melin-server-runtime 0.20.0

//! Replication — synchronous input-stream streaming from primary to replica.
//!
//! The primary's JournalStage encodes each `InputSlot` it just durably
//! journaled into a wire-ready `InputBatch` frame in the replication ring
//! (separately from the journal-codec bytes it writes to disk). The
//! `ReplicationSender` thread forwards those frames as-is over TCP/DPDK
//! — no decode + re-encode on the hot path. The replica decodes the
//! frames straight back into `InputSlot`s, publishes them to its local
//! input disruptor with the primary's pre-assigned sequences and
//! timestamps, and the replica's JournalStage re-encodes them through
//! its own writer for byte-exact-on-replay durability.
//!
//! ## Wire Protocol
//!
//! Length-prefixed frames, little-endian, over a dedicated TCP connection
//! (or DPDK pipe). The full `InputBatch` payload layout lives in
//! `melin_transport_core::replication::protocol` /
//! `melin_transport_core::replication_wire`.
//!
//! ### Auth (before handshake)
//! - **Challenge** (Primary → Replica): `[len:u32][0x03][nonce:[u8;32]]`
//! - **ChallengeResponse** (Replica → Primary): `[len:u32][0x04][signature:[u8;64]][pubkey:[u8;32]]`
//! - **AuthOk** (Primary → Replica): `[len:u32][0x05]`
//! - **AuthFailed** (Primary → Replica): `[len:u32][0x06]`
//!
//! ### Replica → Primary
//! - **Handshake**: `[len:u32][0x01][last_sequence:u64][chain_hash:[u8;32]]`
//! - **Ack**: `[len:u32][0x02][acked_sequence:u64][in_memory_sequence:u64]`
//!
//! ### Primary → Replica
//! - **StreamStart**: `[len:u32][0x10][start_sequence:u64][segment_start_sequence:u64][anchor_hash:[u8;32]][epoch:u64][genesis_known:u8][genesis_entries:u64]`
//!   — the segment header identity a fresh replica creates its journal
//!   with (lineage origin for full catch-up, the seeded segment's
//!   identity after a snapshot transfer), and the lineage's genesis
//!   length, which a fresh replica records and one with a journal checks
//!   against its own
//! - **NeedSnapshot**: `[len:u32][0x11]`
//! - **HashMismatch**: `[len:u32][0x12]` — divergent replica journal;
//!   the replica archives its lineage, then the snapshot flow follows
//! - **SnapshotBegin**: `[len:u32][0x13][snapshot_len:u64][snap_sequence:u64][snap_chain_hash:[u8;32]]`
//! - **SnapshotChunk**: `[len:u32][0x14][data...]`
//! - **SnapshotEnd**: `[len:u32][0x15][crc32c:u32]`
//! - **Rotate**: `[len:u32][0x16][boundary_seq:u64][tail_hash:[u8;32]]`
//!   — primary-driven rotation, verified + adopted by the replica
//! - **ChainCheck**: `[len:u32][0x17][sequence:u64][chain_hash:[u8;32]]`
//!   — periodic live-stream chain validation
//! - **SegmentSeedBegin**: `[len:u32][0x18][seed_len:u64]` — raw byte
//!   prefix of the primary's segment containing the snapshot boundary;
//!   body rides SnapshotChunk frames, ends with SnapshotEnd
//! - **InputBatch**: `[len:u32][0x21][count:u16][slot...]` — see
//!   `transport-core::replication_wire` for the per-slot layout
//! - **Heartbeat**: `[len:u32][0x30][sequence:u64]`
//!
//! ## Limitations
//!
//! - Dual replication (up to 2 replicas in parallel)
//!
//! See `docs/replication.md` for the full design document and limitation details.

use std::sync::Arc;
#[cfg(test)]
use std::sync::atomic::AtomicU64;
use std::sync::atomic::{AtomicBool, Ordering};

use melin_journal::{BufferedWriter, JournalWrite};

use melin_app::Application;
use melin_transport_core::lineage::SharedLineagePolicy;
use melin_transport_core::pipeline::{InputSlot, OutputSlot};
use melin_transport_core::replication::archive::{ArchiveReason, archive_local_lineage};
use melin_transport_core::replication::protocol::{MAX_CONTROL_FRAME, decode_primary_message};

mod auth;
#[cfg(feature = "dpdk")]
mod dpdk;
mod receiver_transport;
mod tcp_receiver;
mod tcp_sender;
// Only the DPDK sender uses the worker, but it is pure thread/channel
// plumbing with no libdpdk dependency — so it is also compiled under
// `test`, keeping its mechanics (and the affinity trap it exists to
// avoid) testable on a build without the `dpdk` feature.
#[cfg(any(feature = "dpdk", test))]
mod validation_worker;
// The DPDK sender's join worker: likewise thread/channel plumbing over the
// transport-agnostic catch-up steps, compiled under `test` so it is tested
// without libdpdk.
#[cfg(any(feature = "dpdk", test))]
mod join_worker;

use receiver_transport::{ControlFrameSource, SessionExit, StreamingResult, receive_chunked_body};

/// Log a replica's join ending, on either sender: from here its slot
/// streams live and the replica's acks count. Without it, a join that
/// never ends and one that ended look alike in the log.
pub(crate) fn log_replica_live(slot: usize, sent: &SentHighWater) {
    tracing::info!(
        slot,
        sent_sequence = sent.get(),
        "replica caught up, streaming live"
    );
}

// Wire-protocol types, auth, catch-up, ack queueing, dual-track
// cursor management, and per-replica metrics now live in
// `melin_transport_core::replication`. Re-export the public types
// here so the module's public API surface (e.g.
// `melin_server_runtime::replication::Ack` / `::ReplicationMetrics`) is
// unchanged for downstream consumers and tests.
pub use melin_transport_core::replication::ack_queue::{
    PendingAck, PendingAckQueue, try_flush_dual_track, wait_for_journal_cursor,
};
pub use melin_transport_core::replication::protocol::{
    Ack, Handshake, PrimaryMessage, ReplicaMessage,
};
pub use melin_transport_core::replication::{ReplicaCursors, ReplicationMetrics, SentHighWater};

#[cfg(feature = "dpdk")]
pub(crate) use dpdk::REPL_DISPATCH_BURST;
#[cfg(feature = "dpdk")]
pub use dpdk::{DpdkReplicationDriver, run_receiver_dpdk};
pub use tcp_receiver::{ReceiverResult, run_receiver};
pub use tcp_sender::{ReplicationListener, Sender, run_sender};

/// The handles a replica's receive loop shares with the rest of the
/// node — the admin endpoint and the control-plane raft driver.
/// Bundled (rather than five positional parameters) because they
/// always travel together through the kernel-TCP and DPDK receiver
/// signatures, and a transposition between same-typed flags is exactly
/// the bug a bundle prevents.
#[derive(Clone)]
pub struct ReplicaControlPlane {
    /// Promotion request: polled by the receive loop; filed by the
    /// admin `PROMOTE` command or the raft driver (auto-promotion).
    pub promote: crate::promotion::PromotionRequest,
    /// Flipped `true` once journal recovery has seeded the fence epoch,
    /// so the raft driver can trust this node's advertised tip — see
    /// `melin_raft::recency::TipSource`.
    pub tip_ready: std::sync::Arc<AtomicBool>,
    /// Sequence half of the advertised journal tip — see
    /// [`melin_transport_core::AdvertisedJournalTip`].
    pub journal_tip: melin_transport_core::AdvertisedJournalTip,
    /// `true` while this replica holds an authenticated replication
    /// connection to its primary. The raft driver refuses to
    /// auto-promote while set: a live link means the primary is
    /// demonstrably alive, and winning a control-plane election (e.g.
    /// because a *different* node's raft died) must not depose a
    /// healthy primary.
    pub primary_link_up: std::sync::Arc<AtomicBool>,
    /// Set once a primary's `StreamStart` has been accepted since this
    /// process booted, and never cleared: the auto-promotion rule's
    /// "a primary has been seen" input, which tells a blank genesis node
    /// from a replica that followed a primary before it died.
    pub(crate) primary_observed: std::sync::Arc<AtomicBool>,
    /// The ack policy the lineage this replica holds has in force: the
    /// newest journaled `AckPolicyChange` it holds, with the transition to
    /// it (`LineagePolicy`), or `None` when it holds none. The raft
    /// driver's auto-promotion refusal judges by the newest policy, so it
    /// judges the policy the entries this replica holds were acked under,
    /// never one advertised ahead of them. It seeds the replica's shadow
    /// snapshots and a promoted primary's lineage and gate too: the gate
    /// starts under the policy before the transition, with the change
    /// pending, as a restarted primary's does.
    ///
    /// One writer, the receive thread: stored whole from the recovered
    /// lineage at boot and after a divergence recovery, from the snapshot
    /// at a resync, cleared when the local lineage is archived (so a change
    /// in a divergent tail disappears with the tail), and folded as each
    /// change is accepted from the stream, before the advertised tip moves
    /// past it (see `receiver_transport::process_streaming_frames`). Not by
    /// the replica's matching stage, which lags the receive thread: two
    /// writers would let an older value overwrite a newer one.
    pub(crate) lineage_ack_policy: std::sync::Arc<SharedLineagePolicy>,
    /// Whether the replica's pipeline is healthy — fed to the replica's
    /// minimal health endpoint (`/health` OK/ERR and the
    /// `melin_pipeline_healthy` gauge). Process-lifetime (the per-session
    /// pipeline is torn down and rebuilt across resyncs, so its own
    /// `journal_failed` latch cannot be handed to the endpoint):
    /// `true` from boot, latched `false` by the journal stage's failure
    /// wrapper, reset `true` when a fresh pipeline is built.
    pub pipeline_healthy: std::sync::Arc<AtomicBool>,
}

impl ReplicaControlPlane {
    /// Fresh handles for a replica boot: nothing requested, tip not
    /// trustworthy yet, tip sequence 0, primary link down and never seen,
    /// no lineage policy held, pipeline healthy (nothing has failed yet).
    pub fn new() -> Self {
        Self {
            promote: crate::promotion::PromotionRequest::new(),
            tip_ready: std::sync::Arc::new(AtomicBool::new(false)),
            journal_tip: melin_transport_core::AdvertisedJournalTip::new(
                melin_transport_core::WireSeq::new(0),
            ),
            primary_link_up: std::sync::Arc::new(AtomicBool::new(false)),
            primary_observed: std::sync::Arc::new(AtomicBool::new(false)),
            lineage_ack_policy: std::sync::Arc::new(SharedLineagePolicy::new(None)),
            pipeline_healthy: std::sync::Arc::new(AtomicBool::new(true)),
        }
    }
}

impl Default for ReplicaControlPlane {
    fn default() -> Self {
        Self::new()
    }
}

/// Diagnostic: emit a `tcp_info` span at debug level describing the
/// kernel's view of the socket (rtt, cwnd, retrans, unacked, rcv_space,
/// rto). Guarded internally with `tracing::enabled!` so the
/// `getsockopt(TCP_INFO)` syscall is skipped when debug logging is off —
/// the call is free at runtime unless `RUST_LOG=debug` (or a more
/// specific filter) is active. Used to distinguish user-space stalls
/// from TCP-level congestion collapse when diagnosing replication
/// slowdowns.
pub(super) fn log_tcp_info(fd: std::os::unix::io::RawFd, tag: &str, slot: usize) {
    if !tracing::enabled!(tracing::Level::DEBUG) {
        return;
    }
    // SAFETY: all-zero pattern is a valid `tcp_info` (all numeric fields).
    // The kernel fills in whatever the running version supports and
    // returns the written length in `len`.
    let mut info: libc::tcp_info = unsafe { std::mem::zeroed() };
    let mut len = std::mem::size_of::<libc::tcp_info>() as libc::socklen_t;
    let rc = unsafe {
        libc::getsockopt(
            fd,
            libc::IPPROTO_TCP,
            libc::TCP_INFO,
            &mut info as *mut _ as *mut libc::c_void,
            &mut len,
        )
    };
    if rc != 0 {
        tracing::debug!(
            slot,
            tag,
            errno = std::io::Error::last_os_error().raw_os_error().unwrap_or(0),
            "tcp_info getsockopt failed"
        );
        return;
    }
    tracing::debug!(
        slot,
        tag,
        state = info.tcpi_state,
        ca_state = info.tcpi_ca_state,
        rtt_us = info.tcpi_rtt,
        rttvar_us = info.tcpi_rttvar,
        snd_cwnd = info.tcpi_snd_cwnd,
        snd_ssthresh = info.tcpi_snd_ssthresh,
        snd_mss = info.tcpi_snd_mss,
        rcv_mss = info.tcpi_rcv_mss,
        unacked = info.tcpi_unacked,
        retrans = info.tcpi_retrans,
        total_retrans = info.tcpi_total_retrans,
        lost = info.tcpi_lost,
        rcv_space = info.tcpi_rcv_space,
        rto_us = info.tcpi_rto,
        "tcp_info"
    );
}

/// Sleep for the given duration in 100ms increments, checking the shutdown
/// flag and the promotion request between increments. Returns early if
/// either is set.
pub(super) fn sleep_checking_flags(
    duration: std::time::Duration,
    shutdown: &AtomicBool,
    promote: &crate::promotion::PromotionRequest,
) {
    let deadline = std::time::Instant::now() + duration;
    while std::time::Instant::now() < deadline {
        if shutdown.load(Ordering::Relaxed) || promote.is_requested() {
            return;
        }
        std::thread::sleep(std::time::Duration::from_millis(100));
    }
}

/// Wait out one retry delay: sleep for the current `backoff` (checking
/// the shutdown and promotion flags — see [`sleep_checking_flags`]),
/// then double it, capped at [`MAX_BACKOFF`], for the next attempt.
/// Sleep-then-double, so the first retry waits the base delay rather
/// than twice it. Every retry arm in both receivers goes through this —
/// don't hand-roll the idiom, the copies drift.
pub(super) fn sleep_then_double_backoff(
    backoff: &mut std::time::Duration,
    shutdown: &AtomicBool,
    promote: &crate::promotion::PromotionRequest,
) {
    sleep_checking_flags(*backoff, shutdown, promote);
    *backoff = (*backoff * 2).min(MAX_BACKOFF);
}

/// Outcome of shutting down a replica pipeline. All stage threads are
/// joined in every variant — by the time the caller sees this, no
/// pipeline thread can still touch the journal or snapshot files.
pub(super) enum TeardownOutcome<A, W> {
    /// Clean exit: both stages returned their state, reusable for the
    /// next pipeline build (post-snapshot) or promotion.
    Clean(A, W),
    /// The journal stage exited with an error — its writer is gone and
    /// the matching stage's state is unusable (it may have applied
    /// events the journal never persisted). Carries the error so the
    /// orchestrator can distinguish repairable chain divergence
    /// (in-process resync) from journal I/O death (exit).
    JournalFailed(melin_journal::JournalError),
    /// A stage thread panicked; no state survives.
    Panicked,
}

/// Shut down the replica pipeline and extract the application + journal writer
/// from the stage threads.
///
/// Relies on the caller having published a `JournalEvent::Shutdown`
/// sentinel to the input ring before invoking this — the journal and
/// matching stages exit when they consume the sentinel via the normal
/// event-processing path. We deliberately don't flip `shutdown_flag`
/// here: doing so could cause a stage to take its emergency-abort
/// branch *before* consuming the sentinel, hitting the
/// drain-vs-cursor race that the sentinel design exists to avoid.
///
/// `drain_handle` and `shadow_handle` still observe `shutdown_flag`
/// because they don't process the input ring (no sentinel reaches
/// them); set the flag for them only.
pub(super) fn shutdown_pipeline<A: Send + 'static, W: Send + 'static>(
    shutdown_flag: &AtomicBool,
    journal_handle: std::thread::JoinHandle<Result<W, melin_journal::JournalError>>,
    matching_handle: std::thread::JoinHandle<A>,
    drain_handle: std::thread::JoinHandle<()>,
    shadow_handle: Option<std::thread::JoinHandle<()>>,
) -> TeardownOutcome<A, W> {
    // Defense-in-depth: set the flag before joining. The sentinel was
    // already published by `teardown_replica_pipeline` before this call,
    // so no further events can arrive in the input ring — setting the
    // flag here cannot race with new publishes. The flag is the fallback
    // exit signal for paths that don't observe the sentinel: the sequencer
    // in `no-persist` builds, the drain consumer, the shadow stage, and
    // any case where the receiver thread panicked before publishing the
    // sentinel.
    //
    // The flag is also what makes these joins safe when the journal
    // stage is already dead: the matching stage's gate on the (frozen)
    // journal cursor is a non-blocking spin that re-checks the flag
    // every iteration, and its shutdown drain is `try_consume`-bounded.
    shutdown_flag.store(true, Ordering::Release);
    // Join EVERY thread before reporting the outcome, even on failure:
    // the in-process divergence-resync path archives the journal and
    // snapshot right after this returns, and a still-running shadow
    // thread could be mid-snapshot-write during those renames.
    let journal_result = journal_handle.join();
    let matching_result = matching_handle.join();
    let _ = drain_handle.join();
    if let Some(h) = shadow_handle {
        let _ = h.join();
    }
    let writer = match journal_result {
        Ok(Ok(w)) => w,
        // Already error!-logged by the journal thread's spawn wrapper.
        Ok(Err(e)) => return TeardownOutcome::JournalFailed(e),
        Err(_) => return TeardownOutcome::Panicked,
    };
    match matching_result {
        Ok(app) => TeardownOutcome::Clean(app, writer),
        Err(_) => TeardownOutcome::Panicked,
    }
}

/// Live replica pipeline — built once on first connect (or after a snapshot
/// transfer), persists across `Disconnected` reconnects so the orchestrator
/// doesn't pay the journal-recover + thread-spawn cost on every drop.
///
/// Shared between the kernel-TCP and DPDK receivers; both build pipelines
/// with the same shape (journal / matching / drain / optional shadow), and
/// both take the next handshake from [`Self::settled_resume_point`] at
/// reconnect time.
pub(super) struct ReplicaPipelineHandles<A: Application, W: Send + 'static> {
    pub(super) input_producer: melin_pipeline::ring::Producer<InputSlot<A::Event>>,
    pub(super) journal_cursor: Arc<melin_pipeline::padding::Sequence>,
    /// Seqlock-published fsync state (journal seq + chain hash + ring
    /// position), read half: the reconnect handshake's resume point, read
    /// without owning the writer. Seeded with the journal the pipeline
    /// was built over, so it is never behind what is on disk.
    pub(super) chain_hash_lock:
        melin_pipeline::seqlock::SeqLockReader<melin_transport_core::pipeline::FsyncState>,
    /// Primary-announced rotation hand-off: the receiver thread pushes
    /// `Rotate` boundaries here (in stream order), the journal stage
    /// pops and rotates at exactly those sequences. Replicas have no
    /// local rotation triggers.
    pub(super) stream_marks: melin_transport_core::pipeline::StreamMarkQueue,
    /// The lineage's genesis length (`None`: unknown, a pre-v16 header),
    /// read from the writer's header when the pipeline was built. Fixed
    /// for the pipeline's life: rotation carries it forward unchanged and
    /// anything that changes the lineage (a resync) tears the pipeline
    /// down first. Kept here so the reconnect handshake can compare it
    /// against the primary's without re-reading the live segment by path,
    /// which the journal's disk thread may be renaming mid-rotation.
    pub(super) genesis_entries: Option<u64>,
    /// Latched by the journal thread's spawn wrapper when the stage
    /// exits with an error (chain divergence, journal I/O failure, a
    /// refused sequence) or panics (see `run_journal_stage`).
    /// The streaming receiver checks it and tears the session down —
    /// without this, a dead journal stage freezes the journal cursor
    /// and the receiver wedges forever on ring backpressure or the
    /// ack-durability wait.
    pub(super) journal_failed: Arc<AtomicBool>,
    /// Per-pipeline shutdown flag — flipped only on a controlled teardown
    /// (Promote/Shutdown/Fatal/Snapshot). NOT flipped on `Disconnected`.
    pub(super) pipeline_shutdown: Arc<AtomicBool>,
    pub(super) journal_handle: std::thread::JoinHandle<Result<W, melin_journal::JournalError>>,
    pub(super) matching_handle: std::thread::JoinHandle<A>,
    pub(super) drain_handle: std::thread::JoinHandle<()>,
    pub(super) shadow_handle: Option<std::thread::JoinHandle<()>>,
}

/// What [`ReplicaPipelineHandles::settled_resume_point`] found.
pub(super) enum ResumePoint {
    /// The fsync state, covering every slot published into the input
    /// ring: the next handshake's pair.
    Settled(melin_transport_core::pipeline::FsyncState),
    /// The journal stage has died. The pipeline cannot take another
    /// session; the caller tears it down as a fatal session exit would
    /// (see [`journal_failed_while_disconnected`]) instead of dialing.
    JournalFailed,
    /// Shutdown or promotion was requested; the caller handles both
    /// before any handshake.
    Interrupted,
}

impl<A: Application, W: Send + 'static> ReplicaPipelineHandles<A, W> {
    /// The next handshake's resume point: the fsync state, once it covers
    /// every slot published into the input ring.
    ///
    /// A session publishes ahead of durability, so when it ends the ring
    /// can still hold entries the journal has not made durable. A
    /// handshake at the durable position taken before they land has the
    /// primary resend them, and the next session's contiguity gate, which
    /// starts at that position, publishes them a second time: the matching
    /// stage applies them twice, and the journal stage refuses them,
    /// stopping the replica. Once the fsync state covers the ring, its
    /// position is exactly what the replica holds.
    ///
    /// Stops waiting when the journal stage has failed (nothing more will
    /// be journaled) or `interrupted` returns true.
    pub(super) fn settled_resume_point(&self, interrupted: impl Fn() -> bool) -> ResumePoint {
        // Only a streaming session publishes into the ring, and the
        // previous one has ended (kernel TCP joined its thread; DPDK ran
        // it on this one), so the cursor is final for this wait.
        let published = self.input_producer.peek_cursor();
        let mut outcome = ResumePoint::Interrupted;
        // Yielding, whatever the ingress strategy: this is the reconnect
        // path, and what it waits out is a disk stall.
        melin_pipeline::wait::WaitStrategy::SpinThenYield.wait_until(|| {
            // First, even when the ring is covered: a stage that died
            // after its last durable batch would otherwise hand out a
            // ready pair, and the loop would dial the primary and take
            // one of its replica slots only for the session to fail on
            // its first iteration.
            if self.journal_failed.load(Ordering::Acquire) {
                outcome = ResumePoint::JournalFailed;
                return true;
            }
            let state = self.chain_hash_lock.load();
            if state.input_ring_seq.get() >= published {
                outcome = ResumePoint::Settled(state);
                return true;
            }
            interrupted()
        });
        outcome
    }
}

