                        dim: u32::try_from(mean.len()).unwrap_or(u32::MAX),
                        doc_count,
                    });
                    current_mean = Some(mean);
                }
            }
            ProjectionOutcome::Failure { cursor, failure_code } => {
                if terminal_state_for_cursor(&tx, *cursor)?.is_some() {
                    continue;
                }
                let existing: u64 = tx.query_row(
                    "SELECT COUNT(*) FROM operational_mutations
                     WHERE collection_name = 'projection_failures'
                       AND json_extract(payload_json, '$.write_cursor') = ?1",
                    [cursor],
                    |row| row.get(0),
                )?;
                if existing == 0 {
                    let payload = format!(
                        r#"{{"write_cursor":{cursor},"failure_code":"{failure_code}","recorded_at":0}}"#
                    );
                    tx.execute(
                        "INSERT INTO operational_mutations(
                            collection_name, record_key, op_kind, payload_json, schema_id, write_cursor
                         ) VALUES('projection_failures', ?1, 'append', ?2, NULL, ?3)",
                        params![cursor.to_string(), payload, cursor],
                    )?;
                }
                record_projection_terminal(&tx, *cursor, "failed")?;
            }
        }
    }
    // 0.7.2 PR-2bc S2 — the AUTOMATIC in-ingest drift detector (EWMA recent
    // mean + cos-threshold + debounce + 200k cap + `MeanRecomputeDeferred`)
    // was CARVED OUT and DEFERRED to 0.8.x; its recall premise was refuted
    // (the mean is a non-lever) and the benefit is unmeasured. The mean is
    // refreshed only on demand via `Engine::recompute_mean` (the
    // `doctor recompute-mean` verb). See `dev/design/embedder.md` §0.3 and
    // `dev/plans/prompts/0.8.x-auto-mean-drift-DEFERRED.md`. Nothing here
    // mutates `mean_vec` after the initial pin.

    advance_projection_cursor(&tx)?;
    tx.commit()?;
    // EU-5f — publish MeanVecPinned only after the pin tx is durable, so a
    // rolled-back pin never emits a spurious event.
    if !staged_events.is_empty() {
        if let Ok(mut events) = shared.pending_events.lock() {
            events.extend(staged_events);
        }
    }
    Ok(())
}

/// EU-5f — open-time recovery pin (`dev/design/embedder.md` §0.3, Hazard 4).
/// Derives the corpus mean from the existing un-centered `vector_default`
/// rows, pins it, and re-quantizes every row, all in one transaction on the
/// single-threaded open connection (no workers running yet, so no gate is
/// needed). Called only when MC is required, no mean is pinned, and the row
/// count already meets the threshold.
fn recover_mean_vec_pin(
    connection: &mut Connection,
    identity: &EmbedderIdentity,
) -> Result<(), EngineError> {
    let tx = connection.transaction().map_err(|_| EngineError::Storage)?;
    recompute_mean_in_tx(&tx, identity)?;
    tx.commit().map_err(|_| EngineError::Storage)?;
    Ok(())
}

/// 0.7.2 PR-2b — shared mean (re)compute core, run INSIDE the caller's
/// transaction. Derives the FULL-corpus mean from the un-centered
/// `vector_default.embedding` BLOBs, writes `mean_vec`, and re-quantizes
/// EVERY row via the existing [`run_pin_and_requantize_pass`] so no row is
/// left under a stale centering.
///
/// This generalizes the EU-5f open-time recovery pin: it has NO "no mean
/// pinned yet" guard, so it equally serves the FIRST pin (recovery) and a
/// REFRESH of an already-pinned mean (PR-2b drift / `doctor recompute-mean`).
/// The caller owns the transaction boundary, which is what makes a fault
/// between the `mean_vec` UPDATE and re-quantize completion roll back
/// wholesale (`dev/design/embedder.md` §0.5 atomicity). It does NOT publish
/// any event — that is the caller's job, strictly post-durable-commit.
fn recompute_mean_in_tx(
    tx: &rusqlite::Transaction<'_>,
    identity: &EmbedderIdentity,
) -> Result<MeanRecomputeReport, EngineError> {
    recompute_mean_in_tx_inner(tx, identity, false)
}

/// 0.7.2 PR-2b — recompute core with an optional fault-injection point. The
/// `fail_after_mean_update` flag (debug builds only, set via a test seam)
/// errors AFTER the `mean_vec` UPDATE but BEFORE the re-quantize completes,
/// so the caller's tx rolls back the partial recentering.
fn recompute_mean_in_tx_inner(
    tx: &rusqlite::Transaction<'_>,
    identity: &EmbedderIdentity,
    fail_after_mean_update: bool,
) -> Result<MeanRecomputeReport, EngineError> {
    let started = Instant::now();
    let dim = identity.dimension as usize;
    // The previously-pinned mean (if any) is read first so we can report
    // the pre-recompute drift cosine.
    let old_mean = read_pinned_mean_vec(tx, identity.dimension)?;
    let rows: Vec<(i64, Vec<u8>)> = {
        let mut statement = tx
            .prepare("SELECT rowid, embedding FROM vector_default ORDER BY rowid")
            .map_err(|_| EngineError::Storage)?;
        let mapped = statement
            .query_map([], |row| Ok((row.get::<_, i64>(0)?, row.get::<_, Vec<u8>>(1)?)))
            .map_err(|_| EngineError::Storage)?;
        let mut out = Vec::new();
        for r in mapped {
            out.push(r.map_err(|_| EngineError::Storage)?);
        }
        out
    };
    let mut accumulator = MeanAccumulator::new(dim);
    for (_rowid, blob) in &rows {
        if blob.len() != dim * 4 {
            return Err(EngineError::Storage);
        }
        accumulator.add(&decode_vector_blob(blob));
    }
    let old_doc_count = accumulator.count();
    let mean = accumulator.materialize();
    let drift_cos_before = match &old_mean {
        Some(old) => cosine_similarity(&mean, old),
        None => 1.0,
    };
    tx.execute(
        "UPDATE _fathomdb_embedder_profiles SET mean_vec = ?1 WHERE profile = 'default'",
        params![encode_vector_blob(&mean)],
    )
    .map_err(|_| EngineError::Storage)?;
    if fail_after_mean_update {
        // Injected fault: bail before re-quantizing so the caller's tx
        // rolls back the `mean_vec` UPDATE too (crash-atomicity proof).
        return Err(EngineError::Storage);
    }
    let (doc_count, _) = run_pin_and_requantize_pass(tx, &rows, &mean)?;
    Ok(MeanRecomputeReport {
        dim: u32::try_from(dim).unwrap_or(u32::MAX),
        old_doc_count,
        doc_count_requantized: doc_count,
        drift_cos_before,
        mean_was_pinned: old_mean.is_some(),
        elapsed_ms: u64::try_from(started.elapsed().as_millis()).unwrap_or(u64::MAX),
    })
}

fn enforce_provenance_retention(connection: &Connection, cap: u64) -> rusqlite::Result<()> {
    if cap == 0 {
        return Ok(());
    }
    let slack = cap.max(20) / 20;
    let upper = cap.saturating_add(slack.max(1));
    let count: u64 =
        connection.query_row("SELECT COUNT(*) FROM operational_mutations", [], |row| row.get(0))?;
    if count <= upper {
        return Ok(());
    }
    let to_delete = count.saturating_sub(cap);
    connection.execute(
        "DELETE FROM operational_mutations
         WHERE id IN (
             SELECT id FROM operational_mutations
             ORDER BY id
             LIMIT ?1
         )",
        [to_delete],
    )?;
    Ok(())
}

fn projection_status(
    connection: &Connection,
    kind: &str,
) -> Result<lifecycle::ProjectionStatus, EngineError> {
    let latest = connection
        .query_row(
            "SELECT COALESCE(MAX(write_cursor), 0) FROM canonical_nodes WHERE kind = ?1",
            [kind],
            |row| row.get::<_, u64>(0),
        )
        .map_err(|_| EngineError::Storage)?;
    if latest == 0 {
        return Ok(lifecycle::ProjectionStatus::UpToDate);
    }
    let pending: u64 = connection
        .query_row(
            "SELECT COUNT(*)
             FROM canonical_nodes
             LEFT JOIN _fathomdb_projection_terminal
               ON _fathomdb_projection_terminal.write_cursor = canonical_nodes.write_cursor
             WHERE canonical_nodes.kind = ?1
               AND _fathomdb_projection_terminal.write_cursor IS NULL",
            [kind],
            |row| row.get(0),
        )
        .map_err(|_| EngineError::Storage)?;
    if pending > 0 {
        return Ok(lifecycle::ProjectionStatus::Pending);
    }
    match terminal_state_for_cursor(connection, latest).map_err(|_| EngineError::Storage)? {
        Some(state) if state == "failed" => Ok(lifecycle::ProjectionStatus::Failed),
        _ => Ok(lifecycle::ProjectionStatus::UpToDate),
    }
}

fn canonical_database_path(path: &Path) -> Result<PathBuf, EngineOpenError> {
    let parent = path
        .parent()
        .filter(|parent| !parent.as_os_str().is_empty())
        .unwrap_or_else(|| Path::new("."));
    let canonical_parent = parent.canonicalize().map_err(|_| EngineOpenError::Io {
        message: "database parent directory is not accessible".to_string(),
    })?;
    let file_name = path.file_name().ok_or_else(|| EngineOpenError::Io {
        message: "database path has no file name".to_string(),
    })?;

