mbx_cache_core/

agent.rs

use crate::uploads::{ConnectionUploads, UploadQueue, UploadSink};
use crate::{
    ActionPrediction, ActionPromiseCompletion, ActionPromiseState, CacheDigest, CacheDirectory,
    LocalActionCache, LocalCas, MAX_STAGED_BLOB_PACK_BYTES, ManifestPutOutcome, RemoteActionResult,
    RemoteCacheClient, RemoteCacheMode, RustcMetadata, TaskActionManifest, blob_pack_chunk,
    canonical_json,
};
use eyre::{Context, Result, bail};
use futures_util::{FutureExt, StreamExt, future::BoxFuture, stream};
use log::{info, warn};
use std::collections::{BTreeMap, BTreeSet};
use std::fs;
use std::path::{Path, PathBuf};
use std::sync::atomic::{AtomicU64, Ordering};
use std::sync::{Arc, Mutex, Weak};
use std::time::{Duration, Instant, SystemTime};

/// Fleet claims are leases rather than correctness locks. This only bounds how
/// long one shim waits through repeated pending responses before degrading to
/// an ordinary compilation; the server independently expires abandoned claims.
const ACTION_PROMISE_WAIT: Duration = Duration::from_secs(60 * 60);
use tokio::io::{AsyncBufRead, AsyncBufReadExt, AsyncRead, AsyncWrite, AsyncWriteExt, BufReader};

mod file_digest;
mod manifest;
mod prefetch;
mod stats;
mod wire;

#[cfg(test)]
pub(crate) use prefetch::select_prefetch_actions;

pub use file_digest::{
    FileDigestCache, FileDigestScope, FileIdentity, FileSnapshot, NoFileDigestCache,
    RecordedFileDigest,
};
pub use manifest::{is_task_identity, task_manifest_actions};
use manifest::{
    merge_remote_task_manifest, merge_task_manifests, task_manifest_dir, validate_task_identity,
    validate_task_manifest,
};
pub use stats::{AgentStats, CompilerStats};
use wire::MAX_REQUEST_BYTES;
pub use wire::{
    AGENT_PROTOCOL_VERSION, ActionDiagnostic, AgentEvent, AgentEventObserver, AgentRequest,
    AgentResponse, RestoreStats,
};

const MAX_EXECUTABLE_IDENTITIES: usize = 64;
const MAX_EXECUTABLE_IDENTITY_SIZE: usize = 64 * 1024;
const MAX_EXECUTABLE_IDENTITY_BYTES: usize = 256 * 1024;
const TASK_ACTION_MANIFEST_VERSION: u8 = 1;
const MAX_TASK_ACTION_PREDICTIONS: usize = 16 * 1024;
/// Longest shim diagnostic the agent accepts, in bytes.
const MAX_WARNING_BYTES: usize = 4 * 1024;
/// Most distinct shim diagnostics one session surfaces before going quiet.
///
/// A failure mode that repeats across a build tends to repeat with the same
/// message, which deduplication already collapses; the cap only guards
/// against a message that embeds something unique per compilation.
const MAX_WARNINGS: usize = 128;
const ACTION_DIAGNOSTIC_PREFIX: &str = "@mbx-action-diagnostic\t";
/// Most file identities one digest lookup or record may carry.
const MAX_FILE_DIGEST_BATCH: usize = 16 * 1024;
/// Most recorded file digests one session retains across every scope.
///
/// Roughly two orders of magnitude above the largest workspace measured; the
/// cap exists so a pathological build bounds the agent instead of growing it.
const MAX_FILE_DIGEST_ENTRIES: usize = 1024 * 1024;
const MAX_REMOTE_TRANSFERS: usize = 64;
const MAX_PREFETCH_TRANSFERS: usize = 48;
/// Most blob packs downloaded at once.
///
/// Packs are individually bounded to keep their staging footprint predictable.
/// Allowing a few independent streams prevents one large Azure object stream
/// from serializing an entire workspace restore.
const MAX_CONCURRENT_BLOB_PACKS: usize = 8;
/// Most predicted actions whose complete output closures are downloaded
/// speculatively for one task.
///
/// Manifests deliberately retain predictions across compatible builds, so a
/// large workspace can describe far more actions than the next invocation
/// will request. Keep the most expensive recorded actions warm and let
/// foreground lookups fetch only unusually large tails on demand. The remote
/// download-byte budget remains the primary bound on speculative transfer. A
/// 1,024-action ceiling covers the measured large-workspace manifests while
/// retaining a guard against pathological manifests and stale prediction sets.
const MAX_PREFETCH_ACTIONS: usize = 1024;
// Resolve speculative actions progressively so the most valuable predictions
// become usable first and cancellation at the end of a build abandons a small
// tail instead of one workspace-sized transfer wave. Action-result lookup is
// still batched independently; this only bounds each output-closure download.
const MAX_PREFETCH_ACTION_WAVE: usize = 32;
/// Batched action lookups issued at once.
///
/// One request already asks about hundreds of actions. Serial batches keep a
/// fleet of concurrent CI jobs from multiplying metadata pressure while blob
/// transfers from earlier answers are also underway.
const MAX_PREFETCH_BATCH_LOOKUPS: usize = 1;
const PREFETCH_ACTION_BATCH_DELAY: Duration = Duration::from_millis(5);
const MAX_PREFETCH_DIRECTORY_OBJECTS: usize = 100_000;
const MAX_PREFETCH_OBJECTS_PER_WAVE: usize = 100_000;
const DEFAULT_MAX_REMOTE_DOWNLOAD_BYTES: u64 = 5 * 1024 * 1024 * 1024;

/// Names the identities a task may inherit predictions from, consulted only
/// once nothing has been recorded under its own.
pub type TaskFallbacks = Arc<dyn Fn() -> Vec<String> + Send + Sync>;

/// How many of the store's most recently written manifests a task with
/// fallbacks tries after the named identities yield nothing.
///
/// A runner that restored a cache bundle holds the manifests of the builds
/// that produced it and no version-control history to name them by, so what
/// was written last is the best remaining guess at the lockfile before this
/// one. Each candidate costs one parse, and a manifest from an unrelated
/// workspace only fails to match. One is skipped when it fills more than half
/// the prediction limit: an inherited manifest is kept under the new identity,
/// and a foreign one that large would leave this workspace's own recordings
/// no room.
const NEWEST_MANIFEST_CANDIDATES: usize = 8;

/// Remote action-cache access owned by one task session.
pub struct AgentRemoteCache {
    /// Remote protocol client used by the agent.
    pub client: RemoteCacheClient,
    /// Permitted remote read/write operations.
    pub mode: RemoteCacheMode,
    /// Directory used for verified downloads before CAS ingestion.
    pub staging_dir: PathBuf,
}

#[derive(Default)]
struct AtomicAgentStats {
    lookups: AtomicU64,
    unconsulted: AtomicU64,
    hits: AtomicU64,
    stores: AtomicU64,
    stored_bytes: AtomicU64,
    verifications: AtomicU64,
    divergences: AtomicU64,
    downloaded_bytes: AtomicU64,
    uploaded_bytes: AtomicU64,
    background_uploads: AtomicU64,
    background_upload_failures: AtomicU64,
    remote_blob_pack_uploads: AtomicU64,
    remote_blob_pack_upload_blobs: AtomicU64,
    upload_drain_duration_ns: AtomicU64,
    prefetched_actions: AtomicU64,
    predictions_loaded: AtomicU64,
    remote_failures: AtomicU64,
    remote_manifest_lookups: AtomicU64,
    remote_manifest_lookup_duration_ns: AtomicU64,
    remote_action_lookups: AtomicU64,
    remote_action_lookup_duration_ns: AtomicU64,
    remote_blob_requests: AtomicU64,
    remote_blob_pack_requests: AtomicU64,
    remote_blob_pack_blobs: AtomicU64,
    remote_blob_transfer_duration_ns: AtomicU64,
    local_cas_write_duration_ns: AtomicU64,
    prefetch_runs: AtomicU64,
    prefetch_duration_ns: AtomicU64,
    materialization_duration_ns: AtomicU64,
    bypasses: Mutex<BTreeMap<String, u64>>,
    avoided_compiler_duration_ns: AtomicU64,
    compiler: Mutex<BTreeMap<String, CompilerStats>>,
    slow_compilations: Mutex<BTreeMap<String, u64>>,
    restored_output_files: AtomicU64,
    restored_output_bytes: AtomicU64,
    reflinked_output_files: AtomicU64,
    reflinked_output_bytes: AtomicU64,
    copied_output_files: AtomicU64,
    copied_output_bytes: AtomicU64,
    reused_output_files: AtomicU64,
    reused_output_bytes: AtomicU64,
}

struct AtomicDurationTimer<'a> {
    started: Instant,
    target: &'a AtomicU64,
}

impl<'a> AtomicDurationTimer<'a> {
    fn start(target: &'a AtomicU64) -> Self {
        Self {
            started: Instant::now(),
            target,
        }
    }
}

impl Drop for AtomicDurationTimer<'_> {
    fn drop(&mut self) {
        atomic_saturating_add(self.target, duration_ns(self.started));
    }
}

fn duration_ns(started: Instant) -> u64 {
    started.elapsed().as_nanos().try_into().unwrap_or(u64::MAX)
}

fn validate_crate_name(crate_name: Option<&str>) -> Result<()> {
    if let Some(crate_name) = crate_name
        && (crate_name.len() > 256 || crate_name.contains(['\0', '\n', '\r']))
    {
        bail!("invalid compiler crate name");
    }
    Ok(())
}