/// The session exit a reconnect loop dispatches when
/// [`ReplicaPipelineHandles::settled_resume_point`] finds the journal
/// stage dead: the same fatal teardown a streaming session takes when it
/// sees the latch. A chain divergence is repaired in-process; anything
/// else stops the replica. Nothing was heard from a primary, so the
/// backoff stays escalated.
pub(in crate::replication) fn journal_failed_while_disconnected() -> StreamingResult {
    StreamingResult {
        exit: SessionExit::Fatal("replica journal stage failed while disconnected".into()),
        heard_from_primary: false,
    }
}

/// Run the replica's journal stage on its thread, latching `failed` (and
/// clearing `healthy`) if it stops for any reason other than a clean
/// teardown: an error, or a panic.
///
/// Every wait on the journal's progress — the streaming loop's
/// backpressure and ack waits, the reconnect's resume-point wait, the
/// teardown's sentinel publish — gives up on that latch. A panic never
/// returns an error, so without the guard below it would leave them all
/// spinning on a cursor that will never move. (A release build aborts on
/// panic; this matters wherever the profile unwinds.)
fn run_journal_stage<W, E: std::fmt::Display>(
    run: impl FnOnce() -> Result<W, E>,
    failed: &AtomicBool,
    healthy: &AtomicBool,
) -> Result<W, E> {
    /// Latches on drop unless defused: dropped during unwinding, it
    /// reports the panic the result never will.
    struct PanicLatch<'a> {
        failed: &'a AtomicBool,
        healthy: &'a AtomicBool,
        armed: bool,
    }
    impl Drop for PanicLatch<'_> {
        fn drop(&mut self) {
            if self.armed {
                self.failed.store(true, Ordering::Release);
                self.healthy.store(false, Ordering::Release);
                tracing::error!("replica journal stage panicked — session teardown");
            }
        }
    }
    let mut guard = PanicLatch {
        failed,
        healthy,
        armed: true,
    };
    let result = run();
    guard.armed = false;
    if let Err(ref e) = result {
        // Latch before logging so the receiver reacts even if logging
        // stalls. A dead journal stage freezes the journal cursor; every
        // downstream wait on it (ring backpressure, ack durability) would
        // spin forever — the streaming loop polls this latch and tears
        // the session down instead.
        failed.store(true, Ordering::Release);
        // Mirror into the process-lifetime gauge the replica health
        // endpoint serves.
        healthy.store(false, Ordering::Release);
        tracing::error!(error = %e, "replica journal stage failed — session teardown");
    }
    result
}

/// [`ReplicaPipelineHandles`] over the journal's writer — the only
/// instantiation that exists. The struct keeps its writer parameter
/// because the teardown helpers are writer-agnostic plumbing; this
/// alias spares every signature from spelling the concrete type out.
pub(super) type ReplicaHandles<A> =
    ReplicaPipelineHandles<A, BufferedWriter<<A as Application>::Event>>;

/// Build the replica pipeline and spawn its stage threads on the configured
/// cores. Returns the bundle of state the orchestrator keeps across
/// `Disconnected` reconnects.
#[allow(clippy::too_many_arguments)]
pub(super) fn build_replica_pipeline_with_threads<A>(
    mut app: A,
    writer: BufferedWriter<A::Event>,
    cores: crate::layout::PipelineCores,
    // How the replica's segment preparer materialises staged extents.
    // A replica's rotation stall sits on the ack path — under
    // `disk+ram`/`two-disks` it delays the primary's ack gate — so this
    // is not a primary-only tuning knob.
    staging_mode: melin_journal::StagingMode,
    snapshot_interval_ms: u64,
    snapshot_path: std::path::PathBuf,
    group_commit_delay: std::time::Duration,
    fence_state: Arc<melin_transport_core::fence::FenceState>,
    // The lineage's ack policy as recovered (see
    // `ReplicaControlPlane::lineage_ack_policy`), seeding the shadow
    // stage's next to the fence epoch.
    lineage_policy: &SharedLineagePolicy,
    // Process-lifetime health mirror for the replica health endpoint —
    // see `ReplicaControlPlane::pipeline_healthy`. Reset `true` here (a
    // fresh pipeline is healthy) and latched `false` by the journal
    // thread's failure wrapper, alongside the per-pipeline
    // `journal_failed` latch.
    pipeline_healthy: Arc<AtomicBool>,
    // The node's sizing, applied to every instance a pipeline is built
    // around — see `Application::prefault`.
    sizing: &A::Sizing,
) -> Result<ReplicaHandles<A>, Box<dyn std::error::Error>>
where
    A: Application + Send + 'static,
    A::Event: Send + Sync + 'static,
    A::Report: Send + 'static,
    A::QueryResponse: Send + 'static,
{
    // Before the shadow copy is taken, so that neither the stream's
    // first events nor the copy's first snapshot grow the collections on
    // the matching thread. A replica's apply sits on the primary's ack
    // path under `disk+ram`, so its page faults are the primary's tail.
    <A as Application>::prefault(&mut app, sizing);
    let shadow_app = <A as Application>::clone_via_snapshot(&app)?;

    let enable_shadow = snapshot_interval_ms > 0;
    // Shadow snapshot seeds its lineage from the fence state's epoch and
    // the lineage policy cell (both set from the replica's recovered
    // journal, or the snapshot a resync installed, before this builder
    // runs).
    let shadow_initial_lineage = melin_transport_core::lineage::Lineage {
        epoch: fence_state.epoch(),
        ack_policy: lineage_policy.load(),
    };
    // The lineage's genesis length, stamped into the replica's own
    // snapshots — the same value its journal header holds.
    let genesis_entries = writer.read_header_info()?.genesis_entries;
    // The first sequence the stream will bring, which the shadow counts
    // from (see `shadow::run`).
    let starting_wire_seq = writer.next_sequence();
    let pipeline = melin_transport_core::pipeline::build_replica_pipeline(
        app,
        writer,
        4096, // max_journal_batch
        group_commit_delay,
        cores.stage_waits(),
        enable_shadow,
        fence_state,
    );

    let pipeline_shutdown = Arc::new(AtomicBool::new(false));

    let ps = Arc::clone(&pipeline_shutdown);
    let journal_core = cores.journal_seq.core;
    let mut journal_stage = pipeline.journal_stage;
    // Replicas never rotate on local triggers (size or operator
    // command) — they adopt the boundaries the primary announces over
    // the stream, which keeps segment boundaries (and with them chain
    // values and journal bytes) identical across nodes.
    let stream_marks: melin_transport_core::pipeline::StreamMarkQueue =
        Arc::new(std::sync::Mutex::new(std::collections::VecDeque::new()));
    journal_stage.set_stream_marks(Arc::clone(&stream_marks));
    cores.place_journal_children(&mut journal_stage);
    journal_stage.set_staging_mode(staging_mode);
    let journal_failed = Arc::new(AtomicBool::new(false));
    let journal_failed_latch = Arc::clone(&journal_failed);
    // Start the journal's disk and preparer threads here rather than on
    // journal-seq — see `JournalStage::start`. Before the health latch
    // below, so a failed start never reports a healthy pipeline.
    let sequencer = journal_stage.start()?;
    // A fresh pipeline is healthy — this also clears the latch after a
    // successful in-process resync rebuild.
    pipeline_healthy.store(true, Ordering::Release);
    // Every stage thread is created already in its own placement's
    // context, and this thread is put back where it was after each. Left
    // to inherit it, a stage thread starts wherever this thread is — after
    // `start` above, possibly on the disk thread's isolated core, where
    // that thread spins at real-time priority and the stage never runs.
    //
    // A stage thread that cannot be created, or does not start, fails the
    // build. The stages already running are told to stop first, so a
    // failed build leaves nothing spinning behind it.
    let abandon = |e: String| -> Box<dyn std::error::Error> {
        pipeline_shutdown.store(true, Ordering::Release);
        e.into()
    };
    let journal_handle =
        melin_app::affinity::spawn_placed("journal-seq", journal_core, move || {
            run_journal_stage(
                || sequencer.run(&ps),
                &journal_failed_latch,
                &pipeline_healthy,
            )
        })
        .map_err(|e| abandon(format!("start the replica's journal-seq thread: {e}")))?;

    let ps = Arc::clone(&pipeline_shutdown);
    let matching_core = cores.matching.core;
    let matching_stage = pipeline.matching_stage;
    let matching_handle = melin_app::affinity::spawn_placed("matching", matching_core, move || {
        matching_stage.run(&ps)
    })
    .map_err(|e| abandon(format!("start the replica's matching thread: {e}")))?;

    // Drain thread uses the response core — replicas have no response stage,
    // but the consumer needs to be drained so the output ring doesn't fill.
    let ps = Arc::clone(&pipeline_shutdown);
    let drain = cores.response;
    let drain_consumer = pipeline.drain_consumer;
    let drain_handle = melin_app::affinity::spawn_placed("drain", drain.core, move || {
        let mut consumer = drain_consumer;
        let mut batch = vec![OutputSlot::<A::Report, A::QueryResponse>::default(); 256];
        let mut waiter = drain.wait.waiter();
        loop {
            if ps.load(Ordering::Relaxed) {
                return;
            }
            let count = consumer.consume_batch(&mut batch, 256);
            if count == 0 {
                waiter.idle();
            } else {
                waiter.reset();
            }
        }
    })
    .map_err(|e| abandon(format!("start the replica's drain thread: {e}")))?;

    let shadow_handle = if let Some(shadow_cons) = pipeline.shadow_consumer {
        let snap_path = snapshot_path;
        let chain_lock = pipeline.chain_hash_lock.clone();
        let ps = Arc::clone(&pipeline_shutdown);
        let shadow = cores.shadow;
        Some(
            melin_app::affinity::spawn_placed("replica-shadow", shadow.core, move || {
                melin_transport_core::shadow::run(
                    shadow_cons,
                    shadow_app,
                    snap_path,
                    std::time::Duration::from_millis(snapshot_interval_ms),
                    chain_lock,
                    &ps,
                    shadow.wait,
                    shadow_initial_lineage,
                    starting_wire_seq,
                    genesis_entries,
                );
            })
            .map_err(|e| abandon(format!("start the replica's shadow thread: {e}")))?,
        )
    } else {
        None
    };

    Ok(ReplicaPipelineHandles {
        input_producer: pipeline.input_producer,
        journal_cursor: pipeline.cursors.journal_ring_arc(),
        chain_hash_lock: pipeline.chain_hash_lock,
        stream_marks,
        genesis_entries,
        journal_failed,
        pipeline_shutdown,
        journal_handle,
        matching_handle,
        drain_handle,
        shadow_handle,
    })
}

/// Tear down the pipeline: publish the shutdown sentinel, join all
/// threads, return the recovered (App, journal writer) so the orchestrator
/// can use them for the next pipeline build (e.g., post-snapshot) or
/// pass them up on promotion.
pub(super) fn teardown_replica_pipeline<A: Application + Send + 'static, W: Send + 'static>(
    mut handles: ReplicaPipelineHandles<A, W>,
) -> TeardownOutcome<A, W> {
    // The sentinel is published here, not by callers, so no teardown
    // path can forget it — the journal and matching stages exit by
    // consuming it through the normal event path, draining anything
    // received-but-not-yet-journaled (see `shutdown_pipeline` for why
    // the flag alone is only the emergency fallback). Bounded retry
    // rather than the blocking `publish`: if the journal stage dies
    // with the ring full, its gate cursor freezes and a blocking
    // publish would spin forever — re-check the failure latch between
    // attempts and fall through to the flag-only teardown once it
    // trips (the sentinel has no reader then anyway: the matching
    // stage is gated behind the frozen journal cursor).
    //
    // Deliberately a plain yield rather than the node's wait strategy:
    // this is the orchestrator thread on a teardown path, where
    // latency is irrelevant and handing the CPU to the stage threads
    // is unconditionally the right move, whatever the node's policy.
    while !handles.journal_failed.load(Ordering::Acquire) {
        match handles
            .input_producer
            .try_publish(InputSlot::<A::Event>::shutdown_sentinel())
        {
            Ok(_) => break,
            // Ring full — consumers need to make progress first.
            Err(_) => std::thread::yield_now(),
        }
    }
    shutdown_pipeline::<A, W>(
        &handles.pipeline_shutdown,
        handles.journal_handle,
        handles.matching_handle,
        handles.drain_handle,
        handles.shadow_handle,
    )
}

/// How many mid-stream divergence resyncs the receiver attempts
/// in-process (per process lifetime) before giving up. Mid-stream
/// divergence is never expected in a healthy cluster — it means
/// corruption or a serious bug somewhere — so the budget is exactly
/// one: the first occurrence repairs automatically (archive the local
/// lineage as `.divergent.<n>`, re-seed from the primary) and pages
/// the operator via `melin_replica_divergence_total`; a second in the
/// same process lifetime is systematic, and continuing to repair
/// would fill the disk with archives while masking the underlying
/// fault. Exit hard instead.
pub(super) const MAX_INPROCESS_DIVERGENCE_RESYNCS: u32 = 1;

/// Recover replica boot state from disk.
///
/// Recovers whenever any journal segment survives — live OR archived
/// (a crash between rotation's rename and the new live file's creation
/// leaves archives with no live segment, and recovery handles that
/// layout; treating it as a fresh replica would discard local durable
/// history and then fail `create_new` against the surviving lineage).
/// Returns `(None, None, 0, zeros)` for a genuinely fresh replica.
/// Also seeds the fencing epoch from the recovered journal.
///
/// Called at receiver startup, and again after a mid-stream chain
/// divergence tears the pipeline down: the on-disk journal is
/// self-consistent (merely forked from the primary's history), so
/// recovery re-derives a truthful handshake pair `(last_sequence,
/// chain_hash)` and the next connection takes the primary's
/// HashMismatch → archive → reseed path in-process.
#[allow(clippy::type_complexity)]
pub(super) fn recover_replica_state<A, W>(
    journal_path: &std::path::Path,
    snapshot_path: &std::path::Path,
    fence_state: &melin_transport_core::fence::FenceState,
    lineage_policy: &SharedLineagePolicy,
    // Applied to a genesis instance before the journal is replayed into
    // it — see `init_engine`. The pipeline build sizes the result again.
    sizing: &A::Sizing,
) -> Result<(Option<A>, Option<W>, u64, [u8; 32]), Box<dyn std::error::Error>>
where
    A: Application,
    W: melin_journal::JournalWrite<A::Event>,
{
    // Swept every boot, like the genesis staging file: only a later adopt
    // that meets its narrow guard would otherwise remove it.
    remove_restamp_staging(journal_path)?;
    let lineage_exists =
        journal_path.exists() || !melin_journal::segment::list_archives(journal_path)?.is_empty();
    if !lineage_exists {
        // Nothing held, so no policy either (the epoch stays: fencing
        // never moves it back).
        lineage_policy.store(None);
        return Ok((None, None, 0u64, [0u8; 32]));
    }
    let engine = if snapshot_path.exists() {
        tracing::info!("recovering replica from snapshot + journal");
        melin_transport_core::JournaledApp::<A, W>::recover_from_snapshot(
            snapshot_path,
            journal_path,
        )?
    } else {
        let mut app = A::default();
        <A as Application>::prefault(&mut app, sizing);
        melin_transport_core::JournaledApp::<A, W>::recover(app, journal_path)?
    };
    let next = engine.next_sequence();
    let last = next.saturating_sub(1);
    let hash = engine.chain_hash().unwrap_or([0u8; 32]);
    // Seed the lineage from the replica's own recovered journal.
    // Streaming `EpochBump`s and the snapshot-resync path raise the epoch
    // later.
    adopt_recovered_lineage(fence_state, lineage_policy, engine.recovered_lineage());
    let (app, writer) = engine.into_parts();
    Ok((Some(app), Some(writer), last, hash))
}

/// Adopt a lineage recovered from disk (a journal replay, or a snapshot
/// the primary sent) as this replica's. The epoch is observed, a maximum,
/// since fencing never moves it back; the ack policy is stored whole,
/// since the recovered lineage replaces whatever this node held, and a
/// change in a tail it dropped must go with the tail.
fn adopt_recovered_lineage(
    fence_state: &melin_transport_core::fence::FenceState,
    lineage_policy: &SharedLineagePolicy,
    lineage: melin_transport_core::lineage::Lineage,
) {
    fence_state.observe_epoch(lineage.epoch);
    lineage_policy.store(lineage.ack_policy);
}

/// Reconnect backoff cap shared by both receivers — exponential from
/// 1 s, clamped here so a long outage settles to one attempt every 30 s
/// without hammering a flapping primary. Reset to 1 s only when a
/// session heard the primary speak (data or heartbeat) — see
/// [`handle_session_exit`].
pub(super) const MAX_BACKOFF: std::time::Duration = std::time::Duration::from_secs(30);

/// What the receiver's reconnect loop does after a streaming session
/// ends — the transport-independent half of the exit dispatch, returned
/// by [`handle_session_exit`].
pub(in crate::replication) enum AfterSession<A, W> {
    /// Terminal: `run_receiver*` returns this verbatim — clean shutdown
    /// (`Ok(None)`), a promotion hand-off (`Ok(Some(state))`), or a
    /// fatal error.
    Return(ReceiverResult<A, W>),
    /// A mid-stream divergence was repaired in-process: the recovered
    /// on-disk state is the new handshake position. The caller adopts it
    /// and reconnects; the primary's `HashMismatch` verdict then routes
    /// the replica through archive + re-seed.
    Resync {
        app: Option<A>,
        journal_writer: Option<W>,
        last_sequence: u64,
        chain_hash: [u8; 32],
    },
    /// A plain disconnect or a stream contiguity break — backoff has
    /// already been applied (and the flags checked); the caller
    /// reconnects, reusing the still-live pipeline.
    Reconnect,
}

impl<A, W> AfterSession<A, W> {
    /// Fold this outcome into the reconnect loop's state: the result
    /// `run_receiver*` must return, or `None` to go round the loop again
    /// (having adopted a resync's recovered state). One copy for every
    /// dispatch site in both receivers, so they cannot drift apart.
    pub(in crate::replication) fn adopt(
        self,
        app: &mut Option<A>,
        journal_writer: &mut Option<W>,
        last_sequence: &mut u64,
        chain_hash: &mut [u8; 32],
    ) -> Option<ReceiverResult<A, W>> {
        match self {
            AfterSession::Return(r) => Some(r),
            AfterSession::Resync {
                app: recovered_app,
                journal_writer: recovered_writer,
                last_sequence: recovered_sequence,
                chain_hash: recovered_hash,
            } => {
                *app = recovered_app;
                *journal_writer = recovered_writer;
                *last_sequence = recovered_sequence;
                *chain_hash = recovered_hash;
                None
            }
            AfterSession::Reconnect => None,
        }
    }
}

/// Dispatch a finished streaming session — shared by the kernel-TCP and
/// DPDK receivers.
///
/// Folds the behaviours that were previously copied between the two
/// receiver loops (and had drifted — the copies reset the post-resync
/// writer differently, a latent corruption bug): the
/// `Shutdown`/`Promote`/`Fatal` teardown (including the once-per-process
/// in-process divergence-resync policy) and the `Disconnected` reconnect
/// backoff. The shutdown-sentinel publish lives in
/// [`teardown_replica_pipeline`].
///
/// `close` ends the session's connection, transport-specifically. It is
/// invoked exactly once, first, on every exit, terminal or not: the
/// session is over whatever follows, and the primary must learn so now
/// rather than after the pipeline teardown or the backoff. The DPDK
/// receiver resets its smoltcp socket, which tells the primary (a DPDK
/// node is its own TCP stack, so nothing else would once it stops) and
/// frees the local socket-set entry; the kernel-TCP receiver passes a
/// no-op (its `TcpStream` is dropped by the caller, and the kernel tells
/// the primary).
// A long argument list, but each is a distinct piece of the receiver
// loop's state, borrowed from its locals; bundling them would only move
// the noise.
#[allow(clippy::too_many_arguments)]
pub(in crate::replication) fn handle_session_exit<A, W>(
    result: StreamingResult,
    pipeline: &mut Option<ReplicaPipelineHandles<A, W>>,
    divergence_resyncs: &mut u32,
    backoff: &mut std::time::Duration,
    last_sequence: u64,
    journal_path: &std::path::Path,
    snapshot_path: &std::path::Path,
    fence_state: &melin_transport_core::fence::FenceState,
    lineage_policy: &SharedLineagePolicy,
    shutdown: &AtomicBool,
    promote: &crate::promotion::PromotionRequest,
    close: impl FnOnce(),
    // For the resync path, which recovers the local journal afresh — see
    // `recover_replica_state`.
    sizing: &A::Sizing,
) -> AfterSession<A, W>
where
    A: Application + Send + 'static,
    W: JournalWrite<A::Event> + Send + 'static,
{
    let StreamingResult {
        exit,
        heard_from_primary,
    } = result;

    close();

    match exit {
        SessionExit::Shutdown => {
            if let Some(p) = pipeline.take() {
                let _ = teardown_replica_pipeline::<A, W>(p);
            }
            AfterSession::Return(Ok(None))
        }