    Ok(canonical_parent.join(file_name))
}

fn acquire_lock(path: &Path) -> Result<File, EngineOpenError> {
    let lock_path = lock_path(path);
    let mut options = OpenOptions::new();
    options.read(true).write(true).create(true);
    #[cfg(unix)]
    options.mode(0o600);

    let mut file = options.open(&lock_path).map_err(|_| EngineOpenError::Io {
        message: "could not open database lock file".to_string(),
    })?;

    match file.try_lock() {
        Ok(()) => {
            let pid = std::process::id().to_string();
            let _ = file.set_len(0);
            let _ = file.seek(SeekFrom::Start(0));
            let _ = file.write_all(pid.as_bytes());
            Ok(file)
        }
        Err(std::fs::TryLockError::WouldBlock) => {
            Err(EngineOpenError::DatabaseLocked { holder_pid: read_holder_pid(&lock_path) })
        }
        Err(_) => {
            Err(EngineOpenError::Io { message: "could not acquire database lock".to_string() })
        }
    }
}

fn lock_path(path: &Path) -> PathBuf {
    let mut lock_path = path.as_os_str().to_os_string();
    lock_path.push(LOCK_SUFFIX);
    PathBuf::from(lock_path)
}

fn read_holder_pid(path: &Path) -> Option<u32> {
    std::fs::read_to_string(path).ok()?.trim().parse().ok()
}

fn map_migration_error(err: SchemaMigrationError) -> EngineOpenError {
    match err {
        SchemaMigrationError::IncompatibleSchemaVersion { seen, supported } => {
            EngineOpenError::IncompatibleSchemaVersion { seen, supported }
        }
        SchemaMigrationError::MigrationError(report) => EngineOpenError::MigrationError {
            schema_version_before: report.schema_version_before,
            schema_version_current: report.schema_version_current,
            step_id: report.migration_steps.last().map_or(0, |step| step.step_id),
        },
        SchemaMigrationError::Storage { message } => {
            EngineOpenError::Io { message: message.to_string() }
        }
    }
}

/// 0.7.0 perf-experiments hook: process-start `sqlite3_config` calls.
/// Runs exactly once per process; must precede any `Connection::open`.
/// Gated on `FATHOMDB_PERF_EXPERIMENTS=1`. Each individual config
/// option is opt-in via its own env var so unrelated experiments do
/// not implicitly co-fire.
///
/// Currently supports:
/// - `FATHOMDB_PERF_SQLITE_MEMSTATUS_OFF=1`:
///   `sqlite3_config(SQLITE_CONFIG_MEMSTATUS, 0)` — drops the
///   allocator stats locking surface (whitepaper § 7.4). Composes
///   with other levers; small payoff alone.
///
/// Pattern: shutdown → config → initialize, mirroring B.1 attempt #2
/// (`d448263`, reverted). The captured rc for each config call is
/// logged to stderr so experiments can verify the call took effect.
fn init_perf_experiments_runtime() {
    static INIT: Once = Once::new();
    INIT.call_once(|| {
        if std::env::var_os("FATHOMDB_PERF_EXPERIMENTS").is_none() {
            return;
        }
        let memstatus_off =
            std::env::var_os("FATHOMDB_PERF_SQLITE_MEMSTATUS_OFF").is_some_and(|v| v == "1");
        // FATHOMDB_PERF_SQLITE_PAGECACHE=<page_size_bytes>:<page_count>
        // E.g. "4096:5000" => pre-allocate 4096 B × 5000 pages = 20 MB
        // global page-cache backing. SQLite distributes this across
        // connections; reduces global allocator pressure for page
        // cache fills.
        let pagecache = std::env::var("FATHOMDB_PERF_SQLITE_PAGECACHE").ok();
        // FATHOMDB_PERF_SQLITE_PCACHE2=1 installs the per-instance
        // custom page-cache allocator (pcache2.rs). Targets AC-020
        // residual contention on the default pcache1 mutex.
        let pcache2_on =
            std::env::var_os("FATHOMDB_PERF_SQLITE_PCACHE2").is_some_and(|v| v == "1");
        if !memstatus_off && pagecache.is_none() && !pcache2_on {
            return;
        }
        // SAFETY: sqlite3_shutdown / sqlite3_initialize are documented
        // as safe to call before any other SQLite API; sqlite3_config
        // must be called between shutdown and initialize. We pre-empt
        // rusqlite's lazy first-call sqlite3_initialize via this
        // explicit shutdown-then-config-then-initialize sequence,
        // identical to B.1 attempt #2's plumbing.
        unsafe {
            let rc_shutdown = rusqlite::ffi::sqlite3_shutdown();
            let rc_memstatus = if memstatus_off {
                rusqlite::ffi::sqlite3_config(rusqlite::ffi::SQLITE_CONFIG_MEMSTATUS, 0_i32)
            } else {
                -1
            };
            // SQLITE_CONFIG_PAGECACHE = 7 per sqlite3.h. With buffer=NULL,
            // SQLite allocates the backing memory itself but still
            // partitions it for use as the page-cache pool.
            let rc_pagecache = if let Some(spec) = pagecache.as_ref() {
                let mut parts = spec.split(':');
                let sz = parts.next().and_then(|s| s.parse::<i32>().ok()).unwrap_or(0);
                let n = parts.next().and_then(|s| s.parse::<i32>().ok()).unwrap_or(0);
                if sz > 0 && n > 0 {
                    rusqlite::ffi::sqlite3_config(
                        7, // SQLITE_CONFIG_PAGECACHE
                        std::ptr::null_mut::<std::ffi::c_void>(),
                        sz,
                        n,
                    )
                } else {
                    eprintln!(
                        "perf-experiment: bad FATHOMDB_PERF_SQLITE_PAGECACHE spec '{spec}' (expect '<bytes>:<count>')"
                    );
                    -1
                }
            } else {
                -1
            };
            let rc_pcache2 = if pcache2_on {
                // SQLITE_CONFIG_PCACHE2 = 18 per sqlite3.h. The methods
                // table must outlive the SQLite engine; we pass a
                // pointer to our static.
                rusqlite::ffi::sqlite3_config(
                    rusqlite::ffi::SQLITE_CONFIG_PCACHE2,
                    &raw const pcache2::PCACHE2_METHODS.0,
                )
            } else {
                -1
            };
            let rc_init = rusqlite::ffi::sqlite3_initialize();
            eprintln!(
                "perf-experiment: runtime-config rcs shutdown={rc_shutdown} \
                 memstatus={rc_memstatus} pagecache={rc_pagecache} pcache2={rc_pcache2} \
                 initialize={rc_init} (0=SQLITE_OK; 21=SQLITE_MISUSE; -1=not configured)"
            );
        }
    });
}

fn register_sqlite_vec_extension() {
    static REGISTER: Once = Once::new();
    REGISTER.call_once(|| unsafe {
        let entrypoint: unsafe extern "C" fn(
            *mut rusqlite::ffi::sqlite3,
            *mut *const std::os::raw::c_char,
            *const rusqlite::ffi::sqlite3_api_routines,
        ) -> std::os::raw::c_int = std::mem::transmute(sqlite3_vec_init as *const ());
        rusqlite::ffi::sqlite3_auto_extension(Some(entrypoint));
    });
}

fn probe_open_integrity(connection: &Connection) -> Result<(), EngineOpenError> {
    // `SELECT COUNT(*) FROM sqlite_schema` forces a full traversal of the
    // sqlite_schema b-tree; this surfaces page-1 b-tree corruption that a
    // bare `PRAGMA schema_version` (which only reads the schema cookie
    // out of the file header) would miss.
    connection
        .query_row("SELECT COUNT(*) FROM sqlite_schema", [], |row| row.get::<_, i64>(0))
        .map(|_| ())
        .map_err(|err| map_open_sqlite_error(err, OpenStage::SchemaProbe))
}

fn probe_database_header(connection: &Connection) -> Result<(), EngineOpenError> {
    connection
        .query_row("PRAGMA application_id", [], |row| row.get::<_, i64>(0))
        .map(|_| ())
        .map_err(|err| map_open_sqlite_error(err, OpenStage::HeaderProbe))
}