#[derive(serde::Deserialize)]
struct ActionDiagnosticEnvelope {
    outcome: String,
    crate_name: Option<String>,
    diagnostic: ActionDiagnostic,
}

fn parse_action_diagnostic(
    message: &str,
) -> Option<Result<(String, Option<String>, ActionDiagnostic)>> {
    let payload = message.strip_prefix(ACTION_DIAGNOSTIC_PREFIX)?;
    Some((|| {
        let envelope: ActionDiagnosticEnvelope = serde_json::from_str(payload)?;
        if !matches!(envelope.outcome.as_str(), "hit" | "miss") {
            bail!("invalid action diagnostic outcome");
        }
        validate_crate_name(envelope.crate_name.as_deref())?;
        Ok((envelope.outcome, envelope.crate_name, envelope.diagnostic))
    })())
}

fn atomic_saturating_add(target: &AtomicU64, value: u64) {
    let _ = target.fetch_update(Ordering::Relaxed, Ordering::Relaxed, |current| {
        Some(current.saturating_add(value))
    });
}

fn queue_prefetch_digest(
    verified: &BTreeMap<CacheDigest, PathBuf>,
    pending: &mut BTreeMap<CacheDigest, ()>,
    digest: CacheDigest,
) {
    if verified.contains_key(&digest) || pending.contains_key(&digest) {
        return;
    }
    pending.insert(digest, ());
}

fn queue_prefetch_directory(
    seen: &BTreeMap<CacheDigest, ()>,
    pending: &mut BTreeMap<CacheDigest, ()>,
    digest: CacheDigest,
    limit: usize,
) -> bool {
    if seen.contains_key(&digest) || pending.contains_key(&digest) {
        return true;
    }
    if seen.len().saturating_add(pending.len()) >= limit {
        return false;
    }
    pending.insert(digest, ());
    true
}

/// See [`CacheAgent::with_task_loader`].
pub type TaskLoader = dyn for<'a> Fn(&'a CacheAgent, &'a str) -> BoxFuture<'a, ()> + Send + Sync;

/// Shared state for an agent hosted by the process that owns a build session.
///
/// Transport listeners deliberately live in the embedder so the session
/// lifecycle owns them. This type only contains ecosystem-independent CAS and
/// protocol logic.
#[derive(Clone)]
pub struct CacheAgent {
    cas: LocalCas,
    actions: LocalActionCache,
    verified_blobs: Arc<Mutex<BTreeMap<CacheDigest, VerifiedBlob>>>,
    version: Arc<str>,
    write_locks: Arc<Mutex<BTreeMap<CacheDigest, Weak<tokio::sync::Mutex<()>>>>>,
    action_locks: Arc<Mutex<BTreeMap<CacheDigest, Weak<tokio::sync::Mutex<()>>>>>,
    stats: Arc<AtomicAgentStats>,
    observer: Option<Arc<dyn AgentEventObserver>>,
    observer_emission: Arc<Mutex<()>>,
    /// Loads a task the first time a request names it, so a connection that
    /// only reports a bypass never waits for a manifest it will not read.
    task_loader: Option<Arc<TaskLoader>>,
    executable_identities: Arc<Mutex<BTreeMap<ExecutableIdentityKey, Vec<u8>>>>,
    manifest_dir: Arc<PathBuf>,
    task_actions: Arc<Mutex<BTreeMap<String, TaskActionState>>>,
    /// Where a task may inherit predictions from, by task identity.
    task_fallbacks: Arc<Mutex<BTreeMap<String, TaskFallbacks>>>,
    next_task_run: Arc<AtomicU64>,
    manifest_write_lock: Arc<Mutex<()>>,
    remote: Option<Arc<RemoteCacheClient>>,
    remote_mode: RemoteCacheMode,
    remote_staging_dir: Arc<PathBuf>,
    remote_download_limit: u64,
    remote_download_bytes: Arc<AtomicU64>,
    pending_remote_actions: Arc<Mutex<BTreeMap<CacheDigest, RemoteActionResult>>>,
    remote_transfers: Arc<tokio::sync::Semaphore>,
    prefetch_transfers: Arc<tokio::sync::Semaphore>,
    prefetch_tasks: Arc<Mutex<Vec<tokio::task::JoinHandle<()>>>>,
    /// Distinct shim diagnostics already surfaced, so a warning that fires
    /// once per compilation is printed once per session.
    warnings: Arc<Mutex<BTreeSet<String>>>,
    /// Digests of files shims hashed or wrote this session, keyed by scope and
    /// path, each entry standing while its recorded identity matches the disk.
    file_digests: Arc<Mutex<BTreeMap<(FileDigestScope, PathBuf), RecordedFileDigest>>>,
    /// Deferred remote publication, present only when the session may write.
    uploads: Option<UploadQueue>,
}

/// What every file-digest record must satisfy before the ledger will answer
/// with it, whether a shim sent it or an earlier session left it behind.
fn validate_file_digest_record(entry: &RecordedFileDigest) -> Result<()> {
    if !entry.file.path.is_absolute() {
        bail!("file-digest records need absolute paths");
    }
    entry.digest.validate()?;
    if entry.file.len != entry.digest.size {
        bail!("file-digest record length does not match its digest");
    }
    Ok(())
}

/// Records background upload activity against a session's statistics.
struct AgentUploadSink {
    stats: Arc<AtomicAgentStats>,
}

impl UploadSink for AgentUploadSink {
    fn record_blob_uploaded(&self, bytes: u64) {
        self.stats
            .background_uploads
            .fetch_add(1, Ordering::Relaxed);
        self.stats
            .uploaded_bytes
            .fetch_add(bytes, Ordering::Relaxed);
    }

    fn record_action_uploaded(&self) {
        self.stats
            .background_uploads
            .fetch_add(1, Ordering::Relaxed);
    }

    fn record_blob_pack_uploaded(&self, blobs: u64) {
        self.stats
            .remote_blob_pack_uploads
            .fetch_add(1, Ordering::Relaxed);
        self.stats
            .remote_blob_pack_upload_blobs
            .fetch_add(blobs, Ordering::Relaxed);
    }

    fn record_upload_failure(&self) {
        self.stats
            .background_upload_failures
            .fetch_add(1, Ordering::Relaxed);
        self.stats.remote_failures.fetch_add(1, Ordering::Relaxed);
    }
}

/// A CAS blob whose contents this session hashed, and the file identity that
/// says the hash still describes what is on disk.
///
/// Length alone would miss an overwrite that keeps the size; the modification
/// time is what makes a rewrite visible without reading the bytes again.
#[derive(Debug, Clone)]
struct VerifiedBlob {
    path: PathBuf,
    len: u64,
    modified: SystemTime,
}

impl VerifiedBlob {
    /// Describe a file that was just verified, or nothing when the filesystem
    /// will not report a modification time to compare against later.
    fn describe(path: &Path) -> Option<Self> {
        let metadata = std::fs::metadata(path).ok()?;
        Some(Self {
            path: path.to_path_buf(),
            len: metadata.len(),
            modified: metadata.modified().ok()?,
        })
    }

    /// Whether the file still has the identity it had when it was verified.
    fn is_unchanged(&self) -> bool {
        let Ok(metadata) = std::fs::metadata(&self.path) else {
            return false;
        };
        metadata.len() == self.len && metadata.modified().is_ok_and(|now| now == self.modified)
    }
}

#[derive(Debug, Clone, Default)]
struct TaskActionState {
    manifest: String,
    baseline_loaded: bool,
    predictions: BTreeMap<CacheDigest, ActionPrediction>,
    pending_predictions: BTreeMap<CacheDigest, ActionPrediction>,
    prefetched_adapters: BTreeSet<String>,
    remote_etag: Option<String>,
}