        SessionExit::Promote => AfterSession::Return(match pipeline.take() {
            Some(p) => match teardown_replica_pipeline::<A, W>(p) {
                TeardownOutcome::Clean(ex, wr) => Ok(Some((ex, wr))),
                _ => Err("pipeline failed during promotion".into()),
            },
            None => Err("pipeline missing on promote".into()),
        }),

        SessionExit::Fatal(e) => {
            let outcome = match pipeline.take() {
                Some(p) => teardown_replica_pipeline::<A, W>(p),
                // Fatal implies a streaming session, which implies a
                // pipeline — but don't turn a missing one into a resync.
                None => return AfterSession::Return(Err(e)),
            };
            // Mid-stream chain divergence is repairable in-process: the
            // on-disk journal is self-consistent (merely forked from the
            // primary's history), so re-derive the handshake state from
            // disk and reconnect — the primary judges the recovered
            // position divergent and the next session takes the
            // HashMismatch → archive → reseed path, no restart needed.
            // Every other fatal exits as before: protocol violations and
            // journal I/O death (ENOSPC, RO-FS) would fail the same way
            // after a resync. The journal's own error names the cause; the
            // exit's message alone may not (the reconnect wait's does not).
            let je = match outcome {
                TeardownOutcome::JournalFailed(
                    je @ melin_journal::JournalError::ReplicaChainDivergence { .. },
                ) => je,
                TeardownOutcome::JournalFailed(je) => {
                    return AfterSession::Return(Err(format!("{e}: {je}").into()));
                }
                TeardownOutcome::Clean(..) | TeardownOutcome::Panicked => {
                    return AfterSession::Return(Err(e));
                }
            };

            *divergence_resyncs += 1;
            let attempt = *divergence_resyncs;
            if attempt > MAX_INPROCESS_DIVERGENCE_RESYNCS {
                return AfterSession::Return(Err(format!(
                    "mid-stream chain divergence recurred {attempt} times — giving up on \
                     in-process resync (each cycle archives the local journal and re-seeds \
                     from the primary; recurrence at this rate means the primary keeps \
                     streaming history that forks from what it announces): {je}"
                )
                .into()));
            }
            tracing::warn!(
                error = %je,
                attempt,
                max_attempts = MAX_INPROCESS_DIVERGENCE_RESYNCS,
                "mid-stream chain divergence — re-deriving local state for in-process resync"
            );
            match recover_replica_state::<A, W>(
                journal_path,
                snapshot_path,
                fence_state,
                lineage_policy,
                sizing,
            ) {
                Ok((app, journal_writer, seq, hash)) => AfterSession::Resync {
                    app,
                    journal_writer,
                    last_sequence: seq,
                    chain_hash: hash,
                },
                Err(e) => AfterSession::Return(Err(e)),
            }
        }

        SessionExit::StreamGap(e) => {
            // The primary's stream skipped past what we hold; the
            // contiguous prefix is committed and the journal stage keeps
            // flushing it, so re-handshaking at the durable position
            // (re-read from `FsyncState` at the top of the reconnect
            // loop, after the backoff) lets the primary re-stream the
            // hole. The primary evidently spoke, so the backoff resets
            // exactly as a heard-from disconnect does. Not a resync:
            // nothing on disk is wrong.
            *backoff = std::time::Duration::from_secs(1);
            tracing::warn!(
                error = %e,
                last_sequence,
                "replication stream broke contiguity — reconnecting from the durable position"
            );
            sleep_then_double_backoff(backoff, shutdown, promote);
            AfterSession::Reconnect
        }

        SessionExit::Corrupted(e) => {
            // A replicated entry arrived damaged and was refused before
            // anything could apply, journal or acknowledge it; the frames
            // before it are committed. The primary still holds the intact
            // entries, so re-handshaking at the durable position has them
            // sent again. Not a resync: nothing local is wrong.
            //
            // The backoff resets, exactly as for a stream gap: frames
            // arrived, so the primary evidently spoke, and nothing else
            // resets the backoff after a long clean session — leaving it
            // to escalate would make one isolated bit flip, days after a
            // start-up that escalated it, cost a `MAX_BACKOFF` stall
            // (writes blocked under a replica-requiring ack policy). A
            // link that keeps damaging frames is redialled at the floor
            // rate, each time logged below; every redial re-streams
            // useful data, so that costs nothing the primary cannot bear.
            // `warn!`, not `error!`: the server is working as designed —
            // what needs attention is the path between the nodes.
            *backoff = std::time::Duration::from_secs(1);
            tracing::warn!(
                error = %e,
                last_sequence,
                backoff_secs = backoff.as_secs(),
                "replicated entry failed its integrity check — refused, reconnecting to \
                 fetch it again; recurrence points at the network, a NIC or memory"
            );
            sleep_then_double_backoff(backoff, shutdown, promote);
            AfterSession::Reconnect
        }

        SessionExit::Disconnected => {
            // A session in which the primary spoke — data or heartbeat
            // (heartbeats flow even on a quiet system) — proves it
            // alive and serving: treat the drop as transient and reset
            // the backoff. A session with no word from the primary
            // keeps escalating: that covers instant-drop flaps and
            // synthetic results from sessions that never started
            // streaming (a persistent local transport failure must not
            // redial at 1 Hz forever).
            if heard_from_primary {
                *backoff = std::time::Duration::from_secs(1);
            }
            tracing::warn!(
                last_sequence,
                heard_from_primary,
                backoff_secs = backoff.as_secs(),
                "reconnecting to primary"
            );
            sleep_then_double_backoff(backoff, shutdown, promote);
            AfterSession::Reconnect
        }
    }
}

/// Tear the live pipeline down for a promotion that fired while the
/// receiver was disconnected — at the top of the reconnect loop or
/// during reconnect backoff — and return the warm application + writer for
/// the promoted primary. Shared by both receivers.
///
/// A clean teardown hands back the warm state; if there is no pipeline
/// (promotion before the first connect, or after a resync that left the
/// state in the receiver's locals) those locals carry it. A missing
/// pair is a hard error — a promote with nothing to promote.
pub(in crate::replication) fn take_pipeline_for_promotion<A, W>(
    pipeline: &mut Option<ReplicaPipelineHandles<A, W>>,
    app: &mut Option<A>,
    journal_writer: &mut Option<W>,
) -> ReceiverResult<A, W>
where
    A: Application + Send + 'static,
    W: JournalWrite<A::Event> + Send + 'static,
{
    if let Some(p) = pipeline.take()
        && let TeardownOutcome::Clean(e, w) = teardown_replica_pipeline::<A, W>(p)
    {
        *app = Some(e);
        *journal_writer = Some(w);
    }
    match (app.take(), journal_writer.take()) {
        (Some(e), Some(w)) => Ok(Some((e, w))),
        _ => Err("promotion requested but no local state available".into()),
    }
}

/// The facts a successful snapshot + segment-seed transfer yields.
struct ResyncTransfer<A> {
    /// The application loaded from the transferred snapshot.
    app: A,
    /// The snapshot's sequence: where the seeded segment is opened.
    snap_sequence: u64,
    /// The snapshot's chain hash, as the primary announced and the loaded
    /// snapshot confirmed.
    snap_chain_hash: [u8; 32],
    /// The seed's length: the seeded journal's `valid_end`.
    seed_len: u64,
    /// The snapshot's lineage. Adopted by the caller once the whole
    /// transfer has succeeded, so a failed one leaves none behind.
    lineage: melin_transport_core::lineage::Lineage,
}

/// What [`handle_resync_verdict`] resolved a `NeedSnapshot` /
/// `HashMismatch` verdict to.
pub(in crate::replication) enum ResyncDecision {
    /// Resync complete — resume streaming from `resume_sequence` on the
    /// re-seeded lineage `(segment_start_sequence, anchor_hash)`. The
    /// recovered App + writer are left in the receiver's `app` /
    /// `journal_writer` locals.
    Ready {
        segment_start_sequence: u64,
        anchor_hash: [u8; 32],
        /// The lineage's genesis length, as the seeded segment's header
        /// (and the `StreamStart` that matched it) records it.
        genesis_entries: Option<u64>,
        resume_sequence: u64,
    },
    /// The transfer failed network-shaped (drop / restart mid-body); the
    /// caller backs off and reconnects. The pre-resync lineage is already
    /// archived and half-applied transfer state has been cleaned up.
    Retry,
}

/// Receive the snapshot + segment-seed transfer that follows a resync
/// verdict, installing both (snapshot → `snapshot_path`, seed →
/// `journal_path`), and returns what they yield ([`ResyncTransfer`]).
/// Shared by both receivers via [`ControlFrameSource`].
///
/// Errors here are network-shaped — the caller retries (see
/// [`ResyncDecision::Retry`]); the post-transfer install in
/// [`handle_resync_verdict`] is what's fatal.
fn receive_resync_transfer<A, S>(
    source: &mut S,
    snapshot_path: &std::path::Path,
    journal_path: &std::path::Path,
) -> Result<ResyncTransfer<A>, Box<dyn std::error::Error + Send + Sync>>
where
    A: Application + Send,
    S: ControlFrameSource,
{
    let (snap_len, snap_sequence, snap_chain_hash) =
        match decode_primary_message(&source.next_frame(MAX_CONTROL_FRAME)?)? {
            PrimaryMessage::SnapshotBegin {
                snapshot_len,
                snap_sequence,
                snap_chain_hash,
            } => (snapshot_len, snap_sequence, snap_chain_hash),
            other => return Err(format!("expected SnapshotBegin, got {other:?}").into()),
        };

    tracing::info!(snap_sequence, snap_len, "receiving snapshot");
    let tmp_path = snapshot_path.with_extension("snapshot.tmp");
    receive_chunked_body(source, &tmp_path, snap_len, "snapshot")?;
    std::fs::rename(&tmp_path, snapshot_path)?;
    tracing::info!(snap_sequence, snap_len, "snapshot received and verified");

    // The primary is already sending the seed: a large state must not
    // leave the link unanswered while it loads (see `serviced`).
    let (snap_app, snap_header) =
        source.serviced(|| melin_transport_core::snapshot::load::<A>(snapshot_path))?;
    let snap_hash = snap_header.chain_hash;
    if snap_hash != snap_chain_hash {
        return Err(format!(
            "snapshot chain hash mismatch: primary sent {snap_chain_hash:02x?}, \
             loaded snapshot has {snap_hash:02x?}"
        )
        .into());
    }

    // Segment seed: the raw byte prefix of the primary's segment
    // containing `snap_sequence`. Written verbatim as our live segment, it
    // makes our segmentation a byte-copy of the primary's from birth —
    // chain values comparable and `Rotate` verification valid immediately.
    let seed_len = match decode_primary_message(&source.next_frame(MAX_CONTROL_FRAME)?)? {
        PrimaryMessage::SegmentSeedBegin { seed_len } => seed_len,
        other => {
            return Err(format!("expected SegmentSeedBegin after snapshot, got {other:?}").into());
        }
    };
    let seed_tmp = journal_path.with_extension("seed.tmp");
    receive_chunked_body(source, &seed_tmp, seed_len, "segment seed")?;
    // Structural check before installing: the CRC only proves transport
    // integrity, not that the primary sent a well-formed prefix ending at
    // the snapshot sequence. (With hash-chain on, the chain cross-check in
    // `handle_resync_verdict` subsumes this; without it, this is the only
    // guard.) Reads the whole seed, so it runs serviced too.
    source.serviced(|| -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
        if let Err(e) =
            melin_journal::segment::verify_segment_prefix(&seed_tmp, snap_sequence, seed_len)
        {
            let _ = std::fs::remove_file(&seed_tmp);
            return Err(format!("segment seed failed structural verification: {e}").into());
        }
        std::fs::rename(&seed_tmp, journal_path)?;
        melin_journal::segment::fsync_parent_dir(journal_path)?;
        Ok(())
    })?;
    Ok(ResyncTransfer {
        app: snap_app,
        snap_sequence,
        snap_chain_hash,
        seed_len,
        lineage: snap_header.lineage,
    })
}

/// Handle a `NeedSnapshot` / `HashMismatch` resync verdict — shared by
/// both receivers. Tears the pipeline down, archives the local lineage
/// (never deleted), resets the handshake position, receives + installs
/// the snapshot and segment seed via `source`, opens the re-seeded
/// segment and ties its chain to the snapshot, then validates the
/// post-snapshot `StreamStart` inline before resuming.
///
/// On success the recovered App + writer are left in `app` /
/// `journal_writer` and [`ResyncDecision::Ready`] carries the resume
/// lineage. A network-shaped transfer failure yields
/// [`ResyncDecision::Retry`] (the caller backs off and reconnects). An
/// inconsistent primary or a local install failure is fatal (`Err`).
#[allow(clippy::too_many_arguments)]
pub(in crate::replication) fn handle_resync_verdict<A, W, S>(
    divergent: bool,
    source: &mut S,
    pipeline: &mut Option<ReplicaPipelineHandles<A, W>>,
    app: &mut Option<A>,
    journal_writer: &mut Option<W>,
    journal_path: &std::path::Path,
    snapshot_path: &std::path::Path,
    fence_state: &melin_transport_core::fence::FenceState,
    control: &ReplicaControlPlane,
    last_sequence: &mut u64,
    chain_hash: &mut [u8; 32],
) -> Result<ResyncDecision, Box<dyn std::error::Error + Send + Sync>>
where
    A: Application + Send + 'static,
    // The pipeline's rings carry events, and its teardown runs serviced
    // on a helper thread.
    A::Event: Send,
    W: JournalWrite<A::Event> + Send + 'static,
    S: ControlFrameSource,
{
    // HashMismatch is NeedSnapshot plus the verdict that our local journal
    // holds divergent history (forked from the primary's — e.g. an
    // ex-primary rejoining after failover with an acked-but-unreplicated
    // suffix).
    if divergent {
        tracing::warn!(
            last_sequence = *last_sequence,
            "primary reports chain divergence — archiving local journal, resyncing from snapshot"
        );
    } else {
        tracing::info!("primary requires snapshot transfer — receiving snapshot");
    }

    // Invalidate the in-memory App + writer before moving their backing
    // files aside. On the in-process divergence repair path these still
    // hold the recovered handles; a transfer failure returns `Retry`, and
    // without this reset the stale writer — now pointing at an
    // archived-away journal — would survive the fresh-replica create gate
    // and get rebuilt into the next pipeline.
    let old_pipeline = pipeline.take();
    let old_app = app.take();
    let old_writer = journal_writer.take();

    // Move the local lineage aside — never delete. Divergent journals are
    // audit-trail material; stale ones may be the last copy of pruned
    // history.
    //
    // No `check_advertised_genesis` here: a resync adopts the primary's
    // lineage wholesale (its seed carries the primary's header) and keeps
    // ours intact in the archive, so no recorded genesis length is
    // overwritten — the hazard that check guards on the streaming path.
    // The genesis length is treated like the chain anchor, which a resync
    // also replaces.
    let reason = if divergent {
        ArchiveReason::Divergent
    } else {
        ArchiveReason::Resync
    };
    // The primary starts the transfer as soon as it has sent its verdict,
    // and the teardown waits on the journal: serviced (see `serviced`).
    source.serviced(move || {
        if let Some(p) = old_pipeline {
            // What the teardown hands back (the app and writer, or the
            // stage's failure) is discarded: the resync archives this
            // lineage and installs the primary's in its place.
            let _ = teardown_replica_pipeline::<A, W>(p);
        }
        // Dropped after the teardown and before the archive, as before.
        drop(old_app);
        drop(old_writer);
        archive_local_lineage(journal_path, snapshot_path, reason)
    })?;
    // Archived — a retried handshake must present as a fresh replica,
    // holding no lineage, so no policy either.
    *last_sequence = 0;
    *chain_hash = [0u8; 32];
    control.lineage_ack_policy.store(None);
    // The advertised control-plane tip must shrink with the holdings:
    // the archived suffix is either divergent (never acked) or being
    // replaced wholesale, so this is the one legitimate tip regression
    // (see `AdvertisedJournalTip::reset`).
    control
        .journal_tip
        .reset(melin_transport_core::WireSeq::new(0));

    let ResyncTransfer {
        app: snap_app,
        snap_sequence,
        snap_chain_hash,
        seed_len,
        lineage: snap_lineage,
    } = match receive_resync_transfer::<A, S>(source, snapshot_path, journal_path) {
        Ok(v) => v,
        Err(e) => {
            tracing::warn!(error = %e, "snapshot transfer failed — retrying");
            // Half-applied resync state, not audit material — drop it
            // so the retry starts clean (the pre-resync lineage is
            // already archived; this is not it).
            let _ = std::fs::remove_file(snapshot_path);
            return Ok(ResyncDecision::Retry);
        }
    };
    // Adopt the primary's snapshot lineage — the resync rebases this
    // replica onto the primary's lineage, its epoch and ack policy
    // included. Only now: a transfer that failed part-way adopts nothing.
    adopt_recovered_lineage(fence_state, &control.lineage_ack_policy, snap_lineage);
    *app = Some(snap_app);

    // Open the seeded segment for appending at the snapshot position —
    // recovery's resume path: the chain rebuilds from the seeded bytes and
    // its value at `snap_sequence` must equal the (verified) snapshot's
    // chain hash. `chain_hash()` is `None` only with `hash-chain` disabled
    // (nothing to tie); an all-zeros snapshot hash means the primary runs
    // without `hash-chain` (also nothing to tie).
    // Rebuilds the chain over the whole seed: serviced (see `serviced`).
    let writer = source.serviced(|| W::open_append(journal_path, snap_sequence, seed_len))?;
    let seeded_chain = writer.chain_hash().unwrap_or(snap_chain_hash);
    if snap_chain_hash != [0u8; 32] && seeded_chain != snap_chain_hash {
        return Err(format!(
            "segment seed chain at {snap_sequence} disagrees with the transferred snapshot's \
             hash — inconsistent primary"
        )
        .into());
    }
    let seeded_info = melin_journal::segment::read_header_info(journal_path)?;
    *journal_writer = Some(writer);

    // Validate the post-snapshot StreamStart inline before resuming — its
    // lineage must agree with the seed the primary just transferred.
    match decode_primary_message(&source.next_frame(MAX_CONTROL_FRAME)?)? {
        PrimaryMessage::StreamStart {
            start_sequence,
            segment_start_sequence,
            anchor_hash,
            genesis_entries,
            epoch,
        } => {
            if segment_start_sequence != seeded_info.starting_sequence
                || anchor_hash != seeded_info.anchor_hash
                || genesis_entries != seeded_info.genesis_entries
            {
                return Err(format!(
                    "post-snapshot StreamStart lineage (start {segment_start_sequence}, genesis \
                     {}) disagrees with the transferred segment seed (start {}, genesis {}) — \
                     inconsistent primary",
                    describe_genesis(genesis_entries),
                    seeded_info.starting_sequence,
                    describe_genesis(seeded_info.genesis_entries),
                )
                .into());
            }
            // We just rebased onto this primary's snapshot, so adopt its
            // epoch wholesale (no stale-primary refusal — our prior state
            // was discarded).
            // The lineage policy came with the snapshot (adopted above,
            // once the transfer succeeded); the stream's changes keep it
            // current from here.
            fence_state.observe_epoch(epoch);
            control.primary_observed.store(true, Ordering::Release);
            tracing::info!(
                start_sequence,
                epoch,
                "streaming resumed after snapshot transfer"
            );
            // The transfer is installed and validated: the node's holdings
            // are exactly the snapshot position again.
            control
                .journal_tip
                .reset(melin_transport_core::WireSeq::new(snap_sequence));
            Ok(ResyncDecision::Ready {
                segment_start_sequence,
                anchor_hash,
                genesis_entries,
                resume_sequence: snap_sequence,
            })
        }
        other => Err(format!("expected StreamStart after snapshot, got {other:?}").into()),
    }
}

/// A genesis length for a log line or an error: the count, or that it
/// is unknown.
fn describe_genesis(genesis_entries: Option<u64>) -> String {
    match genesis_entries {
        Some(n) => format!("{n} entries"),
        None => "unknown".to_string(),
    }
}

/// The genesis length this replica's journal records, for the
/// `StreamStart` cross-check: `None` when the replica has no journal yet
/// (nothing to compare), `Some(None)` for a lineage whose length is
/// unknown, `Some(Some(n))` otherwise.
///
/// Never reads the live segment by path while a pipeline runs: the
/// journal's disk thread owns the writer then and a rotation renames the
/// live segment away before installing its successor, so a read in that
/// window would find no file. The pipeline carries the value captured at
/// build time instead.
pub(in crate::replication) fn local_lineage_genesis<A, W>(
    pipeline: &Option<ReplicaPipelineHandles<A, W>>,
    journal_writer: &Option<W>,
) -> Result<Option<Option<u64>>, melin_journal::JournalError>
where
    A: Application,
    W: JournalWrite<A::Event> + Send + 'static,
{
    match (pipeline, journal_writer) {
        (Some(p), _) => Ok(Some(p.genesis_entries)),
        // No pipeline: this thread owns the writer, so the header read
        // cannot race a rotation.
        (None, Some(w)) => Ok(Some(w.read_header_info()?.genesis_entries)),
        (None, None) => Ok(None),
    }
}

/// Where [`adopt_genesis_into_empty_unknown_journal`] stages the
/// rewritten segment before renaming it over the journal.
fn restamp_staging_path(journal_path: &std::path::Path) -> std::path::PathBuf {
    journal_path.with_extension("restamp.tmp")
}

/// Remove a restamp staging file left by a crash before its rename. The
/// rename had not happened, so the live journal is the old segment and
/// the staged copy is only debris.
fn remove_restamp_staging(journal_path: &std::path::Path) -> std::io::Result<()> {
    match std::fs::remove_file(restamp_staging_path(journal_path)) {
        Ok(()) => Ok(()),
        // No leftover from an interrupted earlier attempt.
        Err(e) if e.kind() == std::io::ErrorKind::NotFound => Ok(()),
        Err(e) => Err(e),
    }
}