/// Pre-`pragma WAL` sidecar validation. SQLite silently discards a WAL
/// file whose header magic is wrong or whose advertised page size is
/// outside `[512, SQLITE_MAX_PAGE_SIZE]`, which would cause us to lose
/// committed frames at open time. AC-035a requires that we instead
/// refuse to open with `Corruption(WalReplayFailure)` rather than
/// silently rebuild from a truncated WAL.
fn probe_wal_sidecar(db_path: &Path) -> Result<(), EngineOpenError> {
    let mut wal_path = db_path.as_os_str().to_owned();
    wal_path.push("-wal");
    let wal_path = PathBuf::from(wal_path);
    // Bounded read: the WAL header is fixed-layout in the first 32
    // bytes (magic + format + page-size + checkpoint-seq + salts +
    // checksums); frame data starts at offset 32 and is irrelevant to
    // the magic + page-size pre-check. A `std::fs::read` of the whole
    // sidecar would force an unclean-shutdown open path to allocate
    // and copy the entire WAL into memory before SQLite touches
    // recovery — a real latency + RSS regression on AC-035.
    use std::io::Read;
    let mut file = match std::fs::File::open(&wal_path) {
        Ok(file) => file,
        Err(err) if err.kind() == std::io::ErrorKind::NotFound => return Ok(()),
        Err(_) => return Ok(()),
    };
    let mut bytes = [0u8; 32];
    if file.read_exact(&mut bytes).is_err() {
        // A short (< 32-byte) sidecar carries no committed frames;
        // SQLite treats it as empty and re-initializes WAL state.
        return Ok(());
    }
    let magic = u32::from_be_bytes([bytes[0], bytes[1], bytes[2], bytes[3]]);
    let page_size = u32::from_be_bytes([bytes[8], bytes[9], bytes[10], bytes[11]]);
    // WAL_MAGIC mask per SQLite `walIndexRecover`: low bit distinguishes
    // big-endian vs little-endian checksum encoding; the rest of the
    // magic is fixed.
    const WAL_MAGIC_MASK: u32 = 0xFFFF_FFFE;
    const WAL_MAGIC: u32 = 0x377F_0682;
    const SQLITE_MAX_PAGE_SIZE: u32 = 65536;
    let magic_ok = (magic & WAL_MAGIC_MASK) == WAL_MAGIC;
    let page_size_ok =
        page_size.is_power_of_two() && (512..=SQLITE_MAX_PAGE_SIZE).contains(&page_size);
    if magic_ok && page_size_ok {
        return Ok(());
    }
    Err(EngineOpenError::Corruption(CorruptionDetail {
        kind: CorruptionKind::WalReplayFailure,
        stage: OpenStage::WalReplay,
        locator: CorruptionLocator::FileOffset { offset: if !magic_ok { 0 } else { 8 } },
        recovery_hint: RecoveryHint {
            code: "E_CORRUPT_WAL_REPLAY",
            doc_anchor: "design/recovery.md#wal-replay-failures",
        },
    }))
}

fn reject_legacy_shape(connection: &Connection) -> Result<(), EngineOpenError> {
    let has_legacy_table = table_exists(connection, "fathom_nodes")
        || table_exists(connection, "fathom_edges")
        || table_exists(connection, "fathom_chunks");
    if !has_legacy_table {
        return Ok(());
    }

    let seen =
        connection.query_row("PRAGMA user_version", [], |row| row.get::<_, u32>(0)).unwrap_or(0);
    Err(EngineOpenError::IncompatibleSchemaVersion { seen, supported: SCHEMA_VERSION })
}

fn table_exists(connection: &Connection, table: &str) -> bool {
    connection
        .query_row(
            "SELECT 1 FROM sqlite_schema WHERE type = 'table' AND name = ?1",
            [table],
            |_row| Ok(()),
        )
        .is_ok()
}

fn read_schema_objects(
    connection: &Connection,
    obj_type: &str,
) -> Result<Vec<SchemaObject>, EngineError> {
    let mut stmt = connection
        .prepare(
            "SELECT name, sql FROM sqlite_schema
             WHERE type = ?1 AND name NOT LIKE 'sqlite_%' AND sql IS NOT NULL
             ORDER BY name",
        )
        .map_err(|_| EngineError::Storage)?;
    let rows = stmt
        .query_map([obj_type], |row| {
            Ok(SchemaObject { name: row.get::<_, String>(0)?, sql: row.get::<_, String>(1)? })
        })
        .map_err(|_| EngineError::Storage)?;
    let mut out = Vec::new();
    for row in rows {
        out.push(row.map_err(|_| EngineError::Storage)?);
    }
    Ok(out)
}

fn order_canonical_first(mut objects: Vec<SchemaObject>) -> Vec<SchemaObject> {
    let mut canonical: Vec<SchemaObject> = Vec::new();
    for name in CANONICAL_TABLES {
        if let Some(pos) = objects.iter().position(|o| o.name == *name) {
            canonical.push(objects.remove(pos));
        }
    }
    canonical.extend(objects);
    canonical
}

fn load_default_profile(connection: &Connection) -> rusqlite::Result<EmbedderIdentity> {
    connection.query_row(
        "SELECT name, revision, dimension FROM _fathomdb_embedder_profiles WHERE profile = ?1",
        [DEFAULT_VECTOR_PROFILE],
        |row| {
            Ok(EmbedderIdentity::new(
                row.get::<_, String>(0)?,
                row.get::<_, String>(1)?,
                row.get::<_, u32>(2)?,
            ))
        },
    )
}

fn default_profile_dimension(connection: &Connection) -> Result<u32, EngineError> {
    load_default_profile(connection)
        .map(|identity| identity.dimension)
        .map_err(|_| EngineError::Storage)
}

fn kind_is_vector_indexed(connection: &Connection, kind: &str) -> Result<bool, EngineError> {
    connection
        .query_row("SELECT 1 FROM _fathomdb_vector_kinds WHERE kind = ?1", [kind], |_row| Ok(()))
        .map(|_| true)
        .or_else(|err| match err {
            rusqlite::Error::QueryReturnedNoRows => Ok(false),
            _ => Err(EngineError::Storage),
        })
}

fn ensure_vector_partition(connection: &mut Connection, dimension: u32) -> rusqlite::Result<()> {
    // 0.7.0 Pack 1 schema per dev/design/0.7.0-vector-quant-pack1.md D1/D2:
    // f32 `embedding` + binary-quant sibling `embedding_bin` + `source_type`
    // partition key + `kind` + `created_at`. The vec0 column type is
    // dim-parameterized, so the reshape lives here rather than in the
    // SQL-only migration framework — see fathomdb-schema migration step 9
    // and dev/plans/runs/0.7.0-PVQ-P1-IMPL-output.json for the deviation
    // from the design memo's "Choose (a)" guidance.
    //
    // Three paths:
    //   (1) no vector_default       -> CREATE at new shape.
    //   (2) old single-column shape -> stage + drop + recreate at new shape
    //                                  + repopulate with vec_quantize_binary.
    //   (3) already new shape       -> no-op.
    let existing_sql: Option<String> = connection
        .query_row(
            "SELECT sql FROM sqlite_master WHERE type='table' AND name=?1",
            [DEFAULT_VECTOR_PARTITION],
            |row| row.get::<_, String>(0),
        )
        .optional()?;

    match existing_sql {
        None => create_vector_partition(connection, dimension),
        Some(sql) if sql.contains("embedding_bin") => Ok(()),
        Some(_) => migrate_vector_partition_to_pack1(connection, dimension),
    }
}

fn create_vector_partition(connection: &Connection, dimension: u32) -> rusqlite::Result<()> {
    let sql = format!(
        "CREATE VIRTUAL TABLE IF NOT EXISTS {DEFAULT_VECTOR_PARTITION} USING vec0(\
            embedding float[{dimension}],\
            embedding_bin bit[{dimension}],\
            source_type TEXT partition key,\
            kind TEXT,\
            created_at INTEGER\
         )"
    );
    connection.execute_batch(&sql)
}

/// SQL fragment implementing the D3 `kind -> source_type` map.
/// Used both by the Pack 1 reshape migration and by the drift-detection
/// unit test that pins it to [`resolve_source_type`].
const KIND_TO_SOURCE_TYPE_CASE_SQL: &str = "CASE s.kind
    WHEN 'email'   THEN 'email'
    WHEN 'article' THEN 'article'
    WHEN 'paper'   THEN 'paper'
    WHEN 'meeting' THEN 'meeting'
    WHEN 'note'    THEN 'note'
    WHEN 'todo'    THEN 'todo'
    WHEN 'doc'     THEN 'article'
    ELSE 'article'
END";

/// Pack 1 in-place reshape of `vector_default`. Stages the existing
/// f32 corpus + each row's `kind`, drops the old single-column vec0
/// table, recreates at the runtime `dimension` with the Pack 1
/// columns, then repopulates with SQL-side `vec_quantize_binary` +
/// the D3 `kind -> source_type` mapping. The preflight CHECK on
/// unknown kinds has already run as migration step 9 by the time we
/// get here.
///
/// Atomicity: the DROP+CREATE+repopulate sequence runs inside a
/// rusqlite `Connection::transaction()` (DEFERRED begin per rusqlite
/// `transaction.rs:417`). Cross-process serialization is provided by
/// the engine's sidecar `acquire_lock` at `open_with_migrations`
/// (`lib.rs:1127` area); reader handles are not opened until
/// `ensure_vector_partition` returns (`lib.rs:1241` area), so readers
/// never observe a partial reshape.
fn migrate_vector_partition_to_pack1(
    connection: &mut Connection,
    dimension: u32,
) -> rusqlite::Result<()> {
    let tx = connection.transaction()?;
    tx.execute_batch(
        "CREATE TABLE _fathomdb_vector_migration_v0_7_0 (
             rowid     INTEGER PRIMARY KEY,
             embedding BLOB NOT NULL,
             kind      TEXT NOT NULL
         );
         INSERT INTO _fathomdb_vector_migration_v0_7_0(rowid, embedding, kind)
             SELECT v.rowid, v.embedding, r.kind
             FROM vector_default v
             JOIN _fathomdb_vector_rows r ON r.rowid = v.rowid;
         DROP TABLE vector_default;",
    )?;
    let create_sql = format!(
        "CREATE VIRTUAL TABLE {DEFAULT_VECTOR_PARTITION} USING vec0(\
            embedding float[{dimension}],\
            embedding_bin bit[{dimension}],\
            source_type TEXT partition key,\
            kind TEXT,\
            created_at INTEGER\
         )"
    );
    tx.execute_batch(&create_sql)?;
    let repopulate_sql = format!(
        "INSERT INTO vector_default(
             rowid, embedding, embedding_bin, source_type, kind, created_at
         )
         SELECT
             s.rowid,
             s.embedding,
             vec_quantize_binary(s.embedding),
             {KIND_TO_SOURCE_TYPE_CASE_SQL},
             s.kind,
             strftime('%s', 'now')
         FROM _fathomdb_vector_migration_v0_7_0 s;
         DROP TABLE _fathomdb_vector_migration_v0_7_0;"
    );
    tx.execute_batch(&repopulate_sql)?;
    tx.commit()
}