/// Match an invocation and, on an adapter's first match, select its prefetch wave.
fn activate_prediction_adapter(
    state: &mut TaskActionState,
    invocation: &CacheDigest,
) -> (Option<ActionPrediction>, Option<Vec<ActionPrediction>>) {
    let prediction = state.predictions.get(invocation).cloned();
    let prefetch = prediction.as_ref().and_then(|prediction| {
        if !state.prefetched_adapters.insert(prediction.adapter.clone()) {
            return None;
        }
        Some(
            state
                .predictions
                .values()
                .filter(|candidate| candidate.adapter == prediction.adapter)
                .cloned()
                .collect(),
        )
    });
    (prediction, prefetch)
}

struct PrefetchedAction {
    adapter: String,
    result: RemoteActionResult,
}

struct RemoteDownloadReservation {
    counter: Arc<AtomicU64>,
    reserved: u64,
    committed: bool,
}

impl RemoteDownloadReservation {
    fn bytes(&self) -> u64 {
        self.reserved
    }

    fn commit(mut self, bytes: u64) {
        debug_assert!(bytes <= self.reserved);
        self.counter
            .fetch_sub(self.reserved.saturating_sub(bytes), Ordering::AcqRel);
        self.committed = true;
    }
}

impl Drop for RemoteDownloadReservation {
    fn drop(&mut self) {
        if !self.committed {
            self.counter.fetch_sub(self.reserved, Ordering::AcqRel);
        }
    }
}

#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord)]
struct ExecutableIdentityKey {
    executable: PathBuf,
    environment: BTreeMap<String, Option<String>>,
}

impl CacheAgent {
    /// Create an agent backed by the cache rooted at `cache_dir`.
    pub fn new(cache_dir: impl Into<PathBuf>, version: impl Into<Arc<str>>) -> Self {
        Self::build(cache_dir.into(), version.into(), None, 0)
    }

    /// Create an agent with local-first access to a remote action cache.
    pub fn new_remote(
        cache_dir: impl Into<PathBuf>,
        version: impl Into<Arc<str>>,
        remote: AgentRemoteCache,
    ) -> Self {
        Self::build(
            cache_dir.into(),
            version.into(),
            Some(remote),
            DEFAULT_MAX_REMOTE_DOWNLOAD_BYTES,
        )
    }

    /// Create a remote agent with a cumulative download budget for the session.
    pub fn new_remote_with_download_limit(
        cache_dir: impl Into<PathBuf>,
        version: impl Into<Arc<str>>,
        remote: AgentRemoteCache,
        max_remote_download_bytes: u64,
    ) -> Self {
        Self::build(
            cache_dir.into(),
            version.into(),
            Some(remote),
            max_remote_download_bytes,
        )
    }

    fn build(
        cache_dir: PathBuf,
        version: Arc<str>,
        remote: Option<AgentRemoteCache>,
        remote_download_limit: u64,
    ) -> Self {
        let remote_mode = remote
            .as_ref()
            .map_or(RemoteCacheMode::ReadOnly, |remote| remote.mode);
        let remote_staging_dir = remote.as_ref().map_or_else(
            || cache_dir.join("remote"),
            |remote| remote.staging_dir.clone(),
        );
        let remote = remote.map(|remote| Arc::new(remote.client));
        let stats = Arc::new(AtomicAgentStats::default());
        let remote_transfers = Arc::new(tokio::sync::Semaphore::new(MAX_REMOTE_TRANSFERS));
        // A session that cannot publish never queues an upload, so it does not
        // need somewhere to queue one.
        let uploads = remote
            .clone()
            .filter(|_| remote_mode.writes())
            .map(|client| {
                UploadQueue::new(
                    client,
                    Arc::new(AgentUploadSink {
                        stats: stats.clone(),
                    }),
                    remote_transfers.clone(),
                )
            });
        Self {
            cas: LocalCas::new(cache_dir.clone()),
            actions: LocalActionCache::new(cache_dir.clone()),
            verified_blobs: Arc::new(Mutex::new(BTreeMap::new())),
            version,
            write_locks: Arc::new(Mutex::new(BTreeMap::new())),
            action_locks: Arc::new(Mutex::new(BTreeMap::new())),
            stats,
            observer: None,
            observer_emission: Arc::new(Mutex::new(())),
            task_loader: None,
            executable_identities: Arc::new(Mutex::new(BTreeMap::new())),
            manifest_dir: Arc::new(task_manifest_dir(&cache_dir)),
            task_actions: Arc::new(Mutex::new(BTreeMap::new())),
            task_fallbacks: Arc::new(Mutex::new(BTreeMap::new())),
            next_task_run: Arc::new(AtomicU64::new(0)),
            manifest_write_lock: Arc::new(Mutex::new(())),
            remote,
            remote_mode,
            remote_staging_dir: Arc::new(remote_staging_dir),
            remote_download_limit,
            remote_download_bytes: Arc::new(AtomicU64::new(0)),
            pending_remote_actions: Arc::new(Mutex::new(BTreeMap::new())),
            remote_transfers,
            prefetch_transfers: Arc::new(tokio::sync::Semaphore::new(MAX_PREFETCH_TRANSFERS)),
            prefetch_tasks: Arc::new(Mutex::new(Vec::new())),
            warnings: Arc::new(Mutex::new(BTreeSet::new())),
            file_digests: Arc::new(Mutex::new(BTreeMap::new())),
            uploads,
        }
    }

    /// Report each accounted cache decision to `observer` as it happens.
    #[must_use]
    pub fn with_observer(mut self, observer: Arc<dyn AgentEventObserver>) -> Self {
        self.observer = Some(observer);
        self
    }

    /// Load tasks on demand.
    ///
    /// The loader is called with the agent and the task's identity before the
    /// first request that names a task this agent has not begun, and is
    /// expected to begin it; a second call for the same task must be a no-op.
    /// Requests that name no task never wait on it.
    pub fn with_task_loader(mut self, loader: Arc<TaskLoader>) -> Self {
        self.task_loader = Some(loader);
        self
    }

    /// Make sure `task` has been begun before a request reads or records
    /// under it.
    async fn ensure_task_loaded(&self, task: &str) {
        let Some(loader) = &self.task_loader else {
            return;
        };
        if self.task_actions.lock().unwrap().contains_key(task) {
            return;
        }
        loader(self, task).await;
    }

    fn emit(&self, event: impl FnOnce() -> AgentEvent) {
        if let Some(observer) = &self.observer {
            let _emission = self.observer_emission.lock().unwrap();
            observer.event(event());
        }
    }

    fn emit_action(&self, diagnostic: Option<AgentEvent>, action: AgentEvent) {
        if let Some(observer) = &self.observer {
            let _emission = self.observer_emission.lock().unwrap();
            if let Some(diagnostic) = diagnostic {
                observer.event(diagnostic);
            }
            observer.event(action);
        }
    }

    fn reserve_remote_download(&self, bytes: u64) -> Result<RemoteDownloadReservation> {
        let mut current = self.remote_download_bytes.load(Ordering::Acquire);
        loop {
            let next = current
                .checked_add(bytes)
                .ok_or_else(|| eyre::eyre!("remote cache download budget overflowed"))?;
            if next > self.remote_download_limit {
                bail!(
                    "remote cache download budget exceeded: {} bytes requested with {} of {} bytes already used",
                    bytes,
                    current,
                    self.remote_download_limit
                );
            }
            match self.remote_download_bytes.compare_exchange_weak(
                current,
                next,
                Ordering::AcqRel,
                Ordering::Acquire,
            ) {
                Ok(_) => {
                    return Ok(RemoteDownloadReservation {
                        counter: self.remote_download_bytes.clone(),
                        reserved: bytes,
                        committed: false,
                    });
                }
                Err(observed) => current = observed,
            }
        }
    }

    fn reserve_remote_download_up_to(&self, requested: u64) -> Result<RemoteDownloadReservation> {
        let mut current = self.remote_download_bytes.load(Ordering::Acquire);
        loop {
            let reserved = requested.min(self.remote_download_limit.saturating_sub(current));
            let next = current + reserved;
            match self.remote_download_bytes.compare_exchange_weak(
                current,
                next,
                Ordering::AcqRel,
                Ordering::Acquire,
            ) {
                Ok(_) => {
                    return Ok(RemoteDownloadReservation {
                        counter: self.remote_download_bytes.clone(),
                        reserved,
                        committed: false,
                    });
                }
                Err(observed) => current = observed,
            }
        }
    }

    /// Load the last committed action manifest for a task into this session.
    pub async fn begin_task(&self, task: &str) -> Result<String> {
        self.begin_task_with_remote_errors(task, false, None, true)
            .await
    }

    /// Load a task but defer each adapter's speculative downloads until its
    /// first prediction matches the current build.
    pub async fn begin_task_on_prediction(&self, task: &str) -> Result<String> {
        self.begin_task_with_remote_errors(task, false, None, false)
            .await
    }