/// Record the primary's genesis length in this replica's journal when
/// the journal holds no entry and its header has no length (`None`,
/// written by a release before the field existed).
///
/// The case it closes: a cluster created under such a release whose
/// primary stopped before journaling anything, after this replica copied
/// its empty journal. The upgraded primary journals its genesis into a
/// new header that keeps the anchor (see `genesis_target` in the server
/// boot path), so this replica's chain still matches at sequence 0 and it
/// streams the genesis — but, left with `None`, it would skip the
/// promotion check for exactly the partial genesis that check exists to
/// refuse, and its segment would no longer be a copy of the primary's.
///
/// Only an empty segment starting at sequence 1 with no archive and the
/// primary's own starting sequence and anchor is rewritten: nothing was
/// journaled, acknowledged or served from it, so replacing its header
/// changes no history. The new segment is staged and renamed over the old
/// one, so a crash leaves one or the other. Anything else is left alone.
///
/// Requires no running pipeline: the caller owns `journal_writer`. So it
/// covers the upgrade that restarts the replica as well as the primary; a
/// replica left running across the primary's re-genesis keeps the unknown
/// length — the unchecked behaviour of earlier releases, never a wrong
/// refusal.
pub(in crate::replication) fn adopt_genesis_into_empty_unknown_journal<E, W>(
    journal_writer: &mut Option<W>,
    lineage: (u64, [u8; 32], Option<u64>),
) -> Result<(), melin_journal::JournalError>
where
    E: melin_app::AppEvent,
    W: JournalWrite<E>,
{
    let (segment_start_sequence, anchor_hash, advertised) = lineage;
    let Some(writer) = journal_writer.as_ref() else {
        return Ok(());
    };
    if advertised.is_none()
        || segment_start_sequence != 1
        || writer.segment_starting_sequence() != 1
        || writer.next_sequence() != 1
    {
        return Ok(());
    }
    let info = writer.read_header_info()?;
    if info.genesis_entries.is_some() || info.anchor_hash != anchor_hash {
        return Ok(());
    }
    let journal_path = writer.path().to_path_buf();
    if !melin_journal::segment::list_archives(&journal_path)?.is_empty() {
        return Ok(());
    }

    let staging = restamp_staging_path(&journal_path);
    remove_restamp_staging(&journal_path)?;
    let staged = W::create_continuing(&staging, 1, anchor_hash, advertised)?;
    let valid_end = staged.valid_end();
    drop(staged);
    // Close the old segment before its name is taken over.
    drop(journal_writer.take());
    std::fs::rename(&staging, &journal_path)?;
    melin_journal::segment::fsync_parent_dir(&journal_path)?;
    *journal_writer = Some(W::open_append(&journal_path, 0, valid_end)?);
    tracing::info!(
        journal = %journal_path.display(),
        genesis_entries = ?advertised,
        "recorded the primary's genesis length in this replica's empty journal"
    );
    Ok(())
}

/// The replica's answer to a live `StreamStart` (not the post-snapshot
/// one, which `handle_resync_verdict` validates itself).
pub(in crate::replication) enum StreamStartVerdict {
    /// The primary is behind this node's fencing epoch: a stale
    /// ex-primary. Nothing was adopted; the caller drops the connection
    /// and retries with backoff. The handshake already carried our higher
    /// epoch, so the stale primary fences itself on its side.
    StalePrimary,
    /// Follow the primary. Its epoch is adopted, and the primary recorded
    /// as observed; the lineage — `(segment start, anchor, genesis length)` — is what a
    /// fresh replica creates its journal from.
    Follow((u64, [u8; 32], Option<u64>)),
}

/// The fields of a live `StreamStart` that [`accept_stream_start`] acts
/// on, grouped so both receivers hand them over by name rather than as a
/// run of positional integers.
pub(in crate::replication) struct StreamStart {
    /// The sequence the primary resumes streaming from (logged only).
    pub start_sequence: u64,
    /// The primary's lineage: `(segment start, anchor, genesis length)`.
    /// A tuple because it is handed back whole in
    /// [`StreamStartVerdict::Follow`] and to the fresh-journal path.
    pub lineage: (u64, [u8; 32], Option<u64>),
    /// The primary's fencing epoch.
    pub epoch: u64,
}

/// Accept or refuse a primary's `StreamStart`, shared by the kernel-TCP
/// and DPDK receivers so the two cannot drift on what following a
/// primary means.
///
/// In order: refuse a primary behind our fencing epoch — following its
/// divergent lineage on top of our more-current state would corrupt the
/// journal — before anything is touched; record the primary's genesis
/// length in an empty pre-upgrade journal; refuse a lineage whose genesis
/// length disagrees with ours; then adopt the primary's epoch (streamed
/// `EpochBump`s keep it current thereafter) and record that a primary has
/// been observed. The ack policy is not taken from the primary: the
/// replica learns it from the changes its lineage holds.
///
/// An `Err` is fatal for the receive loop: the journal could not be read
/// or rewritten, or the two journals do not share one lineage.
///
/// `transport` names the receiver (`"tcp"` or `"dpdk"`) in the refusal
/// and streaming-started logs, so an operator running both can tell
/// which path a line came from.
pub(in crate::replication) fn accept_stream_start<A, W>(
    start: StreamStart,
    transport: &'static str,
    pipeline: &Option<ReplicaPipelineHandles<A, W>>,
    journal_writer: &mut Option<W>,
    fence_state: &melin_transport_core::fence::FenceState,
    control: &ReplicaControlPlane,
) -> Result<StreamStartVerdict, Box<dyn std::error::Error>>
where
    A: Application,
    W: JournalWrite<A::Event> + Send + 'static,
{
    let StreamStart {
        start_sequence,
        lineage,
        epoch,
    } = start;
    if fence_state.refuses_primary(epoch) {
        tracing::warn!(
            transport,
            primary_epoch = epoch,
            our_epoch = fence_state.epoch(),
            "primary is behind our fencing epoch — refusing to follow stale primary"
        );
        return Ok(StreamStartVerdict::StalePrimary);
    }
    // A replica with a journal already records its lineage's genesis
    // length; the primary's must agree with it.
    if pipeline.is_none() {
        adopt_genesis_into_empty_unknown_journal(journal_writer, lineage)?;
    }
    if let Some(local) = local_lineage_genesis(pipeline, journal_writer)? {
        check_advertised_genesis(local, lineage.2)?;
    }
    fence_state.observe_epoch(epoch);
    control.primary_observed.store(true, Ordering::Release);
    tracing::info!(transport, start_sequence, epoch, "streaming started");
    Ok(StreamStartVerdict::Follow(lineage))
}

/// Refuse to follow a primary whose lineage records a different genesis
/// length than this replica's journal does.
///
/// The length is lineage metadata, fixed when the journal was created
/// and copied into every segment and every replica's journal since, so
/// within one lineage two known values always agree. A difference means
/// the two journals do not share the lineage the matching chain hashes
/// suggest — a header damaged or edited by hand, or a bug — and nothing
/// says which side is right: overwriting ours would silently change what
/// a later promotion checks against. Exit and let the operator decide.
///
/// An unknown value on either side is no conflict: it is a lineage begun
/// before the length was recorded (or a peer that kept such a header),
/// and it stays as it is — headers are written once.
pub(in crate::replication) fn check_advertised_genesis(
    local: Option<u64>,
    advertised: Option<u64>,
) -> Result<(), String> {
    match (local, advertised) {
        (Some(local), Some(advertised)) if local != advertised => Err(format!(
            "refusing to follow the primary: its journal records a genesis of {advertised} \
             entries but this replica's records {local} — the two journals do not share one \
             lineage (a damaged or hand-edited journal header, or a bug); inspect both journals, \
             then resync this replica from scratch (move its journal, archives and snapshots \
             aside) or restore the primary"
        )),
        _ => Ok(()),
    }
}

#[cfg(test)]
mod tests {
    use std::io::Write;
    use std::sync::Arc;

    use super::auth::{authenticate_replica, authenticate_with_primary};
    use super::*;
    // Any `AppEvent` works as the pipeline's event type here — these
    // protocol-level tests never construct a real app event (the slot's
    // event is always `JournalEvent::Tick`), so the counter example's
    // event type stands in for a production application's event and keeps
    // the runtime's test deps free of any application crate but the example.
    use counter_server::CounterEvent;
    type InputSlot = melin_transport_core::pipeline::InputSlot<CounterEvent>;
    use melin_transport_core::replication::protocol::{
        MAX_CONTROL_FRAME, MAX_DATA_FRAME, MSG_AUTH_OK, MSG_CHALLENGE_RESPONSE, MSG_SNAPSHOT_BEGIN,
        MSG_SNAPSHOT_CHUNK, MSG_SNAPSHOT_END, decode_auth_result, decode_challenge,
        decode_challenge_response, decode_primary_message, decode_replica_message, encode_ack,
        encode_auth_failed, encode_auth_ok, encode_chain_check, encode_challenge,
        encode_challenge_response, encode_handshake, encode_hash_mismatch, encode_heartbeat,
        encode_input_batch, encode_need_snapshot, encode_rotate, encode_segment_seed_begin,
        encode_snapshot_begin, encode_snapshot_chunk, encode_snapshot_end, encode_stream_start,
        read_frame, try_decode_input_batch,
    };

    /// Build a wire-ready `InputBatch` frame containing a single `Tick`
    /// slot at the given sequence — the protocol-level tests don't need
    /// real journal payloads, just something with a known max sequence.
    fn encode_input_batch_with_seq(end_sequence: u64, buf: &mut Vec<u8>) {
        let slot = InputSlot {
            connection_id: 0,
            key_hash: 0,
            sequence: end_sequence,
            timestamp_ns: 0,
            event: melin_journal::JournalEvent::Tick { now_ns: 0 },
            publish_ts: Default::default(),
            recv_ts: Default::default(),
        };
        encode_input_batch(&[slot], buf).expect("encode InputBatch");
    }

    #[test]
    fn handshake_encode_decode_round_trip() {
        let handshake = Handshake {
            last_sequence: 42,
            chain_hash: [0xAB; 32],
            // Non-zero so a dropped/zeroed epoch field is caught.
            epoch: 9,
        };
        let mut buf = Vec::new();
        encode_handshake(&handshake, &mut buf);

        // Read frame: skip 4-byte length prefix.
        let payload = &buf[4..];
        let msg = decode_replica_message(payload).unwrap();
        match msg {
            ReplicaMessage::Handshake(h) => {
                assert_eq!(h.last_sequence, 42);
                assert_eq!(h.chain_hash, [0xAB; 32]);
                assert_eq!(h.epoch, 9);
            }
            _ => panic!("expected Handshake"),
        }
    }

    #[test]
    fn ack_encode_decode_round_trip() {
        let ack = Ack {
            acked_sequence: 1000,
            in_memory_sequence: 1024,
        };
        let mut buf = Vec::new();
        encode_ack(&ack, &mut buf);

        let payload = &buf[4..];
        let msg = decode_replica_message(payload).unwrap();
        match msg {
            ReplicaMessage::Ack(a) => {
                assert_eq!(a.acked_sequence, 1000);
                assert_eq!(a.in_memory_sequence, 1024);
            }
            _ => panic!("expected Ack"),
        }
    }

    /// Pin the exact on-the-wire byte layout of an Ack frame. A future
    /// `repr(C)` field reorder, alignment change, or accidental
    /// big-endian wrapper substitution would silently break replica/
    /// primary compatibility — the const `size_of` assert in
    /// `protocol.rs` only catches size changes, not layout changes.
    /// This test pins the expected bytes so any such break shows up
    /// loudly on the next CI run.
    #[test]
    fn ack_wire_byte_pattern() {
        let ack = Ack {
            acked_sequence: 0xDEAD_BEEF_CAFE_F00D,
            in_memory_sequence: 0x1122_3344_5566_7788,
        };
        let mut buf = Vec::new();
        encode_ack(&ack, &mut buf);
        // [length:u32 LE = 17][tag:u8 = MSG_ACK = 0x02]
        // [acked_sequence:u64 LE][in_memory_sequence:u64 LE]
        let expected: &[u8] = &[
            0x11, 0x00, 0x00, 0x00, // length = 17 LE
            0x02, // MSG_ACK
            0x0D, 0xF0, 0xFE, 0xCA, 0xEF, 0xBE, 0xAD, 0xDE, // acked_sequence LE
            0x88, 0x77, 0x66, 0x55, 0x44, 0x33, 0x22, 0x11, // in_memory_sequence LE
        ];
        assert_eq!(buf.as_slice(), expected, "Ack wire layout drifted");
    }

    /// Two known genesis lengths must agree; an unknown one on either
    /// side is no conflict (a lineage begun before the length was
    /// recorded), and nothing is ever adopted over a known one.
    #[test]
    fn an_advertised_genesis_length_must_match_a_known_one() {
        check_advertised_genesis(Some(3), Some(3)).expect("the same lineage");
        check_advertised_genesis(Some(0), Some(0)).expect("no genesis on both sides");
        let err = check_advertised_genesis(Some(3), Some(4)).expect_err("different lengths");
        assert!(err.contains("refusing to follow the primary"), "{err}");
        assert!(
            err.contains("genesis of 4") && err.contains("records 3"),
            "{err}"
        );
        check_advertised_genesis(Some(0), Some(2)).expect_err("none against some");
        check_advertised_genesis(None, Some(3)).expect("ours unknown");
        check_advertised_genesis(Some(3), None).expect("theirs unknown");
        check_advertised_genesis(None, None).expect("both unknown");
    }

    /// An empty journal written before the genesis length existed takes
    /// the primary's, so the promotion check guards it; a journal that
    /// holds an entry, already has a length, or carries another anchor is
    /// left as it is.
    #[test]
    fn an_empty_unknown_journal_adopts_the_primarys_genesis_length() {
        type Writer = melin_journal::BufferedWriter<CounterEvent>;
        let dir = tempfile::tempdir().expect("tempdir");
        let anchor = [9u8; 32];
        let header = |w: &Option<Writer>| {
            w.as_ref()
                .expect("writer")
                .read_header_info()
                .expect("header")
                .genesis_entries
        };

        // The pre-upgrade empty journal: adopted, still empty, same anchor.
        let path = dir.path().join("empty.journal");
        let mut w = Some(Writer::create_continuing(&path, 1, anchor, None).expect("create"));
        adopt_genesis_into_empty_unknown_journal(&mut w, (1, anchor, Some(3))).expect("adopt");
        assert_eq!(header(&w), Some(3));
        let adopted = w.as_ref().expect("writer");
        assert_eq!(adopted.next_sequence(), 1);
        assert_eq!(adopted.path(), path.as_path());
        assert_eq!(
            melin_journal::segment::read_header_info(&path)
                .expect("on disk")
                .anchor_hash,
            anchor
        );
        assert!(!path.with_extension("restamp.tmp").exists());

        // The primary's length is unknown too: nothing to adopt.
        let path = dir.path().join("both-unknown.journal");
        let mut w = Some(Writer::create_continuing(&path, 1, anchor, None).expect("create"));
        adopt_genesis_into_empty_unknown_journal(&mut w, (1, anchor, None)).expect("no-op");
        assert_eq!(header(&w), None);

        // Another lineage's anchor: not this primary's journal.
        let path = dir.path().join("other-anchor.journal");
        let mut w = Some(Writer::create_continuing(&path, 1, [1; 32], None).expect("create"));
        adopt_genesis_into_empty_unknown_journal(&mut w, (1, anchor, Some(3))).expect("no-op");
        assert_eq!(header(&w), None);

        // A known length is never rewritten (a mismatch is refused by
        // `check_advertised_genesis` instead).
        let path = dir.path().join("known.journal");
        let mut w = Some(Writer::create_continuing(&path, 1, anchor, Some(2)).expect("create"));
        adopt_genesis_into_empty_unknown_journal(&mut w, (1, anchor, Some(3))).expect("no-op");
        assert_eq!(header(&w), Some(2));

        // A journal holding an entry has history: its header stays.
        let path = dir.path().join("history.journal");
        let mut writer = Writer::create_continuing(&path, 1, anchor, None).expect("create");
        let seq = writer.allocate_sequence();
        writer
            .encode_event(seq, 0, &melin_journal::JournalEvent::Tick { now_ns: 0 }, 0)
            .expect("encode");
        writer.flush_batch_sync().expect("flush");
        let mut w = Some(writer);
        adopt_genesis_into_empty_unknown_journal(&mut w, (1, anchor, Some(3))).expect("no-op");
        assert_eq!(header(&w), None);

        // No journal: nothing to do.
        let mut none: Option<Writer> = None;
        adopt_genesis_into_empty_unknown_journal(&mut none, (1, anchor, Some(3))).expect("no-op");
        assert!(none.is_none());
    }

    /// A restamp staging file left by a crash before its rename is swept
    /// at replica recovery, whatever the journal's state — not only by a
    /// later adopt that happens to meet its guard.
    #[test]
    fn replica_recovery_sweeps_a_leftover_restamp_staging_file() {
        type Writer = melin_journal::BufferedWriter<CounterEvent>;
        let dir = tempfile::tempdir().expect("tempdir");
        let fence = melin_transport_core::fence::FenceState::new(0);
        let journal = dir.path().join("replica.journal");
        let snapshot = dir.path().join("replica.snapshot");
        let staging = journal.with_extension("restamp.tmp");

        // No lineage at all.
        std::fs::write(&staging, b"debris").expect("write staging");
        let (app, writer, _, _) = recover_replica_state::<counter_server::Counter, Writer>(
            &journal,
            &snapshot,
            &fence,
            &SharedLineagePolicy::new(None),
            &(),
        )
        .expect("recover");
        assert!(app.is_none() && writer.is_none());
        assert!(!staging.exists(), "swept without a journal");

        // A journal that holds history, which the adopt guard never touches.
        let mut w = Writer::create_continuing(&journal, 1, [0; 32], None).expect("create");
        let seq = w.allocate_sequence();
        w.encode_event(seq, 0, &melin_journal::JournalEvent::Tick { now_ns: 0 }, 0)
            .expect("encode");
        w.flush_batch_sync().expect("flush");
        drop(w);
        std::fs::write(&staging, b"debris").expect("write staging");
        let (_, writer, last, _) = recover_replica_state::<counter_server::Counter, Writer>(
            &journal,
            &snapshot,
            &fence,
            &SharedLineagePolicy::new(None),
            &(),
        )
        .expect("recover");
        assert!(writer.is_some());
        assert_eq!(last, 1);
        assert!(!staging.exists(), "swept beside a journal with history");
    }

    /// Replica recovery adopts the recovered lineage: the epoch as a
    /// maximum, the ack policy stored whole over whatever the cell held —
    /// the journaled change (a lower byte than the cell's: stored, not a
    /// maximum), or nothing when the journal holds none, or when there is
    /// no lineage at all.
    #[test]
    fn replica_recovery_stores_the_lineage_policy_whole() {
        use melin_journal::{AckPolicyByte, AckPolicyChange, JournalEvent};
        use melin_transport_core::lineage::LineagePolicy;
        type Writer = melin_journal::BufferedWriter<CounterEvent>;
        let dir = tempfile::tempdir().expect("tempdir");
        let fence = melin_transport_core::fence::FenceState::new(0);
        let journal = dir.path().join("replica.journal");
        let snapshot = dir.path().join("replica.snapshot");
        // A value no recovery below produces, so a record left alone
        // shows.
        let stale = Some(LineagePolicy {
            change: AckPolicyChange {
                policy: AckPolicyByte(2),
                unwitnessed: true,
            },
            wire_seq: melin_transport_core::WireSeq::new(9),
            previous: Some(AckPolicyByte(1)),
        });
        let cell = SharedLineagePolicy::new(stale);
        let recover = |cell: &SharedLineagePolicy| {
            recover_replica_state::<counter_server::Counter, Writer>(
                &journal,
                &snapshot,
                &fence,
                cell,
                &(),
            )
            .expect("recover")
        };

        // No lineage: nothing held.
        recover(&cell);
        assert_eq!(cell.load(), None);

        // A journal with no change: nothing held either.
        let mut w = Writer::create_continuing(&journal, 1, [0; 32], None).expect("create");
        let append = |w: &mut Writer, event: JournalEvent<CounterEvent>| {
            let seq = w.allocate_sequence();
            w.encode_event(seq, 0, &event, 0).expect("encode");
            w.flush_batch_sync().expect("flush");
        };
        append(&mut w, JournalEvent::Tick { now_ns: 0 });
        cell.store(stale);
        recover(&cell);
        assert_eq!(cell.load(), None);

        append(&mut w, JournalEvent::EpochBump { epoch: 4 });
        append(
            &mut w,
            JournalEvent::AckPolicyChange(AckPolicyChange {
                policy: AckPolicyByte(0),
                unwitnessed: false,
            }),
        );
        drop(w);
        cell.store(stale);
        recover(&cell);
        assert_eq!(
            cell.load(),
            Some(LineagePolicy {
                change: AckPolicyChange {
                    policy: AckPolicyByte(0),
                    unwitnessed: false,
                },
                wire_seq: melin_transport_core::WireSeq::new(3),
                previous: None,
            }),
            "the journal's own record, at the change's sequence"
        );
        assert_eq!(fence.epoch(), 4);
    }

    /// A primary behind this node's fencing epoch is refused before
    /// anything is adopted: not its epoch, not its genesis length into our
    /// journal, and it does not count as a primary observed.
    #[test]
    fn a_stale_primary_is_refused_before_anything_is_adopted() {
        type Writer = melin_journal::BufferedWriter<CounterEvent>;
        let dir = tempfile::tempdir().expect("tempdir");
        let path = dir.path().join("replica.journal");
        let anchor = [9u8; 32];
        let mut journal_writer =
            Some(Writer::create_continuing(&path, 1, anchor, None).expect("create"));
        let fence = melin_transport_core::fence::FenceState::new(5);
        let control = ReplicaControlPlane::new();

        let verdict = accept_stream_start::<counter_server::Counter, Writer>(
            StreamStart {
                start_sequence: 1,
                lineage: (1, anchor, Some(3)),
                epoch: 4,
            },
            "test",
            &None,
            &mut journal_writer,
            &fence,
            &control,
        )
        .expect("a refusal is not an error");

        assert!(matches!(verdict, StreamStartVerdict::StalePrimary));
        assert_eq!(fence.epoch(), 5);
        assert!(!control.primary_observed.load(Ordering::Acquire));
        let header = journal_writer
            .as_ref()
            .expect("writer")
            .read_header_info()
            .expect("header");
        assert_eq!(header.genesis_entries, None, "journal left untouched");
    }