fn encode_vector_blob(vector: &[f32]) -> Vec<u8> {
    vector.iter().flat_map(|value| value.to_le_bytes()).collect()
}

fn decode_vector_blob(bytes: &[u8]) -> Vec<f32> {
    debug_assert_eq!(bytes.len() % 4, 0, "f32 BLOB length must be multiple of 4");
    bytes.chunks_exact(4).map(|c| f32::from_le_bytes([c[0], c[1], c[2], c[3]])).collect()
}

/// EU-5a2 — does the live embedder identity request mean-centering?
/// Identity-name compare per EU-5a1's BGE_SMALL_EMBEDDER_NAME constant
/// (`dev/design/embedder.md` §0.6). NoopEmbedder returns `false`.
fn identity_requires_mean_centering(identity: &EmbedderIdentity) -> bool {
    identity.name == BGE_SMALL_EMBEDDER_NAME
}

/// EU-5a2 — read the pinned mean vector from
/// `_fathomdb_embedder_profiles.mean_vec` for the default profile.
/// Returns `Ok(None)` when the column is NULL or the row is missing;
/// returns `Err(EngineError::Storage)` on dimension drift (the open-time
/// `check_embedder_profile` already fails closed for this, so a runtime
/// drift here would be an internal-inconsistency signal).
fn read_pinned_mean_vec(
    connection: &Connection,
    dimension: u32,
) -> Result<Option<Vec<f32>>, EngineError> {
    let bytes: Option<Vec<u8>> = connection
        .query_row(
            "SELECT mean_vec FROM _fathomdb_embedder_profiles WHERE profile = 'default'",
            [],
            |row| row.get::<_, Option<Vec<u8>>>(0),
        )
        .or_else(|err| match err {
            rusqlite::Error::QueryReturnedNoRows => Ok(None),
            other => Err(other),
        })
        .map_err(|_| EngineError::Storage)?;
    let Some(bytes) = bytes else { return Ok(None) };
    let expected_len = (dimension as usize).saturating_mul(4);
    if bytes.len() != expected_len {
        return Err(EngineError::Storage);
    }
    let mut out = Vec::with_capacity(dimension as usize);
    for chunk in bytes.chunks_exact(4) {
        let arr = [chunk[0], chunk[1], chunk[2], chunk[3]];
        out.push(f32::from_le_bytes(arr));
    }
    Ok(Some(out))
}

/// EU-5a2 — pointwise `v - mean`. Length-checked debug-assert; caller
/// guarantees equal length via `read_pinned_mean_vec` + dimension check.
fn subtract_mean(v: &[f32], mean: &[f32]) -> Vec<f32> {
    debug_assert_eq!(v.len(), mean.len(), "subtract_mean dim mismatch");
    v.iter().zip(mean.iter()).map(|(a, b)| *a - *b).collect()
}

/// Maps the writer-facing `kind` value to the locked Pack 1
/// `source_type` partition-key vocabulary. Must stay in lockstep with
/// the CASE WHEN inlined in migration step 9
/// (`fathomdb-schema/src/lib.rs`); the drift-detection unit test in
/// this module's `tests` mod enforces that. Per
/// `dev/design/0.7.0-vector-quant-pack1.md` D3.
fn resolve_source_type(kind: &str) -> Result<&'static str, EngineError> {
    Ok(match kind {
        "email" => "email",
        "article" => "article",
        "paper" => "paper",
        "meeting" => "meeting",
        "note" => "note",
        "todo" => "todo",
        // Synthetic AC-013 test fixture; coerced so the 6-value HITL lock holds.
        "doc" => "article",
        _ => return Err(EngineError::Storage),
    })
}

fn map_runtime_embedder_error(err: RuntimeEmbedderError) -> EngineError {
    match err {
        RuntimeEmbedderError::Failed { .. } | RuntimeEmbedderError::Timeout => {
            EngineError::Embedder
        }
    }
}

fn default_embedder_identity() -> EmbedderIdentity {
    EmbedderIdentity::new(
        DEFAULT_EMBEDDER_NAME,
        DEFAULT_EMBEDDER_REVISION,
        DEFAULT_EMBEDDER_DIMENSION,
    )
}

fn check_embedder_profile(
    connection: &Connection,
    supplied: &EmbedderIdentity,
) -> Result<bool, EngineOpenError> {
    // Returns `true` iff `_fathomdb_embedder_profiles.mean_vec IS NOT NULL`
    // for the default profile (and its byte length matches `4 * dimension`
    // per `dev/design/embedder.md` §0.2). EU-5a2: column lands in step 10.
    let mut statement = match connection.prepare(
        "SELECT name, revision, dimension, mean_vec FROM _fathomdb_embedder_profiles WHERE profile = 'default'",
    ) {
        Ok(statement) => statement,
        Err(_) => return Ok(false),
    };
    let mut rows = statement.query([]).map_err(|_| {
        EngineOpenError::Corruption(CorruptionDetail {
            kind: CorruptionKind::EmbedderIdentityDrift,
            stage: OpenStage::EmbedderIdentity,
            locator: CorruptionLocator::OpaqueSqliteError { sqlite_extended_code: 0 },
            recovery_hint: RecoveryHint {
                code: "E_CORRUPT_EMBEDDER_IDENTITY",
                doc_anchor: "design/recovery.md#embedder-identity-drift",
            },
        })
    })?;

    let Some(row) = rows.next().map_err(|_| {
        EngineOpenError::Corruption(CorruptionDetail {
            kind: CorruptionKind::EmbedderIdentityDrift,
            stage: OpenStage::EmbedderIdentity,
            locator: CorruptionLocator::OpaqueSqliteError { sqlite_extended_code: 0 },
            recovery_hint: RecoveryHint {
                code: "E_CORRUPT_EMBEDDER_IDENTITY",
                doc_anchor: "design/recovery.md#embedder-identity-drift",
            },
        })
    })?
    else {
        connection
            .execute(
                "INSERT INTO _fathomdb_embedder_profiles(profile, name, revision, dimension)
                 VALUES(?1, ?2, ?3, ?4)",
                params![
                    DEFAULT_VECTOR_PROFILE,
                    supplied.name,
                    supplied.revision,
                    supplied.dimension
                ],
            )
            .map_err(|_| EngineOpenError::Io {
                message: "could not persist embedder profile".to_string(),
            })?;
        return Ok(false);
    };

    let stored_name = row.get::<_, String>(0).map_err(|_| {
        EngineOpenError::Corruption(CorruptionDetail {
            kind: CorruptionKind::EmbedderIdentityDrift,
            stage: OpenStage::EmbedderIdentity,
            locator: CorruptionLocator::TableRow { table: "_fathomdb_embedder_profiles", rowid: 0 },
            recovery_hint: RecoveryHint {
                code: "E_CORRUPT_EMBEDDER_IDENTITY",
                doc_anchor: "design/recovery.md#embedder-identity-drift",
            },
        })
    })?;
    let stored_revision = row.get::<_, String>(1).map_err(|_| {
        EngineOpenError::Corruption(CorruptionDetail {
            kind: CorruptionKind::EmbedderIdentityDrift,
            stage: OpenStage::EmbedderIdentity,
            locator: CorruptionLocator::TableRow { table: "_fathomdb_embedder_profiles", rowid: 0 },
            recovery_hint: RecoveryHint {
                code: "E_CORRUPT_EMBEDDER_IDENTITY",
                doc_anchor: "design/recovery.md#embedder-identity-drift",
            },
        })
    })?;
    let dimension = row.get::<_, u32>(2).map_err(|_| {
        EngineOpenError::Corruption(CorruptionDetail {
            kind: CorruptionKind::EmbedderIdentityDrift,
            stage: OpenStage::EmbedderIdentity,
            locator: CorruptionLocator::TableRow { table: "_fathomdb_embedder_profiles", rowid: 0 },
            recovery_hint: RecoveryHint {
                code: "E_CORRUPT_EMBEDDER_IDENTITY",
                doc_anchor: "design/recovery.md#embedder-identity-drift",
            },
        })
    })?;

    let stored = EmbedderIdentity::new(stored_name, stored_revision, dimension);

    if stored.name != supplied.name || stored.revision != supplied.revision {
        return Err(EngineOpenError::EmbedderIdentityMismatch {
            stored,
            supplied: supplied.clone(),
        });
    }
    if dimension != supplied.dimension {
        return Err(EngineOpenError::EmbedderDimensionMismatch {
            stored: dimension,
            supplied: supplied.dimension,
        });
    }