    /// Load a task using its identity as the run key.
    ///
    /// A task-scoped process has only one run for an identity, so it can defer
    /// this work until its first client connects while putting the identity in
    /// the client's environment ahead of time.
    pub async fn begin_session_task(&self, task: &str) -> Result<()> {
        self.begin_task_with_remote_errors(task, false, Some(task.to_string()), false)
            .await?;
        Ok(())
    }

    /// Name where a task's predictions may come from when nothing has been
    /// recorded under its own identity.
    ///
    /// The identities are produced on demand rather than taken up front,
    /// because finding them can cost a few version-control commands and most
    /// builds never need them. The first manifest found, locally and then on
    /// the remote, is adopted under the task's own identity, so the commands
    /// that follow, tests and lints included, start from it too, and a trusted
    /// build publishes it there. When none of the named identities has one,
    /// the store's newest manifests are tried.
    pub fn register_task_fallbacks(
        &self,
        task: &str,
        fallbacks: impl Fn() -> Vec<String> + Send + Sync + 'static,
    ) -> Result<()> {
        validate_task_identity(task)?;
        self.task_fallbacks
            .lock()
            .unwrap()
            .insert(task.to_string(), Arc::new(fallbacks));
        Ok(())
    }

    /// Load a task and finish its prefetch, surfacing remote lookup failures.
    pub async fn prefetch_task(&self, task: &str) -> Result<String> {
        let run = self
            .begin_task_with_remote_errors(task, true, None, true)
            .await?;
        self.wait_for_prefetches().await;
        Ok(run)
    }

    async fn begin_task_with_remote_errors(
        &self,
        task: &str,
        strict: bool,
        run: Option<String>,
        eager_prefetch: bool,
    ) -> Result<String> {
        validate_task_identity(task)?;
        // The local manifest is already enough to start useful work. Do not
        // leave those predictions idle while a high-latency remote answers the
        // manifest lookup; the authoritative snapshot is loaded again below
        // before it is merged, so another process can still update it while
        // this request is in flight.
        let early_manifest = {
            let _write_guard = self.manifest_write_lock.lock().unwrap();
            let _file_guard = self.lock_task_manifest(task)?;
            self.load_task_manifest(task)?
        };
        let early_actions: BTreeSet<_> = early_manifest
            .iter()
            .flat_map(|manifest| manifest.predictions.iter())
            .map(|prediction| prediction.action.clone())
            .collect();
        if eager_prefetch {
            self.spawn_prefetch_predictions(
                early_manifest
                    .as_ref()
                    .map(|manifest| manifest.predictions.clone())
                    .unwrap_or_default(),
            );
        }
        let (remote_manifest, mut remote_etag) = if self.remote_mode.reads() {
            match self.get_remote_task_manifest(task).await {
                Ok(Some((manifest, etag))) => (Some(manifest), Some(etag)),
                Ok(None) => (None, None),
                Err(error) => {
                    if strict {
                        return Err(error).wrap_err_with(|| {
                            format!("remote task action manifest lookup failed for {task}")
                        });
                    }
                    self.note_remote_failure();
                    warn!("remote task action manifest lookup failed for {task}: {error}");
                    (None, None)
                }
            }
        } else {
            (None, None)
        };
        // Without a remote there is nothing to reconcile: the manifest just
        // read is the whole truth, and writing it back byte for byte would
        // serialize every prediction for nothing.
        let manifest = if self.remote.is_some() && self.remote_mode.reads() {
            let _write_guard = self.manifest_write_lock.lock().unwrap();
            let _file_guard = self.lock_task_manifest(task)?;
            let local_manifest = self.load_task_manifest(task)?;
            let manifest = match (remote_manifest, local_manifest) {
                (Some(remote), Some(local)) => {
                    let (manifest, merged) = merge_remote_task_manifest(task, remote, local);
                    if !merged {
                        remote_etag = None;
                    }
                    Some(manifest)
                }
                (Some(remote), None) => Some(remote),
                (None, local) => local,
            };
            if let Some(manifest) = &manifest {
                self.persist_task_manifest(manifest)?;
            }
            manifest
        } else {
            early_manifest
        };
        let (manifest, remote_etag) = match manifest {
            Some(manifest) => (Some(manifest), remote_etag),
            // Inherited from another identity, so the remote's copy of that
            // one is no precondition for publishing under this one.
            None => (self.inherit_task_manifest(task, strict).await?, None),
        };
        let mut state = if let Some(manifest) = manifest {
            TaskActionState {
                manifest: task.to_string(),
                baseline_loaded: true,
                predictions: manifest
                    .predictions
                    .into_iter()
                    .map(|prediction| (prediction.invocation.clone(), prediction))
                    .collect(),
                pending_predictions: BTreeMap::new(),
                prefetched_adapters: BTreeSet::new(),
                remote_etag,
            }
        } else {
            TaskActionState {
                manifest: task.to_string(),
                baseline_loaded: true,
                remote_etag,
                ..TaskActionState::default()
            }
        };
        let run = run.unwrap_or_else(|| {
            let sequence = self.next_task_run.fetch_add(1, Ordering::Relaxed);
            CacheDigest::blake3(format!("{task}\0{}\0{sequence}", std::process::id()).as_bytes())
                .hash
        });
        // The largest baseline rather than a sum: beginning the same task again
        // in one session reloads the same manifest, and counting it twice would
        // overstate what there was to match.
        self.stats.predictions_loaded.fetch_max(
            state.predictions.len().try_into().unwrap_or(u64::MAX),
            Ordering::Relaxed,
        );
        if eager_prefetch {
            state.prefetched_adapters.extend(
                state
                    .predictions
                    .values()
                    .map(|prediction| prediction.adapter.clone()),
            );
        }
        // The early local wave already owns these actions. Only launch a
        // second wave for predictions learned from the remote or from a local
        // writer that committed while its lookup was in flight.
        let predictions = state
            .predictions
            .values()
            .filter(|prediction| !early_actions.contains(&prediction.action))
            .cloned()
            .collect();
        self.task_actions.lock().unwrap().insert(run.clone(), state);
        if eager_prefetch {
            self.spawn_prefetch_predictions(predictions);
        }
        Ok(run)
    }

    /// Adopt the manifest of a fallback identity for a task that has none.
    ///
    /// The named identities are tried in the order given, each in the local
    /// store and then on the remote, and then the store's newest manifests,
    /// locally only. The first one found is persisted under `task` so the
    /// next command sees a complete baseline rather than only what this one
    /// recorded, and a later session finds it without asking again.
    async fn inherit_task_manifest(
        &self,
        task: &str,
        strict: bool,
    ) -> Result<Option<TaskActionManifest>> {
        let fallbacks = self.task_fallbacks.lock().unwrap().get(task).cloned();
        let Some(fallbacks) = fallbacks else {
            return Ok(None);
        };
        let named = tokio::task::spawn_blocking(move || fallbacks()).await?;
        let mut tried = BTreeSet::from([task.to_string()]);
        let candidates = named.into_iter().map(|identity| (identity, true)).chain(
            self.newest_task_identities()
                .into_iter()
                .map(|identity| (identity, false)),
        );
        for (identity, named) in candidates {
            if !tried.insert(identity.clone()) || validate_task_identity(&identity).is_err() {
                continue;
            }
            let mut found = self.load_task_manifest(&identity)?;
            if found.is_none() && named && self.remote_mode.reads() {
                found = match self.get_remote_task_manifest(&identity).await {
                    Ok(manifest) => manifest.map(|(manifest, _)| manifest),
                    Err(error) => {
                        if strict {
                            return Err(error).wrap_err_with(|| {
                                format!("remote task action manifest lookup failed for {identity}")
                            });
                        }
                        self.note_remote_failure();
                        warn!("remote task action manifest lookup failed for {identity}: {error}");
                        None
                    }
                };
            }
            let Some(mut manifest) = found else {
                continue;
            };
            if manifest.predictions.is_empty()
                || (!named && manifest.predictions.len() > MAX_TASK_ACTION_PREDICTIONS / 2)
            {
                continue;
            }
            info!(
                "no predictions were recorded for {task}; inheriting {} from {identity}",
                manifest.predictions.len()
            );
            manifest.task = task.to_string();
            validate_task_manifest(&manifest, task)?;
            let _write_guard = self.manifest_write_lock.lock().unwrap();
            let _file_guard = self.lock_task_manifest(task)?;
            // Another process may have recorded this identity while the
            // fallbacks were being found. What it wrote describes this
            // lockfile and wins over an inheritance.
            if let Some(recorded) = self.load_task_manifest(task)? {
                return Ok(Some(recorded));
            }
            self.persist_task_manifest(&manifest)?;
            return Ok(Some(manifest));
        }
        Ok(None)
    }