    /// Following a primary adopts its epoch, records it as observed and
    /// its genesis length in an empty pre-upgrade journal, and hands back
    /// the lineage the fresh-journal path creates from. The lineage's ack
    /// policy is left to the entries this replica holds.
    #[test]
    fn following_a_primary_adopts_its_epoch_and_genesis() {
        type Writer = melin_journal::BufferedWriter<CounterEvent>;
        let dir = tempfile::tempdir().expect("tempdir");
        let path = dir.path().join("replica.journal");
        let anchor = [9u8; 32];
        let mut journal_writer =
            Some(Writer::create_continuing(&path, 1, anchor, None).expect("create"));
        let fence = melin_transport_core::fence::FenceState::new(5);
        let control = ReplicaControlPlane::new();
        let held = Some(melin_transport_core::lineage::LineagePolicy::after(
            None,
            melin_transport_core::WireSeq::new(1),
            melin_journal::AckPolicyChange {
                policy: crate::ack_policy::AckPolicy::DiskAndRam.to_byte(),
                unwitnessed: false,
            },
        ));
        control.lineage_ack_policy.store(held);

        let verdict = accept_stream_start::<counter_server::Counter, Writer>(
            StreamStart {
                start_sequence: 1,
                lineage: (1, anchor, Some(3)),
                epoch: 7,
            },
            "test",
            &None,
            &mut journal_writer,
            &fence,
            &control,
        )
        .expect("same lineage");

        let StreamStartVerdict::Follow(lineage) = verdict else {
            panic!("expected Follow");
        };
        assert_eq!(lineage, (1, anchor, Some(3)));
        assert_eq!(fence.epoch(), 7);
        assert!(control.primary_observed.load(Ordering::Acquire));
        assert_eq!(control.lineage_ack_policy.load(), held, "untouched");
        let header = journal_writer
            .as_ref()
            .expect("writer")
            .read_header_info()
            .expect("header");
        assert_eq!(header.genesis_entries, Some(3));
    }

    /// A primary whose genesis length disagrees with ours is fatal, and
    /// its epoch is not adopted on the way out.
    #[test]
    fn a_disagreeing_genesis_length_is_fatal() {
        type Writer = melin_journal::BufferedWriter<CounterEvent>;
        let dir = tempfile::tempdir().expect("tempdir");
        let path = dir.path().join("replica.journal");
        let anchor = [9u8; 32];
        let mut journal_writer =
            Some(Writer::create_continuing(&path, 1, anchor, Some(2)).expect("create"));
        let fence = melin_transport_core::fence::FenceState::new(5);
        let control = ReplicaControlPlane::new();

        let err = accept_stream_start::<counter_server::Counter, Writer>(
            StreamStart {
                start_sequence: 1,
                lineage: (1, anchor, Some(3)),
                epoch: 7,
            },
            "test",
            &None,
            &mut journal_writer,
            &fence,
            &control,
        )
        .err()
        .expect("different lineages");

        assert!(err.to_string().contains("refusing to follow the primary"));
        assert_eq!(fence.epoch(), 5);
    }

    /// The handshake's local genesis length comes from the running
    /// pipeline, never from the live segment's path: a rotation renames
    /// that file away before installing its successor, and a read in the
    /// gap would fail the session. Removing the file stands in for that
    /// gap. Without a pipeline the writer's own header is read, and with
    /// no journal at all there is nothing to compare.
    #[test]
    fn the_local_genesis_length_never_reads_the_live_segment_under_a_pipeline() {
        type Writer = melin_journal::BufferedWriter<CounterEvent>;
        let dir = tempfile::tempdir().expect("tempdir");
        let path = dir.path().join("replica.journal");

        let writer = Writer::create_continuing(&path, 1, [7; 32], Some(5)).expect("create");
        let mut journal_writer = Some(writer);
        assert_eq!(
            local_lineage_genesis::<counter_server::Counter, Writer>(&None, &journal_writer)
                .expect("header read"),
            Some(Some(5)),
        );

        let (input_producer, mut consumers) =
            melin_pipeline::ring::DisruptorBuilder::<InputSlot>::new(4)
                .add_consumer()
                .build(melin_pipeline::wait::WaitStrategy::SpinThenYield);
        let _consumer = consumers.pop().expect("one consumer");
        let owned = journal_writer.take().expect("writer");
        let pipeline = Some(ReplicaPipelineHandles {
            input_producer,
            journal_cursor: Arc::new(make_journal_cursor(0)),
            chain_hash_lock: fsync_state_at(0, 0).1,
            stream_marks: Arc::new(std::sync::Mutex::new(std::collections::VecDeque::new())),
            genesis_entries: Some(5),
            journal_failed: Arc::new(AtomicBool::new(false)),
            pipeline_shutdown: Arc::new(AtomicBool::new(false)),
            journal_handle: std::thread::spawn(
                move || -> Result<Writer, melin_journal::JournalError> { Ok(owned) },
            ),
            matching_handle: std::thread::spawn(counter_server::Counter::default),
            drain_handle: std::thread::spawn(|| {}),
            shadow_handle: None,
        });
        std::fs::remove_file(&path).expect("simulate the rotation gap");
        assert_eq!(
            local_lineage_genesis(&pipeline, &journal_writer).expect("no file read"),
            Some(Some(5)),
        );

        assert_eq!(
            local_lineage_genesis::<counter_server::Counter, Writer>(&None, &None)
                .expect("nothing to read"),
            None,
        );
    }

    #[test]
    fn stream_start_encode_decode_round_trip() {
        let mut buf = Vec::new();
        // Non-zero epoch so a dropped/zeroed field is caught.
        encode_stream_start(99, 42, [0xAA; 32], Some(7), 5, &mut buf);

        let payload = &buf[4..];
        let msg = decode_primary_message(payload).unwrap();
        match msg {
            PrimaryMessage::StreamStart {
                start_sequence,
                segment_start_sequence,
                anchor_hash,
                genesis_entries,
                epoch,
            } => {
                assert_eq!(start_sequence, 99);
                assert_eq!(segment_start_sequence, 42);
                assert_eq!(anchor_hash, [0xAA; 32]);
                assert_eq!(genesis_entries, Some(7));
                assert_eq!(epoch, 5);
            }
            _ => panic!("expected StreamStart"),
        }
    }

    #[test]
    fn heartbeat_encode_decode_round_trip() {
        let mut buf = Vec::new();
        encode_heartbeat(123, &mut buf);

        let payload = &buf[4..];
        let msg = decode_primary_message(payload).unwrap();
        match msg {
            PrimaryMessage::Heartbeat { sequence } => assert_eq!(sequence, 123),
            _ => panic!("expected Heartbeat"),
        }
    }

    #[test]
    fn need_snapshot_encode_decode_round_trip() {
        let mut buf = Vec::new();
        encode_need_snapshot(&mut buf);

        let payload = &buf[4..];
        let msg = decode_primary_message(payload).unwrap();
        assert!(matches!(msg, PrimaryMessage::NeedSnapshot));
    }

    #[test]
    fn hash_mismatch_encode_decode_round_trip() {
        let mut buf = Vec::new();
        encode_hash_mismatch(&mut buf);

        let payload = &buf[4..];
        let msg = decode_primary_message(payload).unwrap();
        assert!(matches!(msg, PrimaryMessage::HashMismatch));
    }

    #[test]
    fn rotate_encode_decode_round_trip() {
        let mut buf = Vec::new();
        encode_rotate(7_000_000, &[0xBB; 32], &mut buf);

        let payload = &buf[4..];
        let msg = decode_primary_message(payload).unwrap();
        match msg {
            PrimaryMessage::Rotate {
                boundary_seq,
                tail_hash,
            } => {
                assert_eq!(boundary_seq, 7_000_000);
                assert_eq!(tail_hash, [0xBB; 32]);
            }
            _ => panic!("expected Rotate"),
        }
    }

    #[test]
    fn chain_check_encode_decode_round_trip() {
        let mut buf = Vec::new();
        encode_chain_check(555, &[0xCC; 32], &mut buf);

        let payload = &buf[4..];
        let msg = decode_primary_message(payload).unwrap();
        match msg {
            PrimaryMessage::ChainCheck {
                sequence,
                chain_hash,
            } => {
                assert_eq!(sequence, 555);
                assert_eq!(chain_hash, [0xCC; 32]);
            }
            _ => panic!("expected ChainCheck"),
        }
    }

    /// Pin the wire layout of the shared `(sequence, hash)` frame body —
    /// same rationale as `ack_wire_byte_pattern`. Rotate and ChainCheck
    /// share the layout; only the tag differs.
    #[test]
    fn rotate_wire_byte_pattern() {
        let mut hash = [0u8; 32];
        for (i, b) in hash.iter_mut().enumerate() {
            *b = i as u8;
        }
        let mut buf = Vec::new();
        encode_rotate(0xDEAD_BEEF_CAFE_F00D, &hash, &mut buf);
        // [length:u32 LE = 41][tag:u8 = MSG_ROTATE = 0x16]
        // [boundary_seq:u64 LE][tail_hash:32 bytes verbatim]
        let mut expected = vec![
            0x29, 0x00, 0x00, 0x00, // length = 41 LE
            0x16, // MSG_ROTATE
            0x0D, 0xF0, 0xFE, 0xCA, 0xEF, 0xBE, 0xAD, 0xDE, // boundary_seq LE
        ];
        expected.extend_from_slice(&hash);
        assert_eq!(
            buf.as_slice(),
            expected.as_slice(),
            "Rotate wire layout drifted"
        );
    }

    #[test]
    fn segment_seed_begin_encode_decode_round_trip() {
        let mut buf = Vec::new();
        encode_segment_seed_begin(123_456, &mut buf);

        let payload = &buf[4..];
        let msg = decode_primary_message(payload).unwrap();
        match msg {
            PrimaryMessage::SegmentSeedBegin { seed_len } => {
                assert_eq!(seed_len, 123_456);
            }
            _ => panic!("expected SegmentSeedBegin"),
        }
    }

    #[test]
    fn unknown_replica_message_type_is_error() {
        let payload = [0xFF, 0, 0, 0, 0, 0, 0, 0, 0];
        let result = decode_replica_message(&payload);
        assert!(result.is_err());
    }

    #[test]
    fn unknown_primary_message_type_is_error() {
        let payload = [0xFF, 0, 0, 0, 0, 0, 0, 0, 0];
        let result = decode_primary_message(&payload);
        assert!(result.is_err());
    }

    #[test]
    fn read_frame_enforces_max_size() {
        // Create a buffer with a length prefix claiming 1000 bytes.
        let mut buf = Vec::new();
        buf.extend_from_slice(&1000u32.to_le_bytes());
        buf.extend_from_slice(&[0u8; 100]); // not enough data, but max_size check comes first

        let mut cursor = std::io::Cursor::new(buf);
        let result = read_frame(&mut cursor, 64);
        assert!(result.is_err());
    }

    #[test]
    fn challenge_encode_decode_round_trip() {
        let nonce = [0x42; 32];
        let mut buf = Vec::new();
        encode_challenge(&nonce, &mut buf);

        let payload = &buf[4..];
        let decoded = decode_challenge(payload).unwrap();
        assert_eq!(decoded, nonce);
    }

    #[test]
    fn challenge_response_encode_decode_round_trip() {
        let sig = [0xAA; 64];
        let pubkey = [0xBB; 32];
        let mut buf = Vec::new();
        encode_challenge_response(&sig, &pubkey, &mut buf);

        let payload = &buf[4..];
        let (decoded_sig, decoded_pubkey) = decode_challenge_response(payload).unwrap();
        assert_eq!(decoded_sig, sig);
        assert_eq!(decoded_pubkey, pubkey);
    }

    #[test]
    fn auth_ok_encode_decode_round_trip() {
        let mut buf = Vec::new();
        encode_auth_ok(&mut buf);

        let payload = &buf[4..];
        assert!(decode_auth_result(payload).unwrap());
    }

    #[test]
    fn auth_failed_encode_decode_round_trip() {
        let mut buf = Vec::new();
        encode_auth_failed(&mut buf);

        let payload = &buf[4..];
        assert!(!decode_auth_result(payload).unwrap());
    }

    #[test]
    fn decode_challenge_rejects_wrong_tag() {
        let mut payload = [0u8; 33];
        payload[0] = MSG_AUTH_OK;
        assert!(decode_challenge(&payload).is_err());
    }

    #[test]
    fn decode_challenge_response_rejects_short_payload() {
        let payload = [MSG_CHALLENGE_RESPONSE; 10]; // too short
        assert!(decode_challenge_response(&payload).is_err());
    }

    #[test]
    fn auth_round_trip_valid_key() {
        use ed25519_dalek::SigningKey;
        use std::os::unix::net::UnixStream;

        let repl_key = SigningKey::from_bytes(&[0xFC; 32]);
        let pub_b64 = base64::Engine::encode(
            &base64::engine::general_purpose::STANDARD,
            repl_key.verifying_key().to_bytes(),
        );
        let keys_content = format!("replication {pub_b64} test-replica\n");
        let authorized_keys =
            melin_app::auth::AuthorizedKeys::parse::<melin_app::auth::NoRoles>(&keys_content)
                .unwrap();

        let (primary_stream, replica_stream) = UnixStream::pair().unwrap();
        primary_stream
            .set_read_timeout(Some(std::time::Duration::from_secs(5)))
            .unwrap();
        replica_stream
            .set_read_timeout(Some(std::time::Duration::from_secs(5)))
            .unwrap();

        let repl_key_clone = SigningKey::from_bytes(&[0xFC; 32]);
        let replica_handle = std::thread::spawn(move || {
            let mut conn = replica_stream;
            authenticate_with_primary(&mut conn, &repl_key_clone)
        });

        let mut conn = primary_stream;
        authenticate_replica(&mut conn, &authorized_keys).unwrap();

        replica_handle.join().unwrap().unwrap();
    }

    #[test]
    fn auth_rejects_unknown_key() {
        use ed25519_dalek::SigningKey;
        use std::os::unix::net::UnixStream;

        // authorized_keys has one key, but the replica uses a different one.
        let authorized_key = SigningKey::from_bytes(&[0xAA; 32]);
        let rogue_key = SigningKey::from_bytes(&[0xBB; 32]);
        let pub_b64 = base64::Engine::encode(
            &base64::engine::general_purpose::STANDARD,
            authorized_key.verifying_key().to_bytes(),
        );
        let keys_content = format!("replication {pub_b64} authorized-replica\n");
        let authorized_keys =
            melin_app::auth::AuthorizedKeys::parse::<melin_app::auth::NoRoles>(&keys_content)
                .unwrap();

        let (primary_stream, replica_stream) = UnixStream::pair().unwrap();
        primary_stream
            .set_read_timeout(Some(std::time::Duration::from_secs(5)))
            .unwrap();
        replica_stream
            .set_read_timeout(Some(std::time::Duration::from_secs(5)))
            .unwrap();

        let replica_handle = std::thread::spawn(move || {
            let mut conn = replica_stream;
            authenticate_with_primary(&mut conn, &rogue_key)
        });

        let mut conn = primary_stream;
        let result = authenticate_replica(&mut conn, &authorized_keys);
        assert!(result.is_err());
        assert!(result.unwrap_err().to_string().contains("unknown"));

        // Replica should also get a rejection.
        let replica_result = replica_handle.join().unwrap();
        assert!(replica_result.is_err());
    }

    #[test]
    fn auth_rejects_wrong_permission() {
        use ed25519_dalek::SigningKey;
        use std::os::unix::net::UnixStream;

        // Key exists but is listed as trader, not replication.
        let key = SigningKey::from_bytes(&[0xCC; 32]);
        let pub_b64 = base64::Engine::encode(
            &base64::engine::general_purpose::STANDARD,
            key.verifying_key().to_bytes(),
        );
        let authorized_keys = crate::test_roles::desk_keys("trader", &pub_b64);

        let (primary_stream, replica_stream) = UnixStream::pair().unwrap();
        primary_stream
            .set_read_timeout(Some(std::time::Duration::from_secs(5)))
            .unwrap();
        replica_stream
            .set_read_timeout(Some(std::time::Duration::from_secs(5)))
            .unwrap();

        let replica_handle = std::thread::spawn(move || {
            let mut conn = replica_stream;
            authenticate_with_primary(&mut conn, &key)
        });

        let mut conn = primary_stream;
        let result = authenticate_replica(&mut conn, &authorized_keys);
        assert!(result.is_err());
        assert_eq!(
            result.unwrap_err().to_string(),
            "key listed as trader, expected replication"
        );

        let replica_result = replica_handle.join().unwrap();
        assert!(replica_result.is_err());
    }

    /// A replica that sends a validly-formatted but tampered signature
    /// (correct public key, wrong signature bytes) is rejected.
    #[test]
    fn auth_rejects_invalid_signature() {
        use ed25519_dalek::SigningKey;
        use std::os::unix::net::UnixStream;

        // Register the correct key.
        let correct_key = SigningKey::from_bytes(&[0xDD; 32]);
        let pub_b64 = base64::Engine::encode(
            &base64::engine::general_purpose::STANDARD,
            correct_key.verifying_key().to_bytes(),
        );
        let keys_content = format!("replication {pub_b64} test-replica\n");
        let authorized_keys =
            melin_app::auth::AuthorizedKeys::parse::<melin_app::auth::NoRoles>(&keys_content)
                .unwrap();

        let (primary_stream, replica_stream) = UnixStream::pair().unwrap();
        primary_stream
            .set_read_timeout(Some(std::time::Duration::from_secs(5)))
            .unwrap();
        replica_stream
            .set_read_timeout(Some(std::time::Duration::from_secs(5)))
            .unwrap();

        // Replica side: read challenge, but sign with a DIFFERENT key,
        // then send the response with the correct public key (spoofing).
        let replica_handle = std::thread::spawn(move || {
            use melin_transport_core::replication::protocol::*;

            let mut reader = replica_stream.try_clone().unwrap();
            let mut writer = replica_stream;

            // Read the challenge.
            let frame = read_frame(&mut reader, MAX_CONTROL_FRAME).unwrap();
            let nonce = decode_challenge(&frame).unwrap();

            // Sign with a WRONG key but send the CORRECT public key.
            let wrong_key = SigningKey::from_bytes(&[0xEE; 32]);
            let bad_signature = ed25519_dalek::Signer::sign(&wrong_key, &nonce);
            let correct_pubkey = correct_key.verifying_key();

            let mut buf = Vec::with_capacity(128);
            encode_challenge_response(
                &bad_signature.to_bytes(),
                correct_pubkey.as_bytes(),
                &mut buf,
            );
            writer.write_all(&buf).unwrap();
            writer.flush().unwrap();

            // Should receive AuthFailed.
            let result_frame = read_frame(&mut reader, MAX_CONTROL_FRAME).unwrap();
            let ok = decode_auth_result(&result_frame).unwrap();
            assert!(!ok, "should receive auth failure");
        });

        let mut conn = primary_stream;
        let result = authenticate_replica(&mut conn, &authorized_keys);
        assert!(result.is_err());
        assert!(
            result
                .unwrap_err()
                .to_string()
                .contains("signature verification failed"),
            "should fail on signature verification"
        );

        replica_handle.join().unwrap();
    }

    #[test]
    fn sender_receiver_end_to_end() {
        use std::os::unix::net::UnixStream;

        // Create a mock connection.
        let (primary_stream, replica_stream) = UnixStream::pair().unwrap();

        // Spawn a thread simulating the replica side.
        let replica_handle = std::thread::spawn(move || {
            let mut reader = replica_stream.try_clone().unwrap();
            let mut writer = replica_stream;

            // Send handshake.
            let mut buf = Vec::new();
            let handshake = Handshake {
                last_sequence: 0,
                chain_hash: [0u8; 32],
                epoch: 0,
            };
            encode_handshake(&handshake, &mut buf);
            writer.write_all(&buf).unwrap();
            writer.flush().unwrap();
            buf.clear();

            // Read StreamStart.
            let frame = read_frame(&mut reader, MAX_CONTROL_FRAME).unwrap();
            let msg = decode_primary_message(&frame).unwrap();
            assert!(matches!(msg, PrimaryMessage::StreamStart { .. }));

            // Read InputBatch.
            let frame = read_frame(&mut reader, MAX_DATA_FRAME).unwrap();
            let slots: Vec<InputSlot> = try_decode_input_batch(&frame).expect("decode InputBatch");
            let end_seq = slots
                .last()
                .map(|s| s.sequence)
                .expect("InputBatch carried at least one slot");

            // Send ack.
            let ack = Ack {
                acked_sequence: end_seq,
                in_memory_sequence: end_seq,
            };
            encode_ack(&ack, &mut buf);
            writer.write_all(&buf).unwrap();
            writer.flush().unwrap();

            end_seq
        });

        // Primary side: simulate handle_replica_connection partially.
        let mut p_reader = primary_stream.try_clone().unwrap();
        let mut p_writer = primary_stream;

        // Read handshake.
        let frame = read_frame(&mut p_reader, MAX_CONTROL_FRAME).unwrap();
        let handshake = match decode_replica_message(&frame).unwrap() {
            ReplicaMessage::Handshake(h) => h,
            _ => panic!("expected Handshake"),
        };
        assert_eq!(handshake.last_sequence, 0);

        // Send StreamStart.
        let mut buf = Vec::new();
        encode_stream_start(0, 1, [0u8; 32], Some(0), 0, &mut buf); // fake lineage for test
        p_writer.write_all(&buf).unwrap();
        p_writer.flush().unwrap();
        buf.clear();