    // EU-5a2 / `dev/design/embedder.md` §0.2 invariant: if `mean_vec` is
    // populated, byte length MUST equal `4 * dimension`. Debug builds
    // assert; release builds fail closed via EmbedderIdentityMismatch
    // (the same fail-closed channel the rest of profile drift takes).
    let mean_vec: Option<Vec<u8>> = row.get::<_, Option<Vec<u8>>>(3).map_err(|_| {
        EngineOpenError::Corruption(CorruptionDetail {
            kind: CorruptionKind::EmbedderIdentityDrift,
            stage: OpenStage::EmbedderIdentity,
            locator: CorruptionLocator::TableRow { table: "_fathomdb_embedder_profiles", rowid: 0 },
            recovery_hint: RecoveryHint {
                code: "E_CORRUPT_EMBEDDER_IDENTITY",
                doc_anchor: "design/recovery.md#embedder-identity-drift",
            },
        })
    })?;
    let pinned = match mean_vec {
        Some(bytes) => {
            let expected_len = (dimension as usize).saturating_mul(4);
            // `dev/design/embedder.md` §0.2 invariant: when populated,
            // `mean_vec` byte length MUST equal `4 * dimension`. Fail
            // closed via the existing identity-drift channel in both
            // debug and release builds — tests deliberately poke
            // malformed values to exercise this branch.
            if bytes.len() != expected_len {
                return Err(EngineOpenError::EmbedderIdentityMismatch {
                    stored,
                    supplied: supplied.clone(),
                });
            }
            true
        }
        None => false,
    };

    Ok(pinned)
}

#[derive(Clone, Debug, Eq, PartialEq)]
enum WritePlan {
    Node,
    Edge,
    AppendOnlyLog,
    LatestState,
    AdminSchema,
}

fn validate_batch(
    connection: &Connection,
    batch: &[PreparedWrite],
) -> Result<Vec<WritePlan>, EngineError> {
    batch.iter().map(|write| validate_write(connection, write)).collect()
}

fn collect_projection_jobs(
    connection: &Connection,
    batch: &[PreparedWrite],
) -> Result<Vec<ProjectionJob>, EngineError> {
    let mut jobs = Vec::new();
    for write in batch {
        if let PreparedWrite::Node { kind, body, .. } = write {
            if kind_is_vector_indexed(connection, kind)? {
                jobs.push(ProjectionJob { cursor: 0, kind: kind.clone(), body: body.clone() });
            }
        }
    }
    Ok(jobs)
}

fn validate_write(
    connection: &Connection,
    write: &PreparedWrite,
) -> Result<WritePlan, EngineError> {
    match write {
        PreparedWrite::Node { kind, body, source_id } => {
            if kind.trim().is_empty() || body.trim().is_empty() {
                return Err(EngineError::WriteValidation);
            }
            if let Some(source_id) = source_id {
                if source_id.is_empty() {
                    return Err(EngineError::WriteValidation);
                }
            }
            Ok(WritePlan::Node)
        }
        PreparedWrite::Edge { kind, from, to, source_id } => {
            if kind.trim().is_empty() || from.trim().is_empty() || to.trim().is_empty() {
                return Err(EngineError::WriteValidation);
            }
            if let Some(source_id) = source_id {
                if source_id.is_empty() {
                    return Err(EngineError::WriteValidation);
                }
            }
            Ok(WritePlan::Edge)
        }
        PreparedWrite::AdminSchema { name, kind, schema_json, retention_json } => {
            if name.trim().is_empty()
                || !matches!(kind.as_str(), "append_only_log" | "latest_state")
                || serde_json::from_str::<Value>(schema_json).is_err()
                || serde_json::from_str::<Value>(retention_json).is_err()
                || contains_external_ref(schema_json)
            {
                return Err(EngineError::SchemaValidation);
            }
            Ok(WritePlan::AdminSchema)
        }
        PreparedWrite::OpStore { collection, record_key, schema_id, body } => {
            if collection.trim().is_empty() || record_key.trim().is_empty() {
                return Err(EngineError::WriteValidation);
            }
            let (kind, schema_json) = collection_metadata(connection, collection)?;
            if let Some(schema_id) = schema_id {
                if schema_id != collection {
                    return Err(EngineError::SchemaValidation);
                }
                validate_payload(&schema_json, body)?;
            } else if serde_json::from_str::<Value>(body).is_err() {
                return Err(EngineError::SchemaValidation);
            }

            match kind.as_str() {
                "append_only_log" => Ok(WritePlan::AppendOnlyLog),
                "latest_state" => Ok(WritePlan::LatestState),
                _ => Err(EngineError::OpStore),
            }
        }
    }
}

fn collection_metadata(
    connection: &Connection,
    collection: &str,
) -> Result<(String, String), EngineError> {
    connection
        .query_row(
            "SELECT kind, schema_json FROM operational_collections WHERE name = ?1",
            [collection],
            |row| Ok((row.get::<_, String>(0)?, row.get::<_, String>(1)?)),
        )
        .map_err(|_| EngineError::OpStore)
}

fn validate_payload(schema_json: &str, body: &str) -> Result<(), EngineError> {
    let schema =
        serde_json::from_str::<Value>(schema_json).map_err(|_| EngineError::SchemaValidation)?;
    let payload = serde_json::from_str::<Value>(body).map_err(|_| EngineError::SchemaValidation)?;

    let compiled = JSONSchema::compile(&schema).map_err(|_| EngineError::SchemaValidation)?;
    compiled.validate(&payload).map_err(|_| EngineError::SchemaValidation)?;

    Ok(())
}

fn contains_external_ref(schema_json: &str) -> bool {
    let Ok(value) = serde_json::from_str::<Value>(schema_json) else {
        return false;
    };
    value_contains_external_ref(&value)
}

fn value_contains_external_ref(value: &Value) -> bool {
    match value {
        Value::Object(object) => object.iter().any(|(key, value)| {
            if key == "$ref" {
                return value.as_str().is_some_and(|uri| !uri.starts_with('#'));
            }
            value_contains_external_ref(value)
        }),
        Value::Array(values) => values.iter().any(value_contains_external_ref),
        _ => false,
    }
}

fn commit_batch(
    connection: &mut Connection,
    batch: &[PreparedWrite],
    plans: &[WritePlan],
    base_cursor: u64,
    provenance_row_cap: u64,
) -> rusqlite::Result<()> {
    let tx = connection.transaction()?;

    for (i, (write, plan)) in batch.iter().zip(plans).enumerate() {
        // Per-row cursor: row i gets `base_cursor + i + 1`. See the
        // comment in `Engine::write_inner`.
        let cursor = base_cursor.saturating_add((i as u64).saturating_add(1));
        match (write, plan) {
            (PreparedWrite::Node { kind, body, source_id }, WritePlan::Node) => {
                tx.execute(
                    "INSERT INTO canonical_nodes(write_cursor, kind, body, source_id)
                     VALUES(?1, ?2, ?3, ?4)",
                    params![cursor, kind, body, source_id],
                )?;
                tx.execute(
                    "INSERT INTO search_index(body, kind, write_cursor) VALUES(?1, ?2, ?3)",
                    params![body, kind, cursor],
                )?;
                if kind_is_vector_indexed(&tx, kind).unwrap_or(false) {
                    tx.execute(
                        "INSERT INTO _fathomdb_projection_state(kind, last_enqueued_cursor, updated_at)
                         VALUES(?1, ?2, 0)
                         ON CONFLICT(kind) DO UPDATE SET last_enqueued_cursor = excluded.last_enqueued_cursor",
                        params![kind, cursor],
                    )?;
                } else {
                    // Non-vector-indexed nodes will never be projected,
                    // so terminate the cursor up-front to let
                    // `advance_projection_cursor` walk past it.
                    record_projection_terminal(&tx, cursor, "up_to_date")?;
                }
            }
            (PreparedWrite::Edge { kind, from, to, source_id }, WritePlan::Edge) => {
                tx.execute(
                    "INSERT INTO canonical_edges(write_cursor, kind, from_id, to_id, source_id)
                     VALUES(?1, ?2, ?3, ?4, ?5)",
                    params![cursor, kind, from, to, source_id],
                )?;
                record_projection_terminal(&tx, cursor, "up_to_date")?;
            }
            (
                PreparedWrite::AdminSchema { name, kind, schema_json, retention_json },
                WritePlan::AdminSchema,
            ) => {
                tx.execute(
                    "INSERT INTO operational_collections(
                        name, kind, schema_json, retention_json, format_version, created_at
                     ) VALUES(?1, ?2, ?3, ?4, 1, 0)
                     ON CONFLICT(name) DO UPDATE SET
                        schema_json = excluded.schema_json,
                        retention_json = excluded.retention_json",
                    params![name, kind, schema_json, retention_json],
                )?;
                record_projection_terminal(&tx, cursor, "up_to_date")?;
            }
            (
                PreparedWrite::OpStore { collection, record_key, schema_id, body },
                WritePlan::AppendOnlyLog,
            ) => {
                tx.execute(
                    "INSERT INTO operational_mutations(
                        collection_name, record_key, op_kind, payload_json, schema_id, write_cursor
                     ) VALUES(?1, ?2, 'append', ?3, ?4, ?5)",
                    params![collection, record_key, body, schema_id, cursor],
                )?;
                record_projection_terminal(&tx, cursor, "up_to_date")?;
            }
            (
                PreparedWrite::OpStore { collection, record_key, schema_id, body },
                WritePlan::LatestState,
            ) => {
                tx.execute(
                    "INSERT INTO operational_state(
                        collection_name, record_key, payload_json, schema_id, write_cursor
                     ) VALUES(?1, ?2, ?3, ?4, ?5)
                     ON CONFLICT(collection_name, record_key) DO UPDATE SET
                        payload_json = excluded.payload_json,
                        schema_id = excluded.schema_id,
                        write_cursor = excluded.write_cursor",
                    params![collection, record_key, body, schema_id, cursor],
                )?;
                record_projection_terminal(&tx, cursor, "up_to_date")?;
            }
            _ => return Err(rusqlite::Error::InvalidQuery),
        }
    }