    /// The identities of the store's most recently written manifests.
    fn newest_task_identities(&self) -> Vec<String> {
        let Ok(entries) = fs::read_dir(self.manifest_dir.as_path()) else {
            return Vec::new();
        };
        let mut manifests: Vec<(std::time::SystemTime, String)> = entries
            .filter_map(|entry| {
                let entry = entry.ok()?;
                let name = entry.file_name();
                let identity = name.to_str()?.strip_suffix(".json")?.to_string();
                validate_task_identity(&identity).ok()?;
                let modified = entry.metadata().ok()?.modified().ok()?;
                Some((modified, identity))
            })
            .collect();
        manifests.sort_by(|left, right| right.cmp(left));
        manifests
            .into_iter()
            .take(NEWEST_MANIFEST_CANDIDATES)
            .map(|(_, identity)| identity)
            .collect()
    }

    /// Cancel speculative downloads before the owning session exits.
    pub async fn cancel_prefetches(&self) {
        let tasks = std::mem::take(&mut *self.prefetch_tasks.lock().unwrap());
        for task in &tasks {
            task.abort();
        }
        for task in tasks {
            if let Err(error) = task.await
                && !error.is_cancelled()
            {
                warn!("remote action prefetch task failed: {error}");
            }
        }
    }

    /// Publish everything a build queued, before the session stops.
    ///
    /// Store requests return once an object is durable locally, so at this point
    /// the remote cache may still be behind the local one. Uploads run on the
    /// session's runtime and are abandoned if it goes away, so a session that
    /// wants them published has to wait here.
    pub async fn wait_for_uploads(&self) {
        let Some(uploads) = &self.uploads else {
            return;
        };
        let _timer = AtomicDurationTimer::start(&self.stats.upload_drain_duration_ns);
        uploads.drain().await;
    }

    async fn wait_for_prefetches(&self) {
        let tasks = std::mem::take(&mut *self.prefetch_tasks.lock().unwrap());
        for task in tasks {
            if let Err(error) = task.await {
                warn!("remote action prefetch task failed: {error}");
            }
        }
    }

    /// Atomically publish the completed actions collected by a task run.
    pub async fn commit_task(&self, run: &str) -> Result<()> {
        self.commit_task_actions(run).await.map(|_| ())
    }

    /// Publish a task run and return exactly the predictions completed by it.
    ///
    /// The persisted task manifest also carries predictions inherited from
    /// earlier runs. Callers that need a receipt for this one run must not
    /// mistake that cumulative manifest for the work the run completed.
    pub async fn commit_task_actions(&self, run: &str) -> Result<Vec<ActionPrediction>> {
        validate_task_identity(run)?;
        let state = {
            let mut runs = self.task_actions.lock().unwrap();
            let state = runs
                .get(run)
                .ok_or_else(|| eyre::eyre!("task action manifest baseline was not loaded"))?;
            if !state.baseline_loaded {
                bail!("task action manifest baseline was not loaded");
            }
            // A run that predicted nothing new leaves the manifest as it found
            // it. Rewriting it would serialize every inherited prediction to
            // say so, and a remote that is only read has nothing to learn
            // either. Decided before the baseline is cloned: the baseline is
            // every prediction the manifest holds.
            if state.pending_predictions.is_empty()
                && (self.remote.is_none() || !self.remote_mode.writes())
            {
                runs.remove(run);
                return Ok(Vec::new());
            }
            state.clone()
        };
        let task = state.manifest;
        validate_task_identity(&task)?;
        let completed = state
            .pending_predictions
            .values()
            .cloned()
            .collect::<Vec<_>>();
        let (manifest, introduced) = {
            let _write_guard = self.manifest_write_lock.lock().unwrap();
            let _file_guard = self.lock_task_manifest(&task)?;
            let mut predictions = self
                .load_task_manifest(&task)?
                .map(|manifest| {
                    manifest
                        .predictions
                        .into_iter()
                        .map(|prediction| (prediction.invocation.clone(), prediction))
                        .collect::<BTreeMap<_, _>>()
                })
                .unwrap_or_default();
            // Which predictions this run adds, as opposed to inherits. Only a
            // new one may be withheld for a failed upload: retracting an
            // inherited one would un-advertise a result that is plausibly still
            // on the server from whichever session put it there.
            let introduced: BTreeSet<CacheDigest> = state
                .pending_predictions
                .keys()
                .filter(|invocation| !predictions.contains_key(*invocation))
                .cloned()
                .collect();
            // Only publish predictions recorded by this run. `predictions`
            // also contains the baseline loaded by `begin_task`; extending
            // with that snapshot would overwrite newer entries committed by
            // another agent process after this run began.
            predictions.extend(state.pending_predictions);
            let manifest = TaskActionManifest {
                version: TASK_ACTION_MANIFEST_VERSION,
                task: task.clone(),
                predictions: predictions.into_values().collect(),
            };
            validate_task_manifest(&manifest, &task)?;
            self.persist_task_manifest(&manifest)?;
            (manifest, introduced)
        };
        self.task_actions.lock().unwrap().remove(run);
        if self.remote_mode.writes() {
            // A manifest advertises the actions it predicts, so it must not
            // reach the remote cache before the results a reader would then go
            // looking for -- nor name a result that never got there at all.
            let mut manifest = manifest;
            if let Some(uploads) = &self.uploads {
                let actions: Vec<CacheDigest> = manifest
                    .predictions
                    .iter()
                    .map(|prediction| prediction.action.clone())
                    .collect();
                let unpublished = uploads.wait_for_actions(&actions).await;
                let withheld = manifest
                    .predictions
                    .iter()
                    .filter(|prediction| {
                        introduced.contains(&prediction.invocation)
                            && unpublished.contains(&prediction.action)
                    })
                    .count();
                if withheld > 0 {
                    // The local manifest keeps them: this checkout can still use
                    // what it built, and a later session can publish it.
                    warn!(
                        "{withheld} of {} predicted actions were not published, so the remote task action manifest omits them",
                        manifest.predictions.len()
                    );
                    manifest.predictions.retain(|prediction| {
                        !(introduced.contains(&prediction.invocation)
                            && unpublished.contains(&prediction.action))
                    });
                }
            }
            match self
                .put_remote_task_manifest(&task, manifest, state.remote_etag)
                .await
            {
                Ok(remote_manifest) => {
                    let _write_guard = self.manifest_write_lock.lock().unwrap();
                    let reconciliation = (|| {
                        let _file_guard = self.lock_task_manifest(&task)?;
                        let manifest = match self.load_task_manifest(&task)? {
                            Some(local) => {
                                merge_remote_task_manifest(&task, remote_manifest, local).0
                            }
                            None => remote_manifest,
                        };
                        self.persist_task_manifest(&manifest)
                    })();
                    if let Err(error) = reconciliation {
                        warn!(
                            "remote task action manifest reconciliation failed for {task}: {error}"
                        );
                    }
                }
                Err(error) => {
                    self.note_remote_failure();
                    warn!("remote task action manifest upload failed for {task}: {error}");
                }
            }
        }
        Ok(completed)
    }

    fn task_manifest_path(&self, task: &str) -> PathBuf {
        self.manifest_dir.join(format!("{task}.json"))
    }

    fn task_manifest_lock_path(&self, task: &str) -> PathBuf {
        self.manifest_dir.join("locks").join(format!("{task}.lock"))
    }

    fn lock_task_manifest(&self, task: &str) -> Result<fslock::LockFile> {
        let path = self.task_manifest_lock_path(task);
        fs::create_dir_all(path.parent().expect("task manifest lock has a parent"))?;
        let mut lock = fslock::LockFile::open(&path)?;
        lock.lock()?;
        Ok(lock)
    }

    fn load_task_manifest(&self, task: &str) -> Result<Option<TaskActionManifest>> {
        match fs::read(self.task_manifest_path(task)) {
            Ok(contents) => Ok(Some(self.parse_task_manifest(task, &contents, false)?)),
            Err(error) if error.kind() == std::io::ErrorKind::NotFound => Ok(None),
            Err(error) => Err(error.into()),
        }
    }

    fn parse_task_manifest(
        &self,
        task: &str,
        contents: &[u8],
        require_canonical: bool,
    ) -> Result<TaskActionManifest> {
        let manifest: TaskActionManifest = serde_json::from_slice(contents)?;
        validate_task_manifest(&manifest, task)?;
        if require_canonical && canonical_json(&manifest)? != contents {
            bail!("task action manifest is not canonical JSON");
        }
        Ok(manifest)
    }

    fn task_manifest_selector(task: &str) -> Result<(Vec<u8>, CacheDigest)> {
        TaskActionManifest::selector(task)
    }

    fn persist_task_manifest(&self, manifest: &TaskActionManifest) -> Result<()> {
        let bytes = canonical_json(manifest)?;
        fs::create_dir_all(self.manifest_dir.as_path())?;
        let mut temporary = tempfile::NamedTempFile::new_in(self.manifest_dir.as_path())?;
        std::io::Write::write_all(temporary.as_file_mut(), &bytes)?;
        temporary.as_file_mut().sync_all()?;
        temporary
            .persist(self.task_manifest_path(&manifest.task))
            .map_err(|error| error.error)?;
        Ok(())
    }