        // Send an InputBatch with a single Tick slot at seq 42.
        encode_input_batch_with_seq(42, &mut buf);
        p_writer.write_all(&buf).unwrap();
        p_writer.flush().unwrap();
        buf.clear();

        // Read ack.
        let frame = read_frame(&mut p_reader, MAX_CONTROL_FRAME).unwrap();
        let ack = match decode_replica_message(&frame).unwrap() {
            ReplicaMessage::Ack(a) => a,
            _ => panic!("expected Ack"),
        };
        assert_eq!(ack.acked_sequence, 42);

        // Join replica thread.
        let end_seq = replica_handle.join().unwrap();
        assert_eq!(end_seq, 42);
    }

    #[test]
    fn multiple_data_batches_acked_in_order() {
        // Send multiple InputBatch frames, verify replica acks each one
        // and the cursor advances correctly.
        use std::os::unix::net::UnixStream;

        let (primary_stream, replica_stream) = UnixStream::pair().unwrap();

        let replica_handle = std::thread::spawn(move || {
            let mut reader = replica_stream.try_clone().unwrap();
            let mut writer = replica_stream;
            let mut buf = Vec::new();

            // Send handshake.
            encode_handshake(
                &Handshake {
                    last_sequence: 0,
                    chain_hash: [0u8; 32],
                    epoch: 0,
                },
                &mut buf,
            );
            writer.write_all(&buf).unwrap();
            writer.flush().unwrap();
            buf.clear();

            // Read StreamStart.
            let frame = read_frame(&mut reader, MAX_CONTROL_FRAME).unwrap();
            assert!(matches!(
                decode_primary_message(&frame).unwrap(),
                PrimaryMessage::StreamStart { .. }
            ));

            // Read and ack 3 InputBatches.
            let mut acked_seqs = Vec::new();
            for _ in 0..3 {
                let frame = read_frame(&mut reader, MAX_DATA_FRAME).unwrap();
                let slots: Vec<InputSlot> =
                    try_decode_input_batch(&frame).expect("decode InputBatch");
                let end_seq = slots
                    .last()
                    .map(|s| s.sequence)
                    .expect("InputBatch carried at least one slot");
                acked_seqs.push(end_seq);

                encode_ack(
                    &Ack {
                        acked_sequence: end_seq,
                        in_memory_sequence: end_seq,
                    },
                    &mut buf,
                );
                writer.write_all(&buf).unwrap();
                writer.flush().unwrap();
                buf.clear();
            }

            acked_seqs
        });

        // Primary side.
        let mut p_reader = primary_stream.try_clone().unwrap();
        let mut p_writer = primary_stream;
        let mut buf = Vec::new();

        // Read handshake.
        let frame = read_frame(&mut p_reader, MAX_CONTROL_FRAME).unwrap();
        assert!(matches!(
            decode_replica_message(&frame).unwrap(),
            ReplicaMessage::Handshake(_)
        ));

        // Send StreamStart.
        encode_stream_start(0, 1, [0u8; 32], Some(0), 0, &mut buf);
        p_writer.write_all(&buf).unwrap();
        p_writer.flush().unwrap();
        buf.clear();

        // Send 3 InputBatches with increasing sequence numbers.
        for seq in [10u64, 20, 30] {
            encode_input_batch_with_seq(seq, &mut buf);
            p_writer.write_all(&buf).unwrap();
            p_writer.flush().unwrap();
            buf.clear();
        }

        // Read 3 acks.
        for expected_seq in [10u64, 20, 30] {
            let frame = read_frame(&mut p_reader, MAX_CONTROL_FRAME).unwrap();
            let ack = match decode_replica_message(&frame).unwrap() {
                ReplicaMessage::Ack(a) => a,
                other => panic!("expected Ack, got {other:?}"),
            };
            assert_eq!(ack.acked_sequence, expected_seq);
        }

        let acked = replica_handle.join().unwrap();
        assert_eq!(acked, vec![10, 20, 30]);
    }

    #[test]
    fn heartbeat_encode_contains_sequence() {
        // Heartbeat messages carry the last known sequence so the replica
        // can verify it hasn't missed any data.
        let mut buf = Vec::new();
        encode_heartbeat(999, &mut buf);

        let payload = &buf[4..];
        match decode_primary_message(payload).unwrap() {
            PrimaryMessage::Heartbeat { sequence, .. } => {
                assert_eq!(sequence, 999);
            }
            other => panic!("expected Heartbeat, got {other:?}"),
        }
    }

    #[test]
    fn replica_mid_stream_handshake_with_nonzero_sequence() {
        // A replica that already has some data sends a non-zero last_sequence
        // in its handshake. The primary should respond with StreamStart
        // containing that sequence, and the replica should only receive
        // events after that point.
        use std::os::unix::net::UnixStream;

        let (primary_stream, replica_stream) = UnixStream::pair().unwrap();

        let replica_handle = std::thread::spawn(move || {
            let mut reader = replica_stream.try_clone().unwrap();
            let mut writer = replica_stream;
            let mut buf = Vec::new();

            // Replica already has events up to sequence 100.
            encode_handshake(
                &Handshake {
                    last_sequence: 100,
                    chain_hash: [0xBB; 32],
                    epoch: 0,
                },
                &mut buf,
            );
            writer.write_all(&buf).unwrap();
            writer.flush().unwrap();
            buf.clear();

            // Read StreamStart — should echo back our last_sequence.
            let frame = read_frame(&mut reader, MAX_CONTROL_FRAME).unwrap();
            match decode_primary_message(&frame).unwrap() {
                PrimaryMessage::StreamStart { start_sequence, .. } => {
                    assert_eq!(
                        start_sequence, 100,
                        "StreamStart should echo replica's last_sequence"
                    );
                }
                other => panic!("expected StreamStart, got {other:?}"),
            }

            // Read an InputBatch — should be for events AFTER 100.
            let frame = read_frame(&mut reader, MAX_DATA_FRAME).unwrap();
            let slots: Vec<InputSlot> = try_decode_input_batch(&frame).expect("decode InputBatch");
            let end_sequence = slots
                .last()
                .map(|s| s.sequence)
                .expect("InputBatch carried at least one slot");
            assert!(
                end_sequence > 100,
                "InputBatch should be after replica's last_sequence"
            );
        });

        // Primary side.
        let mut p_reader = primary_stream.try_clone().unwrap();
        let mut p_writer = primary_stream;
        let mut buf = Vec::new();

        // Read handshake.
        let frame = read_frame(&mut p_reader, MAX_CONTROL_FRAME).unwrap();
        let handshake = match decode_replica_message(&frame).unwrap() {
            ReplicaMessage::Handshake(h) => h,
            _ => panic!("expected Handshake"),
        };
        assert_eq!(handshake.last_sequence, 100);
        assert_eq!(handshake.chain_hash, [0xBB; 32]);

        // Send StreamStart echoing the replica's sequence.
        encode_stream_start(handshake.last_sequence, 1, [0u8; 32], Some(0), 0, &mut buf);
        p_writer.write_all(&buf).unwrap();
        p_writer.flush().unwrap();
        buf.clear();

        // Send InputBatch with sequence 150 (after replica's 100).
        encode_input_batch_with_seq(150, &mut buf);
        p_writer.write_all(&buf).unwrap();
        p_writer.flush().unwrap();

        replica_handle.join().unwrap();
    }

    /// The receiver's stale-primary refusal: a replica that has observed
    /// epoch 5 must refuse a `StreamStart` advertising a *lower* epoch
    /// rather than follow that (divergent) lineage on top of its newer
    /// state. This reproduces the exact decision `run_receiver` makes on
    /// the normal-resume `StreamStart` — decode the frame off the wire,
    /// then `fence_state.refuses_primary(epoch)` — over a real socket so
    /// the epoch wire field and the policy are exercised together.
    ///
    /// In a current-build cluster this branch is a *second* line of
    /// defense: a primary reads the replica's higher-epoch handshake and
    /// self-demotes before it ever sends a `StreamStart` (see
    /// `tcp_sender::handle_replica_connection`). The receiver check guards
    /// the case where the primary does *not* fence — a non-fencing or
    /// older build — which is exactly what the mock primary below is.
    #[test]
    fn receiver_refuses_stream_start_from_stale_primary() {
        use std::os::unix::net::UnixStream;

        use melin_transport_core::fence::FenceState;

        let (primary_stream, replica_stream) = UnixStream::pair().unwrap();

        // Mock primary that — unlike a current build — does *not* fence on
        // the handshake and streams its stale lineage anyway. It advertises
        // epoch 3 on the StreamStart.
        const STALE_EPOCH: u64 = 3;
        let primary_handle = std::thread::spawn(move || {
            let mut reader = primary_stream.try_clone().unwrap();
            let mut writer = primary_stream;

            let frame = read_frame(&mut reader, MAX_CONTROL_FRAME).unwrap();
            let handshake = match decode_replica_message(&frame).unwrap() {
                ReplicaMessage::Handshake(h) => h,
                _ => panic!("expected Handshake"),
            };
            // The replica truthfully advertises its higher epoch; a correct
            // primary would fence here. This mock deliberately doesn't.
            assert_eq!(handshake.epoch, 5, "replica advertises its real epoch");

            let mut buf = Vec::new();
            encode_stream_start(
                handshake.last_sequence,
                1,
                [0u8; 32],
                Some(0),
                STALE_EPOCH,
                &mut buf,
            );
            writer.write_all(&buf).unwrap();
            writer.flush().unwrap();
        });

        // Replica side: epoch 5, mirroring `run_receiver`'s handshake +
        // StreamStart handling.
        let fence_state = Arc::new(FenceState::new(5));
        let mut r_reader = replica_stream.try_clone().unwrap();
        let mut r_writer = replica_stream;

        let mut buf = Vec::new();
        encode_handshake(
            &Handshake {
                last_sequence: 0,
                chain_hash: [0u8; 32],
                epoch: fence_state.epoch(),
            },
            &mut buf,
        );
        r_writer.write_all(&buf).unwrap();
        r_writer.flush().unwrap();

        let frame = read_frame(&mut r_reader, MAX_CONTROL_FRAME).unwrap();
        let stream_epoch = match decode_primary_message(&frame).unwrap() {
            PrimaryMessage::StreamStart { epoch, .. } => epoch,
            other => panic!("expected StreamStart, got {other:?}"),
        };
        assert_eq!(stream_epoch, STALE_EPOCH, "epoch must survive the wire");

        // This is the receiver's refusal decision. A stale primary must be
        // refused; the replica's own epoch must not have been lowered.
        assert!(
            fence_state.refuses_primary(stream_epoch),
            "replica at epoch 5 must refuse a StreamStart from epoch {STALE_EPOCH}"
        );
        assert_eq!(fence_state.epoch(), 5, "refusal must not lower our epoch");

        // Sanity: an equal or newer epoch is followed, not refused (the
        // ex-primary-rejoin path). Guards against an inverted comparison.
        assert!(!fence_state.refuses_primary(5), "same tenure is followed");
        assert!(!fence_state.refuses_primary(6), "newer primary is followed");

        primary_handle.join().unwrap();
    }

    #[test]
    fn snapshot_begin_encode_decode_round_trip() {
        let mut buf = Vec::new();
        encode_snapshot_begin(1_000_000, 42, &[0xAB; 32], &mut buf);

        let payload = &buf[4..];
        let msg = decode_primary_message(payload).unwrap();
        match msg {
            PrimaryMessage::SnapshotBegin {
                snapshot_len,
                snap_sequence,
                snap_chain_hash,
            } => {
                assert_eq!(snapshot_len, 1_000_000);
                assert_eq!(snap_sequence, 42);
                assert_eq!(snap_chain_hash, [0xAB; 32]);
            }
            _ => panic!("expected SnapshotBegin"),
        }
    }

    #[test]
    fn snapshot_chunk_encode_decode_round_trip() {
        let data = vec![1, 2, 3, 4, 5, 6, 7, 8];
        let mut buf = Vec::new();
        encode_snapshot_chunk(&data, &mut buf);

        let payload = &buf[4..];
        let msg = decode_primary_message(payload).unwrap();
        match msg {
            PrimaryMessage::SnapshotChunk(chunk) => {
                assert_eq!(chunk, data);
            }
            _ => panic!("expected SnapshotChunk"),
        }
    }

    #[test]
    fn snapshot_end_encode_decode_round_trip() {
        let mut buf = Vec::new();
        encode_snapshot_end(0xDEADBEEF, &mut buf);

        let payload = &buf[4..];
        let msg = decode_primary_message(payload).unwrap();
        match msg {
            PrimaryMessage::SnapshotEnd { crc32c } => {
                assert_eq!(crc32c, 0xDEADBEEF);
            }
            _ => panic!("expected SnapshotEnd"),
        }
    }

    /// Simulate the receiver side of a snapshot transfer where the
    /// advertised snap_len doesn't match the actual bytes sent.
    /// The receiver must detect this and return an error.
    #[test]
    fn snapshot_receiver_detects_length_mismatch() {
        use std::os::unix::net::UnixStream;

        let (primary_stream, replica_stream) = UnixStream::pair().unwrap();

        // Receiver thread — reads NeedSnapshot, then the snapshot transfer.
        let receiver = std::thread::spawn(move || -> String {
            let mut reader = replica_stream.try_clone().unwrap();

            // Read NeedSnapshot.
            let frame = read_frame(&mut reader, MAX_CONTROL_FRAME).unwrap();
            assert!(matches!(
                decode_primary_message(&frame).unwrap(),
                PrimaryMessage::NeedSnapshot,
            ));

            // Read SnapshotBegin.
            let frame = read_frame(&mut reader, MAX_CONTROL_FRAME).unwrap();
            let (snap_len, _snap_sequence, _snap_chain_hash) =
                match decode_primary_message(&frame).unwrap() {
                    PrimaryMessage::SnapshotBegin {
                        snapshot_len,
                        snap_sequence,
                        snap_chain_hash,
                    } => (snapshot_len, snap_sequence, snap_chain_hash),
                    other => panic!("expected SnapshotBegin, got {other:?}"),
                };

            // Receive chunks and check length at SnapshotEnd.
            let mut received: u64 = 0;
            loop {
                let frame = read_frame(&mut reader, MAX_DATA_FRAME).unwrap();
                match decode_primary_message(&frame).unwrap() {
                    PrimaryMessage::SnapshotChunk(data) => {
                        received += data.len() as u64;
                    }
                    PrimaryMessage::SnapshotEnd { .. } => {
                        if received != snap_len {
                            return format!(
                                "snapshot length mismatch: expected {snap_len} bytes, got {received}"
                            );
                        }
                        return String::new(); // no error
                    }
                    other => panic!("unexpected message: {other:?}"),
                }
            }
        });

        // Primary side — send snapshot with wrong advertised length.
        let mut writer = primary_stream;
        let mut buf = Vec::new();

        let actual_data = vec![0xAA; 100];
        let wrong_len = 999u64; // advertise 999 bytes, send only 100

        encode_need_snapshot(&mut buf);
        std::io::Write::write_all(&mut writer, &buf).unwrap();
        buf.clear();

        encode_snapshot_begin(wrong_len, 42, &[0xBB; 32], &mut buf);
        std::io::Write::write_all(&mut writer, &buf).unwrap();
        buf.clear();

        encode_snapshot_chunk(&actual_data, &mut buf);
        std::io::Write::write_all(&mut writer, &buf).unwrap();
        buf.clear();

        let crc = crc32c::crc32c(&actual_data);
        encode_snapshot_end(crc, &mut buf);
        std::io::Write::write_all(&mut writer, &buf).unwrap();
        std::io::Write::flush(&mut writer).unwrap();

        let error_msg = receiver.join().unwrap();
        assert!(
            error_msg.contains("length mismatch"),
            "expected length mismatch error, got: {error_msg:?}"
        );
    }

    /// Simulate the receiver side of a snapshot transfer where the CRC
    /// in SnapshotEnd doesn't match the actual data. The receiver must
    /// detect and reject the transfer.
    #[test]
    fn snapshot_receiver_detects_crc_mismatch() {
        use std::os::unix::net::UnixStream;

        let (primary_stream, replica_stream) = UnixStream::pair().unwrap();

        let receiver = std::thread::spawn(move || -> String {
            let mut reader = replica_stream.try_clone().unwrap();

            // Read NeedSnapshot.
            let frame = read_frame(&mut reader, MAX_CONTROL_FRAME).unwrap();
            assert!(matches!(
                decode_primary_message(&frame).unwrap(),
                PrimaryMessage::NeedSnapshot,
            ));

            // Read SnapshotBegin.
            let frame = read_frame(&mut reader, MAX_CONTROL_FRAME).unwrap();
            let snap_len = match decode_primary_message(&frame).unwrap() {
                PrimaryMessage::SnapshotBegin { snapshot_len, .. } => snapshot_len,
                other => panic!("expected SnapshotBegin, got {other:?}"),
            };

            // Receive chunks, verify CRC at SnapshotEnd.
            let mut received_data = Vec::new();
            let mut received: u64 = 0;
            loop {
                let frame = read_frame(&mut reader, MAX_DATA_FRAME).unwrap();
                match decode_primary_message(&frame).unwrap() {
                    PrimaryMessage::SnapshotChunk(data) => {
                        received += data.len() as u64;
                        received_data.extend_from_slice(&data);
                    }
                    PrimaryMessage::SnapshotEnd {
                        crc32c: expected_crc,
                    } => {
                        if received != snap_len {
                            return format!("length mismatch: {snap_len} vs {received}");
                        }
                        let actual_crc = crc32c::crc32c(&received_data);
                        if actual_crc != expected_crc {
                            return format!(
                                "CRC mismatch: expected {expected_crc:#x}, got {actual_crc:#x}"
                            );
                        }
                        return String::new();
                    }
                    other => panic!("unexpected message: {other:?}"),
                }
            }
        });

        // Primary side — send correct length but wrong CRC.
        let mut writer = primary_stream;
        let mut buf = Vec::new();

        let data = vec![0xAA; 100];

        encode_need_snapshot(&mut buf);
        std::io::Write::write_all(&mut writer, &buf).unwrap();
        buf.clear();

        encode_snapshot_begin(data.len() as u64, 42, &[0xBB; 32], &mut buf);
        std::io::Write::write_all(&mut writer, &buf).unwrap();
        buf.clear();

        encode_snapshot_chunk(&data, &mut buf);
        std::io::Write::write_all(&mut writer, &buf).unwrap();
        buf.clear();

        // Send a wrong CRC (flip bits).
        let wrong_crc = !crc32c::crc32c(&data);
        encode_snapshot_end(wrong_crc, &mut buf);
        std::io::Write::write_all(&mut writer, &buf).unwrap();
        std::io::Write::flush(&mut writer).unwrap();

        let error_msg = receiver.join().unwrap();
        assert!(
            error_msg.contains("CRC mismatch"),
            "expected CRC mismatch error, got: {error_msg:?}"
        );
    }

    /// The receiver verifies the chain hash from the loaded snapshot
    /// matches the one advertised in SnapshotBegin. Simulate a mismatch.
    #[test]
    fn snapshot_receiver_detects_chain_hash_mismatch() {
        use std::os::unix::net::UnixStream;

        let (primary_stream, replica_stream) = UnixStream::pair().unwrap();

        let receiver = std::thread::spawn(move || -> String {
            let mut reader = replica_stream.try_clone().unwrap();

            let frame = read_frame(&mut reader, MAX_CONTROL_FRAME).unwrap();
            assert!(matches!(
                decode_primary_message(&frame).unwrap(),
                PrimaryMessage::NeedSnapshot,
            ));

            let frame = read_frame(&mut reader, MAX_CONTROL_FRAME).unwrap();
            let (snap_len, _snap_sequence, snap_chain_hash) =
                match decode_primary_message(&frame).unwrap() {
                    PrimaryMessage::SnapshotBegin {
                        snapshot_len,
                        snap_sequence,
                        snap_chain_hash,
                    } => (snapshot_len, snap_sequence, snap_chain_hash),
                    other => panic!("expected SnapshotBegin, got {other:?}"),
                };

            // Receive the snapshot data.
            let mut received_data = Vec::new();
            let mut received: u64 = 0;
            loop {
                let frame = read_frame(&mut reader, MAX_DATA_FRAME).unwrap();
                match decode_primary_message(&frame).unwrap() {
                    PrimaryMessage::SnapshotChunk(data) => {
                        received += data.len() as u64;
                        received_data.extend_from_slice(&data);
                    }
                    PrimaryMessage::SnapshotEnd {
                        crc32c: expected_crc,
                    } => {
                        assert_eq!(received, snap_len, "length should match");
                        let actual_crc = crc32c::crc32c(&received_data);
                        assert_eq!(actual_crc, expected_crc, "CRC should match");
                        break;
                    }
                    other => panic!("unexpected message: {other:?}"),
                }
            }

            // Simulate chain hash verification: the loaded snapshot would
            // have a different chain hash than what SnapshotBegin advertised.
            let loaded_hash = [0xFF; 32]; // different from snap_chain_hash
            if loaded_hash != snap_chain_hash {
                return format!(
                    "snapshot chain hash mismatch: primary sent {snap_chain_hash:02x?}, \
                     loaded snapshot has {loaded_hash:02x?}"
                );
            }
            String::new()
        });

        // Primary side — send valid snapshot but with a chain hash in
        // SnapshotBegin that won't match what the replica "loads".
        let mut writer = primary_stream;
        let mut buf = Vec::new();

        let data = vec![0xAA; 64];
        // Advertise chain hash [0xBB; 32] — receiver will "load" [0xFF; 32].
        let advertised_hash = [0xBB; 32];

        encode_need_snapshot(&mut buf);
        std::io::Write::write_all(&mut writer, &buf).unwrap();
        buf.clear();

        encode_snapshot_begin(data.len() as u64, 10, &advertised_hash, &mut buf);
        std::io::Write::write_all(&mut writer, &buf).unwrap();
        buf.clear();

        encode_snapshot_chunk(&data, &mut buf);
        std::io::Write::write_all(&mut writer, &buf).unwrap();
        buf.clear();

        let crc = crc32c::crc32c(&data);
        encode_snapshot_end(crc, &mut buf);
        std::io::Write::write_all(&mut writer, &buf).unwrap();
        std::io::Write::flush(&mut writer).unwrap();

        let error_msg = receiver.join().unwrap();
        assert!(
            error_msg.contains("chain hash mismatch"),
            "expected chain hash mismatch error, got: {error_msg:?}"
        );
    }