    enforce_provenance_retention(&tx, provenance_row_cap)?;
    advance_projection_cursor(&tx)?;

    tx.commit()
}

fn load_next_cursor(connection: &Connection) -> u64 {
    let nodes = max_cursor(connection, "canonical_nodes").unwrap_or(0);
    let edges = max_cursor(connection, "canonical_edges").unwrap_or(0);
    let mutations = max_cursor(connection, "operational_mutations").unwrap_or(0);
    let state = max_cursor(connection, "operational_state").unwrap_or(0);
    nodes.max(edges).max(mutations).max(state)
}

fn max_cursor(connection: &Connection, table: &str) -> rusqlite::Result<u64> {
    let sql = format!("SELECT COALESCE(MAX(write_cursor), 0) FROM {table}");
    connection.query_row(&sql, [], |row| row.get::<_, u64>(0))
}

/// Map a rusqlite error to its stable SQLite extended-code name.
///
/// Returns `None` for non-`SqliteFailure` variants (e.g. JSON conversion
/// failures, type mismatches at the rusqlite layer) — those are not
/// SQLite-internal events and should not be surfaced under
/// `EventSource::SqliteInternal`. The names returned here are the
/// canonical `SQLITE_*` symbol names from `sqlite3.h` and are stable
/// dispatch keys for AC-021 / AC-006 binding adapters.
///
/// Only the subset of codes the engine can reach in 0.6.0 is enumerated
/// — bare-extended-code matching covers the rest with a stable
/// `"SQLITE_UNKNOWN"` fallback so subscribers always see a typed code.
///
/// Diagnostic completeness for unmapped codes: when this helper returns
/// `"SQLITE_UNKNOWN"`, the numeric extended code is not lost — it
/// remains on the underlying `rusqlite::Error::SqliteFailure` carried
/// in the engine error chain that subscribers can inspect via
/// `EngineError`'s `source()`. Expanding the enumerated subset (or
/// surfacing the numeric code as a typed payload field) is a 0.7+
/// improvement.
fn sqlite_extended_code_name(err: &rusqlite::Error) -> Option<&'static str> {
    let sqlite_error = err.sqlite_error()?;
    let extended = sqlite_error.extended_code;
    Some(match extended {
        rusqlite::ffi::SQLITE_SCHEMA => "SQLITE_SCHEMA",
        rusqlite::ffi::SQLITE_BUSY => "SQLITE_BUSY",
        rusqlite::ffi::SQLITE_LOCKED => "SQLITE_LOCKED",
        rusqlite::ffi::SQLITE_CORRUPT => "SQLITE_CORRUPT",
        rusqlite::ffi::SQLITE_NOTADB => "SQLITE_NOTADB",
        rusqlite::ffi::SQLITE_IOERR => "SQLITE_IOERR",
        rusqlite::ffi::SQLITE_FULL => "SQLITE_FULL",
        rusqlite::ffi::SQLITE_READONLY => "SQLITE_READONLY",
        rusqlite::ffi::SQLITE_CONSTRAINT => "SQLITE_CONSTRAINT",
        rusqlite::ffi::SQLITE_MISUSE => "SQLITE_MISUSE",
        rusqlite::ffi::SQLITE_INTERRUPT => "SQLITE_INTERRUPT",
        rusqlite::ffi::SQLITE_NOMEM => "SQLITE_NOMEM",
        rusqlite::ffi::SQLITE_PERM => "SQLITE_PERM",
        rusqlite::ffi::SQLITE_ABORT => "SQLITE_ABORT",
        rusqlite::ffi::SQLITE_PROTOCOL => "SQLITE_PROTOCOL",
        rusqlite::ffi::SQLITE_RANGE => "SQLITE_RANGE",
        rusqlite::ffi::SQLITE_TOOBIG => "SQLITE_TOOBIG",
        rusqlite::ffi::SQLITE_MISMATCH => "SQLITE_MISMATCH",
        rusqlite::ffi::SQLITE_AUTH => "SQLITE_AUTH",
        rusqlite::ffi::SQLITE_NOTFOUND => "SQLITE_NOTFOUND",
        rusqlite::ffi::SQLITE_CANTOPEN => "SQLITE_CANTOPEN",
        _ => "SQLITE_UNKNOWN",
    })
}

fn sqlite_extended_code_name_from_int(extended: i32) -> &'static str {
    match extended {
        rusqlite::ffi::SQLITE_SCHEMA => "SQLITE_SCHEMA",
        rusqlite::ffi::SQLITE_BUSY => "SQLITE_BUSY",
        rusqlite::ffi::SQLITE_LOCKED => "SQLITE_LOCKED",
        rusqlite::ffi::SQLITE_CORRUPT => "SQLITE_CORRUPT",
        rusqlite::ffi::SQLITE_NOTADB => "SQLITE_NOTADB",
        rusqlite::ffi::SQLITE_IOERR => "SQLITE_IOERR",
        rusqlite::ffi::SQLITE_FULL => "SQLITE_FULL",
        rusqlite::ffi::SQLITE_READONLY => "SQLITE_READONLY",
        rusqlite::ffi::SQLITE_CONSTRAINT => "SQLITE_CONSTRAINT",
        rusqlite::ffi::SQLITE_MISUSE => "SQLITE_MISUSE",
        rusqlite::ffi::SQLITE_INTERRUPT => "SQLITE_INTERRUPT",
        rusqlite::ffi::SQLITE_NOMEM => "SQLITE_NOMEM",
        rusqlite::ffi::SQLITE_PERM => "SQLITE_PERM",
        rusqlite::ffi::SQLITE_ABORT => "SQLITE_ABORT",
        rusqlite::ffi::SQLITE_PROTOCOL => "SQLITE_PROTOCOL",
        rusqlite::ffi::SQLITE_RANGE => "SQLITE_RANGE",
        rusqlite::ffi::SQLITE_TOOBIG => "SQLITE_TOOBIG",
        rusqlite::ffi::SQLITE_MISMATCH => "SQLITE_MISMATCH",
        rusqlite::ffi::SQLITE_AUTH => "SQLITE_AUTH",
        rusqlite::ffi::SQLITE_NOTFOUND => "SQLITE_NOTFOUND",
        rusqlite::ffi::SQLITE_CANTOPEN => "SQLITE_CANTOPEN",
        _ => "SQLITE_UNKNOWN",
    }
}

fn map_open_sqlite_error(err: rusqlite::Error, stage: OpenStage) -> EngineOpenError {
    let Some(sqlite_error) = err.sqlite_error() else {
        return EngineOpenError::Io { message: "could not open database".to_string() };
    };
    match sqlite_error.extended_code {
        rusqlite::ffi::SQLITE_CORRUPT | rusqlite::ffi::SQLITE_NOTADB => {
            EngineOpenError::Corruption(CorruptionDetail {
                kind: match stage {
                    OpenStage::WalReplay => CorruptionKind::WalReplayFailure,
                    OpenStage::HeaderProbe => CorruptionKind::HeaderMalformed,
                    OpenStage::SchemaProbe => CorruptionKind::SchemaInconsistent,
                    OpenStage::EmbedderIdentity => CorruptionKind::EmbedderIdentityDrift,
                },
                stage,
                locator: CorruptionLocator::OpaqueSqliteError {
                    sqlite_extended_code: sqlite_error.extended_code,
                },
                recovery_hint: RecoveryHint {
                    code: match stage {
                        OpenStage::WalReplay => "E_CORRUPT_WAL_REPLAY",
                        OpenStage::HeaderProbe => "E_CORRUPT_HEADER",
                        OpenStage::SchemaProbe => "E_CORRUPT_SCHEMA",
                        OpenStage::EmbedderIdentity => "E_CORRUPT_EMBEDDER_IDENTITY",
                    },
                    doc_anchor: match stage {
                        OpenStage::WalReplay => "design/recovery.md#wal-replay-failures",
                        OpenStage::HeaderProbe => "design/recovery.md#header-malformed",
                        OpenStage::SchemaProbe => "design/recovery.md#schema-inconsistent",
                        OpenStage::EmbedderIdentity => "design/recovery.md#embedder-identity-drift",
                    },
                },
            })
        }
        _ => EngineOpenError::Io { message: "could not open database".to_string() },
    }
}

fn emit_open_error_event(subscriber: &Arc<dyn lifecycle::Subscriber>, err: &EngineOpenError) {
    if let EngineOpenError::Corruption(detail) = err {
        let code = match detail.locator {
            CorruptionLocator::OpaqueSqliteError { sqlite_extended_code } => {
                Some(sqlite_extended_code_name_from_int(sqlite_extended_code))
            }
            _ => None,
        };
        let event = lifecycle::Event {
            phase: lifecycle::Phase::Failed,
            source: lifecycle::EventSource::SqliteInternal,
            category: lifecycle::EventCategory::Corruption,
            code,
        };
        subscriber.on_event(&event);
    }
}