    async fn get_remote_task_manifest(
        &self,
        task: &str,
    ) -> Result<Option<(TaskActionManifest, String)>> {
        let Some(remote) = &self.remote else {
            return Ok(None);
        };
        let (_, selector) = Self::task_manifest_selector(task)?;
        let _permit = self.remote_transfers.acquire().await?;
        self.stats
            .remote_manifest_lookups
            .fetch_add(1, Ordering::Relaxed);
        let _timer = AtomicDurationTimer::start(&self.stats.remote_manifest_lookup_duration_ns);
        let Some(remote_manifest) = remote.get_action_manifest(&selector).await? else {
            return Ok(None);
        };
        let manifest = self.parse_task_manifest(task, &remote_manifest.bytes, true)?;
        Ok(Some((manifest, remote_manifest.etag)))
    }

    async fn put_remote_task_manifest(
        &self,
        task: &str,
        mut manifest: TaskActionManifest,
        mut expected_etag: Option<String>,
    ) -> Result<TaskActionManifest> {
        let Some(remote) = &self.remote else {
            return Ok(manifest);
        };
        let (_, selector) = Self::task_manifest_selector(task)?;
        for _ in 0..4 {
            let bytes = canonical_json(&manifest)?;
            let outcome = {
                let _permit = self.remote_transfers.acquire().await?;
                remote
                    .put_action_manifest(&selector, &bytes, expected_etag.as_deref())
                    .await?
            };
            match outcome {
                ManifestPutOutcome::Stored => return Ok(manifest),
                ManifestPutOutcome::PreconditionFailed => {
                    let Some((remote_manifest, etag)) = self.get_remote_task_manifest(task).await?
                    else {
                        expected_etag = None;
                        continue;
                    };
                    manifest = merge_task_manifests(task, Some(remote_manifest), manifest)?;
                    expected_etag = Some(etag);
                }
            }
        }
        bail!("remote task action manifest changed too frequently")
    }

    /// Return a snapshot of this session's cache activity.
    pub fn stats(&self) -> AgentStats {
        AgentStats {
            session_duration_ns: 0,
            lookups: self.stats.lookups.load(Ordering::Relaxed),
            unconsulted: self.stats.unconsulted.load(Ordering::Relaxed),
            hits: self.stats.hits.load(Ordering::Relaxed),
            stores: self.stats.stores.load(Ordering::Relaxed),
            stored_bytes: self.stats.stored_bytes.load(Ordering::Relaxed),
            verifications: self.stats.verifications.load(Ordering::Relaxed),
            divergences: self.stats.divergences.load(Ordering::Relaxed),
            downloaded_bytes: self.stats.downloaded_bytes.load(Ordering::Relaxed),
            uploaded_bytes: self.stats.uploaded_bytes.load(Ordering::Relaxed),
            background_uploads: self.stats.background_uploads.load(Ordering::Relaxed),
            background_upload_failures: self
                .stats
                .background_upload_failures
                .load(Ordering::Relaxed),
            remote_blob_pack_uploads: self.stats.remote_blob_pack_uploads.load(Ordering::Relaxed),
            remote_blob_pack_upload_blobs: self
                .stats
                .remote_blob_pack_upload_blobs
                .load(Ordering::Relaxed),
            upload_drain_duration_ns: self.stats.upload_drain_duration_ns.load(Ordering::Relaxed),
            prefetched_actions: self.stats.prefetched_actions.load(Ordering::Relaxed),
            predictions_loaded: self.stats.predictions_loaded.load(Ordering::Relaxed),
            bypasses: self.stats.bypasses.lock().unwrap().clone(),
            avoided_compiler_duration_ns: self
                .stats
                .avoided_compiler_duration_ns
                .load(Ordering::Relaxed),
            compiler: self.stats.compiler.lock().unwrap().clone(),
            slow_compilations: self.stats.slow_compilations.lock().unwrap().clone(),
            remote_failures: self.stats.remote_failures.load(Ordering::Relaxed),
            remote_manifest_lookups: self.stats.remote_manifest_lookups.load(Ordering::Relaxed),
            remote_manifest_lookup_duration_ns: self
                .stats
                .remote_manifest_lookup_duration_ns
                .load(Ordering::Relaxed),
            remote_action_lookups: self.stats.remote_action_lookups.load(Ordering::Relaxed),
            remote_action_lookup_duration_ns: self
                .stats
                .remote_action_lookup_duration_ns
                .load(Ordering::Relaxed),
            remote_blob_requests: self.stats.remote_blob_requests.load(Ordering::Relaxed),
            remote_blob_pack_requests: self.stats.remote_blob_pack_requests.load(Ordering::Relaxed),
            remote_blob_pack_blobs: self.stats.remote_blob_pack_blobs.load(Ordering::Relaxed),
            remote_blob_transfer_duration_ns: self
                .stats
                .remote_blob_transfer_duration_ns
                .load(Ordering::Relaxed),
            local_cas_write_duration_ns: self
                .stats
                .local_cas_write_duration_ns
                .load(Ordering::Relaxed),
            prefetch_runs: self.stats.prefetch_runs.load(Ordering::Relaxed),
            prefetch_duration_ns: self.stats.prefetch_duration_ns.load(Ordering::Relaxed),
            materialization_duration_ns: self
                .stats
                .materialization_duration_ns
                .load(Ordering::Relaxed),
            restored_output_files: self.stats.restored_output_files.load(Ordering::Relaxed),
            restored_output_bytes: self.stats.restored_output_bytes.load(Ordering::Relaxed),
            reflinked_output_files: self.stats.reflinked_output_files.load(Ordering::Relaxed),
            reflinked_output_bytes: self.stats.reflinked_output_bytes.load(Ordering::Relaxed),
            copied_output_files: self.stats.copied_output_files.load(Ordering::Relaxed),
            copied_output_bytes: self.stats.copied_output_bytes.load(Ordering::Relaxed),
            reused_output_files: self.stats.reused_output_files.load(Ordering::Relaxed),
            reused_output_bytes: self.stats.reused_output_bytes.load(Ordering::Relaxed),
        }
    }

    /// Handle requests without a transport connection.
    ///
    /// Persistent wrappers use this entry point when Cargo invokes mbx outside
    /// an orchestrated session. It intentionally has the same response
    /// semantics as [`Self::handle_connection`], while leaving framing and the
    /// version handshake to callers that actually cross a process boundary.
    pub async fn handle_requests(
        &self,
        requests: impl IntoIterator<Item = AgentRequest>,
    ) -> Vec<AgentResponse> {
        let mut connection = ConnectionUploads::default();
        let mut responses = Vec::new();
        for request in requests {
            responses.push(self.respond_on(request, &mut connection).await);
        }
        responses
    }

    fn write_lock(&self, digest: &CacheDigest) -> Arc<tokio::sync::Mutex<()>> {
        Self::digest_lock(&self.write_locks, digest)
    }

    fn action_lock(&self, digest: &CacheDigest) -> Arc<tokio::sync::Mutex<()>> {
        Self::digest_lock(&self.action_locks, digest)
    }

    fn digest_lock(
        locks: &Mutex<BTreeMap<CacheDigest, Weak<tokio::sync::Mutex<()>>>>,
        digest: &CacheDigest,
    ) -> Arc<tokio::sync::Mutex<()>> {
        let mut locks = locks.lock().unwrap();
        locks.retain(|_, lock| lock.strong_count() > 0);
        if let Some(lock) = locks.get(digest).and_then(Weak::upgrade) {
            return lock;
        }
        let lock = Arc::new(tokio::sync::Mutex::new(()));
        locks.insert(digest.clone(), Arc::downgrade(&lock));
        lock
    }