    /// Primary-side magic validation: a file without the SNAP magic
    /// (0x534E4150) must be rejected before transfer.
    #[test]
    fn primary_rejects_snapshot_with_invalid_magic() {
        let tmp = tempfile::tempdir().unwrap();
        let snap_path = tmp.path().join("test.snapshot");

        // Write a file with wrong magic but enough bytes for a header.
        let mut bad_snap = vec![0u8; 64];
        bad_snap[0..4].copy_from_slice(&0xDEAD_BEEFu32.to_le_bytes()); // wrong magic

        std::fs::write(&snap_path, &bad_snap).unwrap();

        // Replicate the primary's validation logic.
        let snap_data = std::fs::read(&snap_path).unwrap();
        assert!(
            snap_data.len() >= 48,
            "file should be big enough for header"
        );

        let magic = u32::from_le_bytes(snap_data[0..4].try_into().unwrap());
        assert_ne!(magic, 0x534E_4150);
        assert_eq!(magic, 0xDEAD_BEEF);
    }

    /// Primary-side: a snapshot file smaller than the 48-byte header
    /// must be rejected.
    #[test]
    fn primary_rejects_snapshot_too_small_for_header() {
        let tmp = tempfile::tempdir().unwrap();
        let snap_path = tmp.path().join("test.snapshot");

        // Write a file smaller than the 48-byte header.
        std::fs::write(&snap_path, [0u8; 20]).unwrap();

        let snap_data = std::fs::read(&snap_path).unwrap();
        assert!(
            snap_data.len() < 48,
            "file must be too small for header validation"
        );
    }

    #[test]
    fn decode_snapshot_begin_too_short() {
        // SnapshotBegin needs type(1) + snapshot_len(8) + snap_sequence(8) + chain_hash(32) = 49.
        // Send only the type byte + a few extra bytes.
        let payload = [MSG_SNAPSHOT_BEGIN, 0x01, 0x02, 0x03];
        let err = decode_primary_message(&payload).unwrap_err();
        assert!(
            err.to_string().contains("SnapshotBegin too short"),
            "expected 'SnapshotBegin too short', got: {err}"
        );
    }

    #[test]
    fn decode_snapshot_end_too_short() {
        // SnapshotEnd needs type(1) + crc32c(4) = 5. Send only the type byte.
        let payload = [MSG_SNAPSHOT_END];
        let err = decode_primary_message(&payload).unwrap_err();
        assert!(
            err.to_string().contains("SnapshotEnd too short"),
            "expected 'SnapshotEnd too short', got: {err}"
        );
    }

    #[test]
    fn decode_snapshot_chunk_empty_data() {
        // SnapshotChunk with just the type byte — valid but empty payload.
        let payload = [MSG_SNAPSHOT_CHUNK];
        let msg = decode_primary_message(&payload).unwrap();
        match msg {
            PrimaryMessage::SnapshotChunk(data) => {
                assert!(data.is_empty());
            }
            _ => panic!("expected SnapshotChunk"),
        }
    }

    // --- PendingAckQueue tests ---

    fn make_journal_cursor(val: u64) -> melin_pipeline::padding::Sequence {
        melin_pipeline::padding::Sequence::new(AtomicU64::new(val))
    }

    #[test]
    fn pending_ack_queue_push_and_pop_ready() {
        let mut q = PendingAckQueue::new(8);
        assert!(q.is_empty());
        assert!(!q.is_full());

        q.push(10, 100);
        q.push(20, 200);
        assert!(!q.is_empty());

        // Cursor at 5 — neither ready.
        let cursor = make_journal_cursor(5);
        assert!(q.pop_ready(&cursor).is_none());

        // Cursor at 15 — first ready, second not.
        cursor.get().store(15, Ordering::Relaxed);
        assert_eq!(q.pop_ready(&cursor), Some(100));
        // Only one popped — second still pending.
        assert!(!q.is_empty());

        // Cursor at 25 — second now ready.
        cursor.get().store(25, Ordering::Relaxed);
        assert_eq!(q.pop_ready(&cursor), Some(200));
        assert!(q.is_empty());
    }

    #[test]
    fn pending_ack_queue_pop_ready_returns_highest_sequence() {
        // When multiple acks become ready simultaneously, pop_ready
        // returns the highest acked_sequence (ack semantics are
        // cumulative — "everything up to this sequence is durable").
        let mut q = PendingAckQueue::new(8);
        q.push(10, 100);
        q.push(20, 200);
        q.push(30, 300);

        let cursor = make_journal_cursor(30);
        assert_eq!(q.pop_ready(&cursor), Some(300));
        assert!(q.is_empty());
    }

    #[test]
    fn pending_ack_queue_capacity_and_full() {
        let mut q = PendingAckQueue::new(8);
        for i in 0..8 {
            assert!(!q.is_full());
            q.push(i as u64 + 1, (i + 1) as u64 * 100);
        }
        assert!(q.is_full());
    }

    /// A full queue must absorb the push by merging it into the newest
    /// entry — never block, never drop, and never release a sequence
    /// before its own journal target. The merge is a *coarsening*: the
    /// entry it absorbed is acked later than it would have been.
    #[test]
    fn pending_ack_queue_merges_into_the_newest_entry_when_full() {
        let mut q = PendingAckQueue::new(4);
        for i in 1..=4u64 {
            q.push(i * 10, i * 100);
        }
        assert!(q.is_full());
        assert_eq!(q.merged(), 0);

        // Fifth batch: target 50, sequence 500. Merges into (40, 400).
        q.push(50, 500);
        assert_eq!(q.merged(), 1);
        assert_eq!(q.len(), 4, "merge must not grow the queue");

        // Cursor at 40 would have released 400 before the merge; now the
        // newest entry needs 50, so 300 is the highest safe ack.
        let cursor = make_journal_cursor(40);
        assert_eq!(q.pop_ready(&cursor), Some(300));
        assert!(!q.is_empty(), "merged entry is not durable yet");

        // Cursor reaches the merged target — the merged entry releases
        // the newer sequence, subsuming the one it absorbed.
        cursor.get().store(50, Ordering::Relaxed);
        assert_eq!(q.pop_ready(&cursor), Some(500));
        assert!(q.is_empty());
    }

    /// Repeated merges keep collapsing onto the same slot, so a journal
    /// stall of any length costs granularity and nothing else.
    #[test]
    fn pending_ack_queue_merges_repeatedly_without_growing() {
        let mut q = PendingAckQueue::new(2);
        for i in 1..=100u64 {
            q.push(i, i * 10);
        }
        assert_eq!(q.len(), 2);
        assert_eq!(q.merged(), 98);

        // Nothing releases before its target...
        let cursor = make_journal_cursor(99);
        assert_eq!(q.pop_ready(&cursor), Some(10), "only the first entry");
        // ...and the merged tail carries the newest pair.
        cursor.get().store(100, Ordering::Relaxed);
        assert_eq!(q.pop_ready(&cursor), Some(1000));
    }

    #[test]
    fn pending_ack_queue_pop_oldest_blocking() {
        let mut q = PendingAckQueue::new(8);
        q.push(10, 100);
        q.push(20, 200);

        // Cursor already past both targets — pop_oldest_blocking
        // returns immediately.
        let cursor = make_journal_cursor(25);
        let seq = q.pop_oldest_blocking(
            &cursor,
            melin_pipeline::wait::WaitStrategy::BusySpin,
            &AtomicBool::new(false),
        );
        // Should pop both (oldest + any others that became ready).
        assert_eq!(seq, Some(200));
        assert!(q.is_empty());
    }

    /// A dead journal stage (abort latch set) must abort the blocking
    /// wait instead of spinning forever on the frozen cursor.
    #[test]
    fn pending_ack_queue_blocking_wait_aborts_on_journal_failure() {
        let mut q = PendingAckQueue::new(8);
        q.push(10, 100);

        // Cursor frozen BELOW the target; abort pre-latched.
        let cursor = make_journal_cursor(5);
        let abort = AtomicBool::new(true);
        assert_eq!(
            q.pop_oldest_blocking(
                &cursor,
                melin_pipeline::wait::WaitStrategy::BusySpin,
                &abort
            ),
            None
        );
        assert!(!q.is_empty(), "aborted wait must not pop the entry");
        assert_eq!(
            q.pop_all_blocking(
                &cursor,
                melin_pipeline::wait::WaitStrategy::BusySpin,
                &abort
            ),
            None
        );
    }

    /// `shutdown_pipeline` must surface the journal stage's error kind —
    /// the orchestrator routes `ReplicaChainDivergence` into in-process
    /// resync and everything else into process exit, so collapsing the
    /// error into a None (the old behavior) breaks that dispatch. It
    /// must also join EVERY thread in the failure arms: the resync path
    /// archives journal + snapshot right after teardown, and a
    /// still-running shadow thread mid-snapshot-write would race the
    /// renames.
    #[test]
    fn shutdown_pipeline_surfaces_journal_error_kind() {
        let flag = AtomicBool::new(false);
        let matching_joined = Arc::new(AtomicBool::new(false));
        let mj = Arc::clone(&matching_joined);

        let journal = std::thread::spawn(|| -> Result<u32, melin_journal::JournalError> {
            Err(melin_journal::JournalError::ReplicaChainDivergence {
                sequence: 42,
                expected: [1u8; 32],
                actual: [2u8; 32],
            })
        });
        let matching = std::thread::spawn(move || {
            mj.store(true, Ordering::Release);
            7u64
        });
        let drain = std::thread::spawn(|| {});

        let outcome = shutdown_pipeline::<u64, u32>(&flag, journal, matching, drain, None);
        assert!(flag.load(Ordering::Acquire), "shutdown flag must be set");
        assert!(
            matching_joined.load(Ordering::Acquire),
            "matching thread must be joined even when the journal failed"
        );
        match outcome {
            TeardownOutcome::JournalFailed(
                melin_journal::JournalError::ReplicaChainDivergence { sequence, .. },
            ) => assert_eq!(sequence, 42),
            _ => panic!("expected JournalFailed(ReplicaChainDivergence)"),
        }
    }

    #[test]
    fn shutdown_pipeline_clean_returns_both_states() {
        let flag = AtomicBool::new(false);
        let journal =
            std::thread::spawn(|| -> Result<u32, melin_journal::JournalError> { Ok(11u32) });
        let matching = std::thread::spawn(|| 7u64);
        let drain = std::thread::spawn(|| {});

        match shutdown_pipeline::<u64, u32>(&flag, journal, matching, drain, None) {
            TeardownOutcome::Clean(app, writer) => {
                assert_eq!(app, 7);
                assert_eq!(writer, 11);
            }
            _ => panic!("expected Clean"),
        }
    }

    #[test]
    fn shutdown_pipeline_panicked_stage_reports_panicked() {
        let flag = AtomicBool::new(false);
        let journal =
            std::thread::spawn(|| -> Result<u32, melin_journal::JournalError> { Ok(11u32) });
        let matching = std::thread::spawn(|| -> u64 { panic!("matching stage died") });
        let drain = std::thread::spawn(|| {});

        assert!(matches!(
            shutdown_pipeline::<u64, u32>(&flag, journal, matching, drain, None),
            TeardownOutcome::Panicked
        ));
    }

    /// `ReplicaPipelineHandles` over a real input ring (one gate
    /// consumer) with immediately-returning stage threads — just enough
    /// structure for `teardown_replica_pipeline` to run for real. The
    /// returned consumer observes what teardown published.
    fn teardown_fixture(
        capacity: usize,
    ) -> (
        ReplicaPipelineHandles<counter_server::Counter, u32>,
        melin_pipeline::ring::Consumer<InputSlot>,
    ) {
        handles_fixture(capacity, fsync_state_at(0, 0).1)
    }

    /// A seqlock holding fsync state at `journal_seq` and ring position
    /// `ring_pos`, as the journal stage would publish it.
    fn fsync_state_at(
        journal_seq: u64,
        ring_pos: u64,
    ) -> (
        melin_pipeline::seqlock::SeqLockWriter<melin_transport_core::pipeline::FsyncState>,
        melin_pipeline::seqlock::SeqLockReader<melin_transport_core::pipeline::FsyncState>,
    ) {
        melin_pipeline::seqlock::split(melin_transport_core::pipeline::FsyncState {
            journal_seq: melin_transport_core::WireSeq::new(journal_seq),
            chain_hash: [0u8; 32],
            input_ring_seq: melin_transport_core::cursors::RingPos::new(ring_pos),
        })
    }

    /// [`teardown_fixture`] reading the given fsync state.
    fn handles_fixture(
        capacity: usize,
        chain_hash_lock: melin_pipeline::seqlock::SeqLockReader<
            melin_transport_core::pipeline::FsyncState,
        >,
    ) -> (
        ReplicaPipelineHandles<counter_server::Counter, u32>,
        melin_pipeline::ring::Consumer<InputSlot>,
    ) {
        let (input_producer, mut consumers) =
            melin_pipeline::ring::DisruptorBuilder::<InputSlot>::new(capacity)
                .add_consumer()
                .build(melin_pipeline::wait::WaitStrategy::SpinThenYield);
        let consumer = consumers.pop().expect("one consumer");
        let handles = ReplicaPipelineHandles {
            input_producer,
            journal_cursor: Arc::new(make_journal_cursor(0)),
            chain_hash_lock,
            stream_marks: Arc::new(std::sync::Mutex::new(std::collections::VecDeque::new())),
            genesis_entries: None,
            journal_failed: Arc::new(AtomicBool::new(false)),
            pipeline_shutdown: Arc::new(AtomicBool::new(false)),
            journal_handle: std::thread::spawn(|| -> Result<u32, melin_journal::JournalError> {
                Ok(11)
            }),
            matching_handle: std::thread::spawn(counter_server::Counter::default),
            drain_handle: std::thread::spawn(|| {}),
            shadow_handle: None,
        };
        (handles, consumer)
    }

    /// The invariant behind moving the sentinel publish into
    /// `teardown_replica_pipeline`: a healthy teardown always puts the
    /// shutdown sentinel on the ring, so the stages exit by consuming it
    /// through the normal event path (draining anything ahead of it),
    /// not via the flag's emergency-abort branch.
    #[test]
    fn teardown_publishes_shutdown_sentinel() {
        let (handles, mut consumer) = teardown_fixture(8);
        assert!(matches!(
            teardown_replica_pipeline::<counter_server::Counter, u32>(handles),
            TeardownOutcome::Clean(..)
        ));
        let (_, slot) = consumer.try_consume().expect("sentinel on the ring");
        assert!(
            slot.event.is_shutdown(),
            "published slot must be the sentinel"
        );
        assert!(consumer.try_consume().is_none(), "exactly one sentinel");
    }

    /// The scheduling policy a running thread has now.
    fn policy_of<T>(handle: &std::thread::JoinHandle<T>) -> libc::c_int {
        use std::os::unix::thread::JoinHandleExt;
        let mut policy: libc::c_int = 0;
        let mut param = libc::sched_param { sched_priority: 0 };
        let ret =
            unsafe { libc::pthread_getschedparam(handle.as_pthread_t(), &mut policy, &mut param) };
        assert_eq!(ret, 0, "pthread_getschedparam failed: {ret}");
        policy
    }

    /// Every thread the replica pipeline builder spawns starts in the
    /// scheduling context of its own placement, not in the builder's.
    ///
    /// A new thread inherits its creator's CPU mask and policy, and Linux
    /// offers no way to change them before it first runs. On a tuned host
    /// the builder can sit on an isolated core: the receiver after a
    /// rebuild, or any thread whose home mask includes isolated CPUs. A
    /// child created there has to run once to pin itself elsewhere, and
    /// if a real-time thread spins on that core it never does — the stage
    /// never starts, and the replica wedges when its input ring fills.
    /// `JournalStage::start` hands the disk thread and the preparer their
    /// context before they exist for that reason; the stage threads need
    /// the same.
    ///
    /// Where a child started cannot be read back once it has pinned
    /// itself, but its policy can (see `melin_test_node::sched`): the
    /// builder runs at `SCHED_BATCH`, and a child that inherited its
    /// context keeps it after its own pin, while one handed its own starts
    /// at `SCHED_OTHER`. The end-to-end form, both nodes on both
    /// transports, is `tests/stall_guards.rs`.
    #[test]
    fn replica_pipeline_threads_start_in_their_own_scheduling_context() {
        let Some(core) = melin_test_node::sched::first_pinnable_cpu() else {
            // No core to place a child on but the `0` sentinel.
            eprintln!("skipped: this thread may run on CPU 0 only");
            return;
        };
        let dir = tempfile::tempdir().expect("tempdir");

        let policies = std::thread::spawn(move || {
            // The context no child may start in.
            melin_test_node::sched::set_batch_policy();

            let writer = melin_journal::BufferedWriter::<CounterEvent>::create_continuing(
                &dir.path().join("replica.journal"),
                1,
                [7; 32],
                Some(5),
            )
            .expect("create journal");
            // Every thread on the same core, yielding: what matters is
            // that each has a core of its own to be placed on, not which.
            let placed = crate::layout::Placement::yielding(core);
            let cores = crate::layout::PipelineCores {
                journal_seq: placed,
                matching: placed,
                response: placed,
                shadow: placed,
                journal_prep: placed,
                journal_disk: placed,
                ..crate::layout::PipelineCores::unpinned()
            };
            let handles = build_replica_pipeline_with_threads(
                counter_server::Counter::default(),
                writer,
                cores,
                melin_journal::StagingMode::Allocate,
                // Non-zero so the shadow thread is spawned; long enough
                // that it never takes a snapshot.
                3_600_000,
                dir.path().join("replica.snapshot"),
                std::time::Duration::ZERO,
                Arc::new(melin_transport_core::fence::FenceState::new(0)),
                &SharedLineagePolicy::new(None),
                Arc::new(AtomicBool::new(true)),
                &(),
            )
            .expect("build the replica pipeline");

            let policies = [
                ("journal-seq", Some(policy_of(&handles.journal_handle))),
                ("matching", Some(policy_of(&handles.matching_handle))),
                ("drain", Some(policy_of(&handles.drain_handle))),
                (
                    "replica-shadow",
                    handles.shadow_handle.as_ref().map(policy_of),
                ),
            ];
            let outcome = teardown_replica_pipeline(handles);
            assert!(
                matches!(outcome, TeardownOutcome::Clean(..)),
                "the pipeline must tear down cleanly"
            );
            policies
        })
        .join()
        .expect("builder thread");

        let inherited: Vec<&str> = policies
            .iter()
            .map(|(name, policy)| {
                (
                    *name,
                    policy.unwrap_or_else(|| panic!("{name} was not spawned")),
                )
            })
            .filter(|&(_, policy)| policy == libc::SCHED_BATCH)
            .map(|(name, _)| name)
            .collect();
        assert!(
            inherited.is_empty(),
            "{inherited:?} started in the builder's scheduling context instead of their \
             own placement; on an isolated core held by a real-time spinner they would \
             never run"
        );
    }

    /// A session left three slots in the ring that the journal has not
    /// made durable. The resume point waits for the fsync that covers
    /// them rather than handing the handshake the older durable position,
    /// from which the primary would resend them and the next session
    /// would publish them a second time.
    #[test]
    fn resume_point_waits_for_the_journal_to_cover_the_ring() {
        let (mut fsync, reader) = fsync_state_at(10, 0);
        let (mut handles, _consumer) = handles_fixture(8, reader);
        for _ in 0..3 {
            handles.input_producer.publish(InputSlot::default());
        }
        let journal = std::thread::spawn(move || {
            std::thread::sleep(std::time::Duration::from_millis(50));
            fsync.store(melin_transport_core::pipeline::FsyncState {
                journal_seq: melin_transport_core::WireSeq::new(13),
                chain_hash: [0u8; 32],
                input_ring_seq: melin_transport_core::cursors::RingPos::new(3),
            });
        });
        let outcome = handles.settled_resume_point(|| false);
        journal.join().unwrap();
        assert!(
            matches!(outcome, ResumePoint::Settled(s) if s.journal_seq.get() == 13),
            "resumes past the ring's slots"
        );
    }

    /// Nothing published since the fsync state was last updated (here,
    /// its seed): the resume point is that state, at once.
    #[test]
    fn resume_point_with_nothing_in_flight_is_immediate() {
        let (_fsync, reader) = fsync_state_at(10, 0);
        let (handles, _consumer) = handles_fixture(8, reader);
        assert!(matches!(
            handles.settled_resume_point(|| false),
            ResumePoint::Settled(s) if s.journal_seq.get() == 10
        ));
    }

    /// A failed journal stage never covers the ring, and neither does one
    /// the node is abandoning: the wait gives up on either, and says
    /// which.
    #[test]
    fn resume_point_gives_up_on_a_failed_journal_or_an_interrupt() {
        let (_fsync, reader) = fsync_state_at(10, 0);
        let (mut handles, _consumer) = handles_fixture(8, reader);
        handles.input_producer.publish(InputSlot::default());

        assert!(matches!(
            handles.settled_resume_point(|| true),
            ResumePoint::Interrupted
        ));
        handles.journal_failed.store(true, Ordering::Release);
        assert!(matches!(
            handles.settled_resume_point(|| false),
            ResumePoint::JournalFailed
        ));
    }

    /// A journal stage that died after its last durable batch leaves the
    /// ring covered. The wait must still report it dead, ahead of the
    /// covered state: handed that state, the reconnect loop would dial
    /// the primary and take one of its replica slots with a pipeline that
    /// cannot take the session.
    #[test]
    fn resume_point_reports_a_dead_journal_even_when_the_ring_is_covered() {
        let (_fsync, reader) = fsync_state_at(10, 0);
        let (handles, _consumer) = handles_fixture(8, reader);
        handles.journal_failed.store(true, Ordering::Release);
        assert!(matches!(
            handles.settled_resume_point(|| false),
            ResumePoint::JournalFailed
        ));
    }