/// Install a `sqlite3_profile` callback on `connection` that dispatches
/// per-statement profile records and slow-statement signals to the
/// engine's subscriber registry.
///
/// Why FFI rather than `rusqlite::Connection::profile`: the safe API
/// (rusqlite 0.31) accepts only a `fn(&str, Duration)` with no
/// environment, so it cannot carry a per-engine subscriber-registry
/// pointer. We use `sqlite3_profile` directly with a leaked-into-`Box`
/// context whose pointer is tied to the engine's lifetime via
/// `Engine::profile_contexts`.
///
/// `sqlite3_profile` is documented as deprecated in favor of
/// `sqlite3_trace_v2`, but it remains supported and is sufficient for
/// the wall-clock + SQL-text payload required by AC-005a/b.
#[allow(clippy::vec_box)]
fn install_profile_callback(
    connection: &Connection,
    subscribers: &Arc<lifecycle::SubscriberRegistry>,
    profiling_enabled: &Arc<AtomicBool>,
    slow_threshold_ms: &Arc<AtomicU64>,
    contexts: &mut Vec<Box<ProfileContext>>,
) {
    let mut ctx = Box::new(ProfileContext {
        subscribers: Arc::clone(subscribers),
        profiling_enabled: Arc::clone(profiling_enabled),
        slow_threshold_ms: Arc::clone(slow_threshold_ms),
    });
    let ctx_ptr: *mut ProfileContext = &mut *ctx;

    // SAFETY: the Box outlives the connection. Rust drops struct fields
    // in declaration order. `connection` and `reader_pool` are declared
    // before `profile_contexts`. `ReaderWorkerPool::Drop` joins every
    // reader worker, and each worker uninstalls and drops its owned
    // connection inside `reader_worker_loop` before the worker thread
    // returns. Therefore all connections — and SQLite's internal
    // profile-callback state with them — are torn down before the
    // `Box<ProfileContext>` allocations are freed. `Engine::close`
    // additionally clears the callback via
    // `sqlite3_profile(handle, None, NULL)` before connection close to
    // drain any in-flight callback dispatch.
    unsafe {
        rusqlite::ffi::sqlite3_profile(
            connection.handle(),
            Some(profile_callback_trampoline),
            ctx_ptr.cast::<std::ffi::c_void>(),
        );
    }
    contexts.push(ctx);
}

/// Uninstall the profile callback so SQLite stops calling into our
/// freed `Box<ProfileContext>` pointer once a connection is being torn
/// down. Call before dropping `profile_contexts`.
fn uninstall_profile_callback(connection: &Connection) {
    // SAFETY: passing `None` as the callback unregisters the previous
    // callback; SQLite documents this as legal and idempotent.
    unsafe {
        rusqlite::ffi::sqlite3_profile(connection.handle(), None, std::ptr::null_mut());
    }
}

/// Pack 6.G G.1 — configure SQLite per-connection lookaside on a reader
/// worker connection. Must be called BEFORE any statement is prepared
/// or any PRAGMA is run on `connection`; per the SQLite docs
/// (https://www.sqlite.org/malloc.html §3) lookaside is silently
/// ignored if reconfigured after the first allocation on the
/// connection. Passing `NULL` for the buffer pointer lets SQLite
/// allocate the lookaside backing memory itself.
///
/// rusqlite 0.31's `set_db_config` only handles the boolean
/// `DbConfig::*` variants; `SQLITE_DBCONFIG_LOOKASIDE` is not surfaced
/// (it is commented out in `rusqlite/src/config.rs`), so we call the
/// raw FFI directly.
///
/// Returns the rc of `sqlite3_db_config` so callers can debug-assert
/// `SQLITE_OK` and surface configuration failure under
/// `debug_assertions` test builds without expanding the public surface.
/// 0.7.0 perf-experiments hook: apply caller-supplied reader PRAGMAs
/// from the `FATHOMDB_PERF_READER_PRAGMAS` env var. Format:
/// comma-separated `name=value` pairs (e.g.
/// `cache_size=-262144,mmap_size=268435456,temp_store=MEMORY`).
///
/// **Gated on `FATHOMDB_PERF_EXPERIMENTS=1`.** No-op if the gate env
/// var is unset, so production paths are never affected. Failures to
/// apply individual PRAGMAs are logged to stderr (via `eprintln!`) but
/// do not error the connection open — experiments are best-effort,
/// not contract.
///
/// Scope: 0.7.0 perf-experiment campaign per
/// `dev/plans/0.7.0-perf-experiments.md`. Once Wave 5 picks the
/// landing combination, the chosen PRAGMAs are hardcoded as the new
/// reader-open default and this hook is removed.
/// 0.7.0 perf-experiments hook: apply writer-side PRAGMAs from
/// `FATHOMDB_PERF_WRITER_PRAGMAS` (same format as reader hook).
/// **Runs BEFORE migrations** so PRAGMAs like `page_size` that must
/// precede any table creation take effect on a fresh DB.
///
/// Gated on `FATHOMDB_PERF_EXPERIMENTS=1`. No-op otherwise.
fn apply_perf_experiment_writer_pragmas(connection: &Connection) {
    if std::env::var_os("FATHOMDB_PERF_EXPERIMENTS").is_none() {
        return;
    }
    let raw = match std::env::var("FATHOMDB_PERF_WRITER_PRAGMAS") {
        Ok(s) if !s.is_empty() => s,
        _ => return,
    };
    for entry in raw.split(',') {
        let entry = entry.trim();
        if entry.is_empty() {
            continue;
        }
        let (name, value) = match entry.split_once('=') {
            Some((n, v)) => (n.trim(), v.trim()),
            None => {
                eprintln!("perf-experiment: bad writer pragma entry (expect name=value): {entry}");
                continue;
            }
        };
        if name.is_empty() {
            eprintln!("perf-experiment: empty pragma name in writer entry: {entry}");
            continue;
        }
        match connection.pragma_update(None, name, value) {
            Ok(()) => {
                eprintln!(
                    "perf-experiment: applied PRAGMA {name}={value} on writer (pre-migration)"
                );
            }
            Err(err) => {
                eprintln!("perf-experiment: writer PRAGMA {name}={value} failed: {err}");
            }
        }
    }
}

fn apply_perf_experiment_reader_pragmas(connection: &Connection) {
    if std::env::var_os("FATHOMDB_PERF_EXPERIMENTS").is_none() {
        return;
    }
    let raw = match std::env::var("FATHOMDB_PERF_READER_PRAGMAS") {
        Ok(s) if !s.is_empty() => s,
        _ => return,
    };
    for entry in raw.split(',') {
        let entry = entry.trim();
        if entry.is_empty() {
            continue;
        }
        let (name, value) = match entry.split_once('=') {
            Some((n, v)) => (n.trim(), v.trim()),
            None => {
                eprintln!("perf-experiment: bad pragma entry (expect name=value): {entry}");
                continue;
            }
        };
        if name.is_empty() {
            eprintln!("perf-experiment: empty pragma name in entry: {entry}");
            continue;
        }
        match connection.pragma_update(None, name, value) {
            Ok(()) => {
                eprintln!("perf-experiment: applied PRAGMA {name}={value} on reader");
            }
            Err(err) => {
                eprintln!("perf-experiment: PRAGMA {name}={value} failed: {err}");
            }
        }
    }
}

fn configure_reader_lookaside(connection: &Connection) -> std::os::raw::c_int {
    // SAFETY: `connection.handle()` returns a valid `*mut sqlite3` for
    // the lifetime of `connection`. The variadic
    // `sqlite3_db_config(LOOKASIDE)` call expects three trailing
    // arguments of types `void*`, `int`, `int` — the prototype shape
    // documented in `sqlite3.h`. We pass a null buffer so SQLite owns
    // the lookaside backing allocation, and the slot size / count from
    // the G.1 constants. No allocations happen on the connection
    // before this call (reader open path is `Connection::open` ->
    // `configure_reader_lookaside` -> first PRAGMA).
    unsafe {
        rusqlite::ffi::sqlite3_db_config(
            connection.handle(),
            rusqlite::ffi::SQLITE_DBCONFIG_LOOKASIDE,
            std::ptr::null_mut::<std::ffi::c_void>(),
            READER_LOOKASIDE_SLOT_SIZE,
            READER_LOOKASIDE_SLOT_COUNT,
        )
    }
}