    /// Answer one request outside any connection.
    ///
    /// An upload queued this way has no sibling requests to order against, so
    /// the queue falls back to treating every blob it holds as a prerequisite.
    #[cfg(test)]
    async fn respond(&self, request: AgentRequest) -> AgentResponse {
        self.respond_on(request, &mut ConnectionUploads::default())
            .await
    }

    async fn respond_on(
        &self,
        request: AgentRequest,
        connection: &mut ConnectionUploads,
    ) -> AgentResponse {
        match &request {
            AgentRequest::FindActionPrediction { task, .. }
            | AgentRequest::RecordActionPrediction { task, .. } => {
                self.ensure_task_loaded(task).await;
            }
            _ => {}
        }
        let result = match request {
            AgentRequest::BeginTask { task } => self
                .begin_task(&task)
                .await
                .map(|run| AgentResponse::TaskBegun { run }),
            AgentRequest::CommitTask { run } => self
                .commit_task(&run)
                .await
                .map(|()| AgentResponse::TaskCommitted),
            AgentRequest::FindBlob { digest } => self.find_blob(&digest).await,
            AgentRequest::FindBlobs { digests } => self.find_blobs(digests).await,
            AgentRequest::StoreBlob { digest, source } => {
                self.store_blob(&digest, &source, connection).await
            }
            AgentRequest::FindActionResult { action } => {
                self.stats.lookups.fetch_add(1, Ordering::Relaxed);
                self.find_action_result(&action).await
            }
            AgentRequest::RecordActionHit {
                action,
                restore,
                crate_name,
            } => {
                let diagnostic = connection
                    .take_action_diagnostic("hit", crate_name.as_deref())
                    .map(|diagnostic| AgentEvent::ActionDiagnostic {
                        outcome: "hit".into(),
                        crate_name: crate_name.clone(),
                        diagnostic,
                    });
                self.record_action_hit(&action, restore, crate_name, diagnostic)
            }
            AgentRequest::RecordBypass { kind } => {
                *self
                    .stats
                    .bypasses
                    .lock()
                    .unwrap()
                    .entry(kind.clone())
                    .or_insert(0) += 1;
                self.emit(|| AgentEvent::Bypass { kind });
                Ok(AgentResponse::BypassRecorded)
            }
            AgentRequest::RecordUnconsulted => {
                self.stats.unconsulted.fetch_add(1, Ordering::Relaxed);
                self.emit(|| AgentEvent::Unconsulted);
                Ok(AgentResponse::UnconsultedRecorded)
            }
            AgentRequest::RecordWarning { message } => match parse_action_diagnostic(&message) {
                Some(Ok((outcome, crate_name, diagnostic))) => {
                    connection.record_action_diagnostic(outcome, crate_name, diagnostic);
                    Ok(AgentResponse::WarningRecorded)
                }
                Some(Err(error)) => Err(error),
                None => self.record_warning(message),
            },
            AgentRequest::FindFileDigests { scope, files } => self.find_file_digests(scope, files),
            AgentRequest::JoinActionPromise {
                adapter,
                invocation,
            } => self.join_action_promise(&adapter, &invocation).await,
            AgentRequest::CompleteActionPromise { claim, prediction } => {
                self.complete_action_promise(&claim, &prediction).await
            }
            AgentRequest::RecordFileDigests { scope, entries } => {
                self.record_file_digests(scope, entries)
            }
            AgentRequest::RecordCompilerInvocation {
                outcome,
                crate_name,
                duration_ns,
            } => {
                let diagnostic = connection
                    .take_action_diagnostic(&outcome, crate_name.as_deref())
                    .map(|diagnostic| AgentEvent::ActionDiagnostic {
                        outcome: outcome.clone(),
                        crate_name: crate_name.clone(),
                        diagnostic,
                    });
                self.record_compiler_invocation(
                    &outcome,
                    crate_name.as_deref(),
                    duration_ns,
                    diagnostic,
                )
            }
            AgentRequest::RecordActionVerification { matched, restore } => {
                self.record_materialization(restore);
                self.stats.verifications.fetch_add(1, Ordering::Relaxed);
                if !matched {
                    self.stats.divergences.fetch_add(1, Ordering::Relaxed);
                }
                self.emit(|| AgentEvent::Verification { matched, restore });
                Ok(AgentResponse::ActionVerificationRecorded)
            }
            AgentRequest::StoreActionResult { result } => {
                self.store_action_result(&result, connection).await
            }
            AgentRequest::FindActionPrediction { task, invocation } => {
                self.find_action_prediction(&task, &invocation)
            }
            AgentRequest::RecordActionPrediction { task, prediction } => {
                self.record_action_prediction(&task, prediction)
            }
            AgentRequest::FindExecutableIdentity {
                executable,
                environment,
            } => self.find_executable_identity(executable, environment),
            AgentRequest::StoreExecutableIdentity {
                executable,
                environment,
                stdout,
            } => self.store_executable_identity(executable, environment, stdout),
            AgentRequest::Hello { .. } => {
                Err(eyre::eyre!("hello is only valid as the first request"))
            }
        };
        result.unwrap_or_else(|error| AgentResponse::Error {
            message: error.to_string(),
        })
    }

    async fn find_blob(&self, digest: &CacheDigest) -> Result<AgentResponse> {
        if let Some(path) = self.find_verified_blob(digest)? {
            return Ok(AgentResponse::Blob { path: Some(path) });
        }
        if !self.remote_mode.reads() {
            return Ok(AgentResponse::Blob { path: None });
        }
        let Some(remote) = &self.remote else {
            return Ok(AgentResponse::Blob { path: None });
        };
        match self.fetch_remote_blob(remote, digest).await {
            Ok(path) => Ok(AgentResponse::Blob { path: Some(path) }),
            Err(error) => {
                warn!(
                    "remote cache blob lookup failed for {}: {error}",
                    digest.hash
                );
                Ok(AgentResponse::Blob { path: None })
            }
        }
    }

    async fn find_blobs(&self, digests: Vec<CacheDigest>) -> Result<AgentResponse> {
        let mut paths = BTreeMap::new();
        let mut missing = Vec::new();
        for digest in &digests {
            match self.find_verified_blob(digest)? {
                Some(path) => {
                    paths.insert(digest.clone(), path);
                }
                None => {
                    missing.push(digest.clone());
                }
            }
        }

        if !missing.is_empty()
            && self.remote_mode.reads()
            && let Some(remote) = &self.remote
        {
            paths.extend(self.fetch_remote_blobs(remote, missing, None).await);
        }

        Ok(AgentResponse::Blobs {
            paths: digests
                .into_iter()
                .map(|digest| paths.get(&digest).cloned())
                .collect(),
        })
    }

    async fn store_blob(
        &self,
        digest: &CacheDigest,
        source: &Path,
        connection: &mut ConnectionUploads,
    ) -> Result<AgentResponse> {
        let path = {
            let lock = self.write_lock(digest);
            let _guard = lock.lock().await;
            if let Some(path) = self.find_verified_blob(digest)? {
                path
            } else {
                let path = self.cas.store_file(digest, source)?;
                self.remember_verified_blob(digest, &path);
                self.stats.stores.fetch_add(1, Ordering::Relaxed);
                self.stats
                    .stored_bytes
                    .fetch_add(digest.size, Ordering::Relaxed);
                path
            }
        };
        // The object is durable locally, so the build has what it needs and the
        // remote publication can happen after this request returns.
        if let Some(uploads) = &self.uploads {
            uploads.queue_blob(digest, path.clone(), connection);
        }
        Ok(AgentResponse::Stored { path })
    }

    fn find_verified_blob(&self, digest: &CacheDigest) -> Result<Option<PathBuf>> {
        let remembered = self.verified_blobs.lock().unwrap().get(digest).cloned();
        if let Some(remembered) = remembered {
            // The contents behind this digest were hashed in full when the
            // session first reached for them. Hashing again on every later
            // lookup would re-read each dependency's rlib once per crate that
            // links it, so what is rechecked here is that the file has not
            // been written since: an overwrite moves the modification time,
            // and truncation or eviction changes the length or removes it.
            // Only a replacement that reproduces both -- which no writer in
            // the store does, since blobs are published by rename under a
            // content-derived name -- would go unnoticed until `mbx cache
            // verify` reads the store back in full.
            if remembered.is_unchanged() {
                return Ok(Some(remembered.path));
            }
            self.verified_blobs.lock().unwrap().remove(digest);
        }
        let path = self.cas.find(digest)?;
        if let Some(path) = &path {
            self.remember_verified_blob(digest, path);
        }
        Ok(path)
    }

    fn remember_verified_blob(&self, digest: &CacheDigest, path: &Path) {
        let Some(verified) = VerifiedBlob::describe(path) else {
            // Without an identity to compare against there is nothing to
            // shortcut safely, so the digest stays unremembered and every
            // lookup re-reads it.
            return;
        };
        self.verified_blobs
            .lock()
            .unwrap()
            .insert(digest.clone(), verified);
    }

    async fn find_action_result(&self, action: &CacheDigest) -> Result<AgentResponse> {
        if let Some(result) = self.actions.find(action)? {
            return Ok(AgentResponse::ActionResult {
                result: Some(result),
            });
        }
        if !self.remote_mode.reads() {
            return Ok(AgentResponse::ActionResult { result: None });
        }
        let Some(remote) = &self.remote else {
            return Ok(AgentResponse::ActionResult { result: None });
        };
        let lock = self.action_lock(action);
        let _guard = lock.lock().await;
        if let Some(result) = self.actions.find(action)? {
            return Ok(AgentResponse::ActionResult {
                result: Some(result),
            });
        }
        if let Some(result) = self
            .pending_remote_actions
            .lock()
            .unwrap()
            .get(action)
            .cloned()
        {
            return Ok(AgentResponse::ActionResult {
                result: Some(result),
            });
        }
        let _permit = self.remote_transfers.acquire().await?;
        match self.get_remote_action_result(remote, action).await {
            Ok(Some(result)) => {
                self.pending_remote_actions
                    .lock()
                    .unwrap()
                    .insert(action.clone(), result.clone());
                Ok(AgentResponse::ActionResult {
                    result: Some(result),
                })
            }
            Ok(None) => Ok(AgentResponse::ActionResult { result: None }),
            Err(error) => {
                self.note_remote_failure();
                warn!(
                    "remote cache action lookup failed for {}: {error}",
                    action.hash
                );
                Ok(AgentResponse::ActionResult { result: None })
            }
        }
    }