    /// A journal thread that panics returns no error, so the stage's
    /// wrapper must latch the failure on the way out: otherwise every
    /// wait on the journal's progress, the reconnect's included, spins on
    /// a cursor that will never move.
    #[test]
    fn a_panicked_journal_stage_latches_the_failure() {
        let failed = Arc::new(AtomicBool::new(false));
        let healthy = Arc::new(AtomicBool::new(true));
        let journal_handle = {
            let (failed, healthy) = (Arc::clone(&failed), Arc::clone(&healthy));
            std::thread::spawn(move || {
                run_journal_stage(
                    || -> Result<u32, melin_journal::JournalError> {
                        panic!("journal stage died without returning an error")
                    },
                    &failed,
                    &healthy,
                )
            })
        };
        // The thread's panic is the point; joining reports it.
        assert!(journal_handle.join().is_err(), "the stage panicked");
        assert!(failed.load(Ordering::Acquire), "the failure is latched");
        assert!(
            !healthy.load(Ordering::Acquire),
            "the node reports unhealthy"
        );
    }

    /// End to end over the handles: a slot the panicked stage will never
    /// cover is in the ring, and the reconnect's wait must end anyway
    /// rather than spin until shutdown.
    #[test]
    fn resume_point_ends_when_the_journal_thread_panicked() {
        let (_fsync, reader) = fsync_state_at(10, 0);
        let (mut handles, _consumer) = handles_fixture(8, reader);
        handles.input_producer.publish(InputSlot::default());
        let failed = Arc::clone(&handles.journal_failed);
        let healthy = Arc::new(AtomicBool::new(true));
        let dead = std::thread::spawn(move || {
            run_journal_stage(
                || -> Result<u32, melin_journal::JournalError> { panic!("journal stage died") },
                &failed,
                &healthy,
            )
        });
        assert!(dead.join().is_err(), "the stage panicked");

        // Bounded, so a regression fails instead of hanging the suite.
        let give_up = AtomicBool::new(false);
        let outcome = std::thread::scope(|s| {
            let wait = s.spawn(|| handles.settled_resume_point(|| give_up.load(Ordering::Relaxed)));
            let deadline = std::time::Instant::now() + std::time::Duration::from_secs(10);
            while !wait.is_finished() && std::time::Instant::now() < deadline {
                std::thread::sleep(std::time::Duration::from_millis(5));
            }
            give_up.store(true, Ordering::Relaxed);
            wait.join().expect("wait thread")
        });
        assert!(
            matches!(outcome, ResumePoint::JournalFailed),
            "the wait must end on the panicked stage, not on the test's give-up"
        );
    }

    /// The reconnect loop's dispatch for a journal found dead by the
    /// wait: the fatal teardown, never a reconnect with the dead
    /// pipeline. Its error stops the replica; the pipeline is gone.
    #[test]
    fn a_journal_found_dead_by_the_wait_takes_the_fatal_path() {
        let dir = tempfile::tempdir().expect("tempdir");
        type Writer = melin_journal::BufferedWriter<CounterEvent>;
        let (input_producer, mut consumers) =
            melin_pipeline::ring::DisruptorBuilder::<InputSlot>::new(4)
                .add_consumer()
                .build(melin_pipeline::wait::WaitStrategy::SpinThenYield);
        let _consumer = consumers.pop().expect("one consumer");
        let handles = ReplicaPipelineHandles {
            input_producer,
            journal_cursor: Arc::new(make_journal_cursor(0)),
            chain_hash_lock: fsync_state_at(0, 0).1,
            stream_marks: Arc::new(std::sync::Mutex::new(std::collections::VecDeque::new())),
            genesis_entries: None,
            journal_failed: Arc::new(AtomicBool::new(true)),
            pipeline_shutdown: Arc::new(AtomicBool::new(false)),
            journal_handle: std::thread::spawn(
                || -> Result<Writer, melin_journal::JournalError> {
                    Err(melin_journal::JournalError::ReplicaSequenceMismatch {
                        expected: 4,
                        actual: 1,
                    })
                },
            ),
            matching_handle: std::thread::spawn(counter_server::Counter::default),
            drain_handle: std::thread::spawn(|| {}),
            shadow_handle: None,
        };
        let mut pipeline = Some(handles);
        let mut divergence_resyncs = 0u32;
        let mut backoff = MAX_BACKOFF;
        let mut closes = 0;
        let after = handle_session_exit::<counter_server::Counter, Writer>(
            journal_failed_while_disconnected(),
            &mut pipeline,
            &mut divergence_resyncs,
            &mut backoff,
            0,
            &dir.path().join("r.journal"),
            &dir.path().join("r.snapshot"),
            &melin_transport_core::fence::FenceState::new(0),
            &SharedLineagePolicy::new(None),
            &AtomicBool::new(false),
            &crate::promotion::PromotionRequest::new(),
            || closes += 1,
            &(),
        );
        let AfterSession::Return(Err(err)) = after else {
            panic!("a refused sequence stops the replica rather than reconnecting or resyncing");
        };
        assert!(
            err.to_string()
                .contains("refused to journal sequence 1: the next sequence is 4"),
            "the returned error names the journal's cause, got: {err}"
        );
        assert!(pipeline.is_none(), "the dead pipeline was torn down");
        assert_eq!(divergence_resyncs, 0);
        // Once, as on every exit (the receivers pass a no-op here, as no
        // session is open while the wait runs).
        assert_eq!(closes, 1);
    }

    /// Every exit ends the connection, exactly once, terminal or not:
    /// a replica that stops or promotes must tell its primary as surely
    /// as one that reconnects (on DPDK nothing else will).
    #[test]
    fn every_session_exit_closes_the_connection_once() {
        type Writer = melin_journal::BufferedWriter<CounterEvent>;
        // A constructor per exit: `SessionExit` is neither `Clone` nor
        // `Debug`, so each case is built fresh and labelled by hand.
        type Case = (&'static str, fn() -> SessionExit);
        let exits: [Case; 6] = [
            ("shutdown", || SessionExit::Shutdown),
            ("promote", || SessionExit::Promote),
            ("fatal", || SessionExit::Fatal("protocol violation".into())),
            ("disconnected", || SessionExit::Disconnected),
            ("stream gap", || SessionExit::StreamGap("gap".into())),
            ("corrupted", || SessionExit::Corrupted("damaged".into())),
        ];
        for (label, exit) in exits {
            let dir = tempfile::tempdir().expect("tempdir");
            let mut pipeline: Option<ReplicaPipelineHandles<counter_server::Counter, Writer>> =
                None;
            let mut divergence_resyncs = 0u32;
            let mut backoff = MAX_BACKOFF;
            // Latched so the reconnecting exits' backoff sleep returns
            // at once.
            let shutdown = AtomicBool::new(true);
            let mut closes = 0;
            // The outcome is beside the point here (with no pipeline the
            // terminal exits return errors); only the close count is.
            let _after = handle_session_exit::<counter_server::Counter, Writer>(
                StreamingResult {
                    exit: exit(),
                    heard_from_primary: true,
                },
                &mut pipeline,
                &mut divergence_resyncs,
                &mut backoff,
                0,
                &dir.path().join("r.journal"),
                &dir.path().join("r.snapshot"),
                &melin_transport_core::fence::FenceState::new(0),
                &SharedLineagePolicy::new(None),
                &shutdown,
                &crate::promotion::PromotionRequest::new(),
                || closes += 1,
                &(),
            );
            assert_eq!(closes, 1, "{label}");
        }
    }

    /// Drive `handle_session_exit` through one `Disconnected` exit with
    /// the given liveness flag and a backoff pre-escalated to
    /// `MAX_BACKOFF`, returning the post-exit backoff. Shutdown is
    /// latched so the backoff sleep returns immediately (the receiver's
    /// loop top would handle it on the next turn); the journal and fence
    /// arguments are inert on the `Disconnected` path.
    fn backoff_after_disconnect(heard_from_primary: bool) -> std::time::Duration {
        backoff_after_exit(SessionExit::Disconnected, heard_from_primary)
    }

    /// As [`backoff_after_disconnect`], for any reconnecting exit.
    fn backoff_after_exit(exit: SessionExit, heard_from_primary: bool) -> std::time::Duration {
        let dir = tempfile::tempdir().expect("tempdir");
        type Writer = melin_journal::BufferedWriter<CounterEvent>;
        let mut pipeline: Option<ReplicaPipelineHandles<counter_server::Counter, Writer>> = None;
        let mut divergence_resyncs = 0u32;
        let mut backoff = MAX_BACKOFF;
        let shutdown = AtomicBool::new(true);
        let promote = crate::promotion::PromotionRequest::new();
        let after = handle_session_exit::<counter_server::Counter, Writer>(
            StreamingResult {
                exit,
                heard_from_primary,
            },
            &mut pipeline,
            &mut divergence_resyncs,
            &mut backoff,
            0,
            &dir.path().join("r.journal"),
            &dir.path().join("r.snapshot"),
            &melin_transport_core::fence::FenceState::new(0),
            &SharedLineagePolicy::new(None),
            &shutdown,
            &promote,
            || {},
            &(),
        );
        assert!(matches!(after, AfterSession::Reconnect));
        backoff
    }

    /// A session in which the primary spoke — even heartbeat-only on a
    /// quiet system — must reset the disconnect backoff. A stale
    /// escalated backoff delays primary-link recovery, and with it the
    /// auto-promotion veto, by up to MAX_BACKOFF.
    #[test]
    fn disconnect_resets_backoff_when_primary_spoke() {
        // Reset to 1s, then doubled for the next attempt by the shared
        // helper — NOT stuck at MAX_BACKOFF.
        assert_eq!(
            backoff_after_disconnect(true),
            std::time::Duration::from_secs(2)
        );
    }

    /// A session with no word from the primary — an instant-drop flap,
    /// or the synthetic result of a session that never started
    /// streaming (local transport init failure) — must keep its
    /// escalated backoff rather than redialing at 1 Hz forever.
    #[test]
    fn disconnect_keeps_escalated_backoff_when_primary_silent() {
        assert_eq!(backoff_after_disconnect(false), MAX_BACKOFF);
    }

    /// A damaged replicated entry proves the primary spoke, so it resets
    /// the backoff: one bit flip after a long clean session (with a
    /// backoff escalated at start-up and never reset since) must not
    /// cost a `MAX_BACKOFF` stall before the entry is fetched again.
    #[test]
    fn corrupted_entry_resets_backoff() {
        assert_eq!(
            backoff_after_exit(SessionExit::Corrupted("damaged slot".into()), true),
            std::time::Duration::from_secs(2)
        );
    }

    /// A stream contiguity break is a reconnect, never a process exit:
    /// the contiguous prefix is committed and the journal is intact, so
    /// the receiver re-handshakes from its durable position and the
    /// primary re-streams the hole. It used to fall through the `Fatal`
    /// arm and take the replica down for a documented-benign primary
    /// handoff corner. The pipeline must be left standing (it is what
    /// the reconnect resumes into) and the backoff reset — the primary
    /// was evidently speaking.
    #[test]
    fn stream_gap_reconnects_and_keeps_pipeline() {
        let dir = tempfile::tempdir().expect("tempdir");
        type Writer = melin_journal::BufferedWriter<CounterEvent>;
        // Same shape as `teardown_fixture`, but with a real writer type:
        // `handle_session_exit` is bounded on `JournalWrite` for the
        // resync arm even though this path never touches it.
        let (input_producer, mut consumers) =
            melin_pipeline::ring::DisruptorBuilder::<InputSlot>::new(4)
                .add_consumer()
                .build(melin_pipeline::wait::WaitStrategy::SpinThenYield);
        let _consumer = consumers.pop().expect("one consumer");
        let writer_path = dir.path().join("w.journal");
        let handles = ReplicaPipelineHandles {
            input_producer,
            journal_cursor: Arc::new(make_journal_cursor(0)),
            chain_hash_lock: fsync_state_at(0, 0).1,
            stream_marks: Arc::new(std::sync::Mutex::new(std::collections::VecDeque::new())),
            genesis_entries: None,
            journal_failed: Arc::new(AtomicBool::new(false)),
            pipeline_shutdown: Arc::new(AtomicBool::new(false)),
            journal_handle: std::thread::spawn(move || Writer::create(&writer_path)),
            matching_handle: std::thread::spawn(counter_server::Counter::default),
            drain_handle: std::thread::spawn(|| {}),
            shadow_handle: None,
        };
        let mut pipeline: Option<ReplicaPipelineHandles<counter_server::Counter, Writer>> =
            Some(handles);
        let mut divergence_resyncs = 0u32;
        let mut backoff = MAX_BACKOFF;
        let shutdown = AtomicBool::new(true);
        let promote = crate::promotion::PromotionRequest::new();
        let mut closed = false;
        let after = handle_session_exit::<counter_server::Counter, Writer>(
            StreamingResult {
                exit: SessionExit::StreamGap("sequence gap: expected 3, got 5".into()),
                heard_from_primary: true,
            },
            &mut pipeline,
            &mut divergence_resyncs,
            &mut backoff,
            2,
            &dir.path().join("r.journal"),
            &dir.path().join("r.snapshot"),
            &melin_transport_core::fence::FenceState::new(0),
            &SharedLineagePolicy::new(None),
            &shutdown,
            &promote,
            || closed = true,
            &(),
        );
        assert!(
            matches!(after, AfterSession::Reconnect),
            "a stream gap must reconnect, not return an error"
        );
        assert!(
            pipeline.is_some(),
            "the pipeline must survive for the reconnect"
        );
        assert!(closed, "the transport must be closed before redialing");
        assert_eq!(
            backoff,
            std::time::Duration::from_secs(2),
            "backoff reset to 1s then doubled by the shared helper"
        );
        assert_eq!(divergence_resyncs, 0, "not a resync");
        if let Some(p) = pipeline.take() {
            let _ = teardown_replica_pipeline::<counter_server::Counter, Writer>(p);
        }
    }

    /// With the journal-failure latch set and the ring full — the state
    /// a dead journal stage leaves behind (its frozen gate cursor means
    /// the ring never drains) — teardown must skip the sentinel and
    /// return, instead of spinning forever on ring backpressure.
    #[test]
    fn teardown_skips_sentinel_when_journal_failed_and_ring_full() {
        let (mut handles, mut consumer) = teardown_fixture(4);
        for seq in 0..4 {
            assert!(
                handles
                    .input_producer
                    .try_publish(InputSlot {
                        sequence: seq,
                        ..InputSlot::default()
                    })
                    .is_ok(),
                "ring must have space while filling"
            );
        }
        assert!(
            handles
                .input_producer
                .try_publish(InputSlot::default())
                .is_err(),
            "ring must be full"
        );
        handles.journal_failed.store(true, Ordering::Release);
        // Would spin forever on the frozen gate without the latch check.
        assert!(matches!(
            teardown_replica_pipeline::<counter_server::Counter, u32>(handles),
            TeardownOutcome::Clean(..)
        ));
        while let Some((_, slot)) = consumer.try_consume() {
            assert!(!slot.event.is_shutdown(), "no sentinel may be published");
        }
    }

    #[test]
    fn pending_ack_queue_wraps_around() {
        let mut q = PendingAckQueue::new(8);
        let cursor = make_journal_cursor(100);

        // Fill and drain multiple times to exercise circular buffer wrap.
        for round in 0..3 {
            for i in 0..8 {
                let target = (round * 8 + i) as u64 + 1;
                q.push(target, target * 10);
            }
            assert!(q.is_full());
            let seq = q.pop_ready(&cursor).expect("should be ready");
            assert_eq!(seq, (round * 8 + 8) as u64 * 10);
            assert!(q.is_empty());
        }
    }

    #[test]
    fn pending_ack_queue_pop_all_blocking_empty() {
        let mut q = PendingAckQueue::new(8);
        let cursor = make_journal_cursor(0);
        assert!(
            q.pop_all_blocking(
                &cursor,
                melin_pipeline::wait::WaitStrategy::BusySpin,
                &AtomicBool::new(false)
            )
            .is_none()
        );
    }

    // --- try_flush_dual_track tests ---

    #[test]
    fn dual_track_returns_none_when_both_tracks_idle() {
        let mut q = PendingAckQueue::new(8);
        let cursor = make_journal_cursor(0);
        assert!(
            try_flush_dual_track(&mut q, &cursor, 0, 0, 0).is_none(),
            "no advance on either track → no ack"
        );
    }

    #[test]
    fn dual_track_fires_on_persisted_advance_only() {
        // Persisted track moves: pop_ready returns 100; in-memory
        // stayed at 50 (last_sent). Expect ack with acked=100,
        // in_memory=50 (no regression — the wire field carries the
        // current sample, not a "no change" marker).
        let mut q = PendingAckQueue::new(8);
        q.push(10, 100);
        let cursor = make_journal_cursor(20);
        let ack = try_flush_dual_track(&mut q, &cursor, 50, 0, 50).expect("persisted advanced");
        assert_eq!(ack.acked_sequence, 100);
        assert_eq!(ack.in_memory_sequence, 50);
    }

    #[test]
    fn dual_track_fires_on_in_memory_advance_only() {
        // Persisted is idle (queue empty): unwrap_or keeps the
        // last-sent acked at 100. In-memory bumped from 100 to 200.
        let mut q = PendingAckQueue::new(8);
        let cursor = make_journal_cursor(0);
        let ack = try_flush_dual_track(&mut q, &cursor, 200, 100, 100).expect("in-memory advanced");
        assert_eq!(ack.acked_sequence, 100);
        assert_eq!(ack.in_memory_sequence, 200);
    }

    #[test]
    fn dual_track_coalesces_until_caller_updates_trackers() {
        // Caller did not advance trackers between calls. The queue
        // popped on call 1; on call 2 it's empty so persisted stays
        // at 100 (unwrap_or). In-memory advanced 50 → 80 between
        // calls. Second call must still fire because tracker is
        // still 50.
        let mut q = PendingAckQueue::new(8);
        q.push(10, 100);
        let cursor = make_journal_cursor(20);
        let ack1 = try_flush_dual_track(&mut q, &cursor, 50, 0, 50).expect("call 1 fires");
        assert_eq!(ack1.acked_sequence, 100);
        // Caller "forgot" to update trackers (simulates send failure).
        let ack2 = try_flush_dual_track(&mut q, &cursor, 80, 0, 50)
            .expect("call 2 fires on in-memory advance");
        assert_eq!(
            ack2.acked_sequence, 0,
            "no new persisted pop → unwrap_or(last_sent)"
        );
        assert_eq!(ack2.in_memory_sequence, 80);
    }

    #[test]
    fn dual_track_no_duplicate_ack_after_backpressure_drain() {
        // Models the backpressure-drain → resume-normal-flush path
        // taken by every receiver when `PendingAckQueue` fills up.
        // The receiver must drain the queue (via the
        // pop_oldest_blocking path in production; pop_ready here),
        // update *both* trackers from the drained ack, and only
        // then resume the normal cursor-driven flush. The bug
        // classes this pins are:
        //   (a) emitting a follow-up ack whose `acked_sequence`
        //       regresses below what the drain already sent
        //       (caught by the debug_assert! in
        //       `try_flush_dual_track`, also asserted at value
        //       level here);
        //   (b) emitting a duplicate ack carrying the same
        //       cursors as the drain when neither track actually
        //       advanced — the leak class fixed by ensuring every
        //       send site updates both `last_sent_acked_seq` and
        //       `last_sent_in_memory_seq`.
        let mut q = PendingAckQueue::new(4);
        // Fill the queue: four pending acks at primary seqs
        // 100, 200, 300, 400 with journal targets 10..=40.
        for i in 1..=4u64 {
            q.push(i * 10, i * 100);
        }
        assert!(q.is_full());

        // Backpressure path: caller drains all ready entries
        // before pushing more. Highest acked seen = 400.
        let cursor = make_journal_cursor(40);
        let drained = q.pop_ready(&cursor).expect("all entries durable");
        assert_eq!(drained, 400);
        assert!(q.is_empty());

        // Caller updates BOTH trackers from the drain. in_memory
        // at the time of the drained batch was 450.
        let mut last_sent_acked = drained;
        let mut last_sent_in_mem = 450u64;

        // Quiescent immediately after drain: no fresh push, no
        // in-memory advance → try_flush must return None.
        // Failing here means we'd emit a duplicate ack carrying
        // the same (400, 450) the backpressure drain already sent.
        assert!(
            try_flush_dual_track(
                &mut q,
                &cursor,
                last_sent_in_mem,
                last_sent_acked,
                last_sent_in_mem,
            )
            .is_none(),
            "no advance on either track after backpressure drain → no duplicate ack",
        );

        // Push a fresh batch (primary seq 500, journal target 50),
        // cursor catches up, in_memory advances to 500.
        q.push(50, 500);
        let cursor2 = make_journal_cursor(50);
        let ack = try_flush_dual_track(&mut q, &cursor2, 500, last_sent_acked, last_sent_in_mem)
            .expect("fresh batch after drain fires a new ack");
        assert!(
            ack.acked_sequence >= last_sent_acked,
            "regression: drain sent {last_sent_acked} but next ack carries {}",
            ack.acked_sequence,
        );
        assert_eq!(ack.acked_sequence, 500);
        assert_eq!(ack.in_memory_sequence, 500);

        last_sent_acked = ack.acked_sequence;
        last_sent_in_mem = ack.in_memory_sequence;

        // Post-resume quiescent: confirms idempotency past the
        // resume point — the second flush sees neither track
        // advance and must stay silent.
        assert!(
            try_flush_dual_track(
                &mut q,
                &cursor2,
                last_sent_in_mem,
                last_sent_acked,
                last_sent_in_mem,
            )
            .is_none(),
            "post-resume idle → no further ack",
        );
    }
}