/// Read the high-water-mark for `SQLITE_DBSTATUS_LOOKASIDE_USED` on
/// `connection`. The `current` out-param is the live checked-out slot
/// count and decays as transactions finalize, so it is unreliable as
/// post-warmup evidence. The `hiwtr` out-param latches the largest
/// observed `current` value since the last reset and is the right
/// signal that lookaside was honored at any point on this connection.
/// Reset flag is `0` so reading does not clear the high-water mark.
#[cfg(debug_assertions)]
fn read_lookaside_used_hiwtr(connection: &Connection) -> std::os::raw::c_int {
    let mut current: std::os::raw::c_int = 0;
    let mut hiwtr: std::os::raw::c_int = 0;
    // SAFETY: handle is valid; both out pointers are to local stack
    // ints; reset flag 0 is documented as legal.
    unsafe {
        rusqlite::ffi::sqlite3_db_status(
            connection.handle(),
            rusqlite::ffi::SQLITE_DBSTATUS_LOOKASIDE_USED,
            &mut current,
            &mut hiwtr,
            0,
        );
    }
    hiwtr
}

/// Pack 6.G G.3.5 — read the three page-cache pressure counters on
/// `connection`: `SQLITE_DBSTATUS_CACHE_HIT`, `_CACHE_MISS`, and
/// `_CACHE_USED`. Returns `(hit, miss, used_bytes)`. Hit/miss are
/// monotonic counters (reset flag = 0 here); used_bytes is the live
/// page-cache memory footprint at call time. The caller is expected to
/// take pre/post snapshots and do delta arithmetic explicitly.
#[cfg(debug_assertions)]
fn read_cache_status(
    connection: &Connection,
) -> (std::os::raw::c_int, std::os::raw::c_int, std::os::raw::c_int) {
    let mut hit_current: std::os::raw::c_int = 0;
    let mut hit_hiwtr: std::os::raw::c_int = 0;
    let mut miss_current: std::os::raw::c_int = 0;
    let mut miss_hiwtr: std::os::raw::c_int = 0;
    let mut used_current: std::os::raw::c_int = 0;
    let mut used_hiwtr: std::os::raw::c_int = 0;
    // SAFETY: `connection.handle()` returns a valid `*mut sqlite3` for
    // the lifetime of `connection`. All out-pointers are to local stack
    // ints. Reset flag 0 is documented as legal (no counter is reset).
    unsafe {
        rusqlite::ffi::sqlite3_db_status(
            connection.handle(),
            rusqlite::ffi::SQLITE_DBSTATUS_CACHE_HIT,
            &mut hit_current,
            &mut hit_hiwtr,
            0,
        );
        rusqlite::ffi::sqlite3_db_status(
            connection.handle(),
            rusqlite::ffi::SQLITE_DBSTATUS_CACHE_MISS,
            &mut miss_current,
            &mut miss_hiwtr,
            0,
        );
        rusqlite::ffi::sqlite3_db_status(
            connection.handle(),
            rusqlite::ffi::SQLITE_DBSTATUS_CACHE_USED,
            &mut used_current,
            &mut used_hiwtr,
            0,
        );
    }
    // CACHE_HIT / CACHE_MISS are monotonic counters reported in the
    // `current` out-param; CACHE_USED is the live byte count, also in
    // `current`. The hiwtr values are unused for this telemetry.
    (hit_current, miss_current, used_current)
}

/// FFI trampoline for `sqlite3_profile`.
///
/// Invoked by SQLite at statement-finish with the SQL text and the
/// statement's wall-clock cost in nanoseconds. We dispatch a
/// `ProfileRecord` (when profiling is enabled) and a `SlowStatement`
/// signal (when `wall_clock_ms` exceeds the configured slow threshold).
///
/// Per `dev/design/lifecycle.md` § Public record shape, the public
/// payload exposes `wall_clock_ms`, `step_count`, and `cache_delta`.
/// `sqlite3_profile` does not surface per-statement step counts or
/// cache-hit deltas in its callback; we emit `0` for those fields and
/// document the hazard. AC-005b requires the fields be typed numeric,
/// not that they carry non-zero values for every backend.
unsafe extern "C" fn profile_callback_trampoline(
    user_data: *mut std::ffi::c_void,
    sql: *const std::os::raw::c_char,
    nanoseconds: u64,
) {
    if user_data.is_null() || sql.is_null() {
        return;
    }
    let ctx = unsafe { &*(user_data.cast::<ProfileContext>()) };
    let sql_text = match unsafe { std::ffi::CStr::from_ptr(sql) }.to_str() {
        Ok(s) => s,
        Err(_) => return,
    };

    let wall_clock_ms = nanoseconds / 1_000_000;

    if ctx.profiling_enabled.load(Ordering::Relaxed) {
        let record = lifecycle::ProfileRecord {
            wall_clock_ms,
            // step_count / cache_delta are not surfaced by
            // sqlite3_profile; placeholder 0 satisfies AC-005b's
            // "typed numeric" contract. A future profiling refactor
            // around sqlite3_stmt_status + sqlite3_db_status would
            // populate them with non-zero deltas.
            step_count: 0,
            cache_delta: 0,
        };
        ctx.subscribers.dispatch_profile(&record);
    }

    let threshold = ctx.slow_threshold_ms.load(Ordering::Relaxed);
    if wall_clock_ms > threshold {
        let signal = lifecycle::SlowStatement { statement: sql_text.to_string(), wall_clock_ms };
        ctx.subscribers.dispatch_slow_statement(&signal);
    }
}

#[cfg(test)]
mod tests {
    use super::{resolve_source_type, Engine, PreparedWrite, KIND_TO_SOURCE_TYPE_CASE_SQL};
    use rusqlite::Connection;
    use tempfile::TempDir;

    // Pack 1 drift-detection: the Rust helper used by the two writer
    // sites must agree with the CASE WHEN used by the Pack 1 reshape
    // migration in `migrate_vector_partition_to_pack1`. The CASE SQL
    // is exported as `KIND_TO_SOURCE_TYPE_CASE_SQL`; this test
    // exercises it against an in-memory SQLite (no sqlite-vec extension
    // required — only the CASE) and asserts byte-equal output with the
    // Rust helper for every kind in the locked Pack 1 vocabulary
    // (incl. the synthetic `doc` -> `article` coercion). See
    // `dev/design/0.7.0-vector-quant-pack1.md` D3 / D4.
    #[test]
    fn resolve_source_type_drift_check() {
        let kinds = ["email", "article", "paper", "meeting", "note", "todo", "doc"];

        // 1. Rust helper return values (table is the contract: changes
        //    here must be reflected in the SQL CASE or this test fails).
        let want: &[(&str, &str)] = &[
            ("email", "email"),
            ("article", "article"),
            ("paper", "paper"),
            ("meeting", "meeting"),
            ("note", "note"),
            ("todo", "todo"),
            ("doc", "article"),
        ];
        for (kind, expected) in want {
            let got = resolve_source_type(kind).unwrap_or_else(|_| {
                panic!("resolve_source_type({kind}) returned Err; want Ok({expected})")
            });
            assert_eq!(got, *expected, "Rust helper drift for kind={kind}");
        }
        assert!(
            resolve_source_type("banana").is_err(),
            "unknown kind must surface as writer error"
        );

        // 2. SQL CASE evaluated against the same kinds. Build a
        //    one-row staging row per kind and SELECT through
        //    KIND_TO_SOURCE_TYPE_CASE_SQL; assert each row equals the
        //    Rust helper's output. Drift in either direction fails.
        let conn = Connection::open_in_memory().expect("in-memory sqlite");
        conn.execute_batch("CREATE TABLE s(kind TEXT NOT NULL)").expect("create s");
        for kind in &kinds {
            conn.execute("INSERT INTO s(kind) VALUES (?1)", [kind]).expect("insert kind");
        }
        let sql = format!("SELECT s.kind, {KIND_TO_SOURCE_TYPE_CASE_SQL} FROM s");
        let mut stmt = conn.prepare(&sql).expect("prepare CASE");
        let rows: Vec<(String, String)> = stmt
            .query_map([], |row| Ok((row.get::<_, String>(0)?, row.get::<_, String>(1)?)))
            .expect("query")
            .map(|r| r.expect("row"))
            .collect();
        assert_eq!(rows.len(), kinds.len(), "row count drift");
        for (kind, sql_result) in &rows {
            let rust_result = resolve_source_type(kind).expect("known kind");
            assert_eq!(
                sql_result, rust_result,
                "SQL CASE vs Rust helper drift for kind={kind}: SQL={sql_result}, Rust={rust_result}"
            );
        }
    }

    #[test]
    fn write_advances_cursor() {
        let dir = TempDir::new().unwrap();
        let opened = Engine::open(dir.path().join("rewrite.sqlite")).expect("engine should open");
        let receipt = opened
            .engine
            .write(&[PreparedWrite::Node {
                kind: "doc".to_string(),
                body: "hello".to_string(),
                source_id: None,
            }])
            .expect("write should succeed");

        assert_eq!(receipt.cursor, 1);
    }
}