    async fn store_action_result(
        &self,
        result: &RemoteActionResult,
        connection: &ConnectionUploads,
    ) -> Result<AgentResponse> {
        let path = self.actions.store(result)?;
        if let Some(uploads) = &self.uploads {
            uploads.queue_action_result(result, connection);
        }
        Ok(AgentResponse::ActionStored { path })
    }

    async fn join_action_promise(
        &self,
        adapter: &str,
        invocation: &CacheDigest,
    ) -> Result<AgentResponse> {
        if !self.remote_mode.reads() || !self.remote_mode.writes() {
            return Ok(AgentResponse::ActionPromise {
                claim: None,
                prediction: None,
            });
        }
        let Some(remote) = &self.remote else {
            return Ok(AgentResponse::ActionPromise {
                claim: None,
                prediction: None,
            });
        };
        let deadline = Instant::now() + ACTION_PROMISE_WAIT;
        loop {
            let _permit = self.remote_transfers.acquire().await?;
            let state = remote.join_action_promise(invocation, adapter).await;
            drop(_permit);
            match state {
                Ok(Some(ActionPromiseState::Claimed { claim })) => {
                    return Ok(AgentResponse::ActionPromise {
                        claim: Some(claim),
                        prediction: None,
                    });
                }
                Ok(Some(ActionPromiseState::Complete { prediction })) => {
                    return Ok(AgentResponse::ActionPromise {
                        claim: None,
                        prediction: Some(prediction),
                    });
                }
                Ok(Some(ActionPromiseState::Pending { retry_after_ms }))
                    if Instant::now() < deadline =>
                {
                    tokio::time::sleep(Duration::from_millis(retry_after_ms.clamp(10, 5_000)))
                        .await;
                }
                Ok(Some(ActionPromiseState::Pending { .. }) | None) => {
                    return Ok(AgentResponse::ActionPromise {
                        claim: None,
                        prediction: None,
                    });
                }
                Ok(Some(_)) => {
                    return Ok(AgentResponse::ActionPromise {
                        claim: None,
                        prediction: None,
                    });
                }
                Err(error) => {
                    self.note_remote_failure();
                    warn!(
                        "remote cache action promise failed for {}: {error}",
                        invocation.hash
                    );
                    return Ok(AgentResponse::ActionPromise {
                        claim: None,
                        prediction: None,
                    });
                }
            }
        }
    }

    async fn complete_action_promise(
        &self,
        claim: &str,
        prediction: &ActionPrediction,
    ) -> Result<AgentResponse> {
        prediction.validate()?;
        let Some(remote) = &self.remote else {
            return Ok(AgentResponse::ActionPromiseCompleted);
        };
        let Some(uploads) = &self.uploads else {
            return Ok(AgentResponse::ActionPromiseCompleted);
        };
        if uploads
            .wait_for_actions(std::slice::from_ref(&prediction.action))
            .await
            .contains(&prediction.action)
        {
            // Never promise an action result the server does not hold. The
            // server lease expires and another runner gets to repair it.
            return Ok(AgentResponse::ActionPromiseCompleted);
        }
        let completion = ActionPromiseCompletion {
            claim: claim.to_string(),
            prediction: prediction.clone(),
        };
        let _permit = self.remote_transfers.acquire().await?;
        if let Err(error) = remote
            .complete_action_promise(&prediction.invocation, &completion)
            .await
        {
            self.note_remote_failure();
            warn!(
                "remote cache action promise completion failed for {}: {error}",
                prediction.invocation.hash
            );
        }
        Ok(AgentResponse::ActionPromiseCompleted)
    }

    /// Record that a remote operation failed and the build carried on without it.
    fn note_remote_failure(&self) {
        self.stats.remote_failures.fetch_add(1, Ordering::Relaxed);
    }

    async fn get_remote_action_result(
        &self,
        remote: &RemoteCacheClient,
        action: &CacheDigest,
    ) -> Result<Option<RemoteActionResult>> {
        self.stats
            .remote_action_lookups
            .fetch_add(1, Ordering::Relaxed);
        let _timer = AtomicDurationTimer::start(&self.stats.remote_action_lookup_duration_ns);
        remote.get_action_result(action).await
    }

    fn record_action_hit(
        &self,
        action: &CacheDigest,
        restore: RestoreStats,
        crate_name: Option<String>,
        diagnostic: Option<AgentEvent>,
    ) -> Result<AgentResponse> {
        validate_crate_name(crate_name.as_deref())?;
        if self.actions.find(action)?.is_none() {
            let pending = self.pending_remote_actions.lock().unwrap().remove(action);
            if let Some(result) = pending {
                self.actions.store(&result)?;
            } else {
                bail!("cannot record a hit for a missing action result");
            }
        }
        self.record_restore(restore);
        self.stats.hits.fetch_add(1, Ordering::Relaxed);
        self.emit_action(
            diagnostic,
            AgentEvent::ActionHit {
                crate_name,
                restore,
            },
        );
        Ok(AgentResponse::ActionHitRecorded)
    }

    fn record_restore(&self, restore: RestoreStats) {
        self.record_materialization(restore);
        atomic_saturating_add(
            &self.stats.avoided_compiler_duration_ns,
            restore.avoided_compiler_duration_ns,
        );
        atomic_saturating_add(&self.stats.restored_output_files, restore.output_files);
        atomic_saturating_add(&self.stats.restored_output_bytes, restore.output_bytes);
        atomic_saturating_add(
            &self.stats.reflinked_output_files,
            restore.reflinked_output_files,
        );
        atomic_saturating_add(
            &self.stats.reflinked_output_bytes,
            restore.reflinked_output_bytes,
        );
        atomic_saturating_add(&self.stats.copied_output_files, restore.copied_output_files);
        atomic_saturating_add(&self.stats.copied_output_bytes, restore.copied_output_bytes);
        atomic_saturating_add(&self.stats.reused_output_files, restore.reused_output_files);
        atomic_saturating_add(&self.stats.reused_output_bytes, restore.reused_output_bytes);
    }

    fn record_compiler_invocation(
        &self,
        outcome: &str,
        crate_name: Option<&str>,
        duration_ns: u64,
        diagnostic: Option<AgentEvent>,
    ) -> Result<AgentResponse> {
        if !matches!(
            outcome,
            "miss" | "unconsulted" | "bypass" | "verification" | "incremental"
        ) {
            bail!("invalid compiler invocation outcome");
        }
        validate_crate_name(crate_name)?;
        let mut compiler = self.stats.compiler.lock().unwrap();
        let stats = compiler.entry(outcome.to_string()).or_default();
        stats.invocations = stats.invocations.saturating_add(1);
        stats.duration_ns = stats.duration_ns.saturating_add(duration_ns);
        drop(compiler);
        if outcome != "verification"
            && let Some(crate_name) = crate_name.filter(|name| !name.is_empty())
        {
            let mut slow = self.stats.slow_compilations.lock().unwrap();
            let duration = slow.entry(crate_name.to_string()).or_default();
            *duration = duration.saturating_add(duration_ns);
        }
        self.emit_action(
            diagnostic,
            AgentEvent::CompilerInvocation {
                outcome: outcome.to_string(),
                crate_name: crate_name.map(str::to_string),
                duration_ns,
            },
        );
        Ok(AgentResponse::CompilerInvocationRecorded)
    }

    fn record_materialization(&self, restore: RestoreStats) {
        atomic_saturating_add(&self.stats.materialization_duration_ns, restore.duration_ns);
    }

    /// Surface a shim diagnostic on this process's stderr.
    ///
    /// The agent lives in the process that owns the session, so printing here
    /// reaches the terminal running the build rather than the stderr of the
    /// compilation the shim stands in for -- which build scripts read as part
    /// of the compiler's answer. Deduplicated because one cause tends to fire
    /// once per compilation, and capped so a message unique per compilation
    /// cannot scroll the build away.
    fn record_warning(&self, message: String) -> Result<AgentResponse> {
        if message.is_empty()
            || message.len() > MAX_WARNING_BYTES
            || message.contains(['\n', '\r', '\0'])
        {
            bail!("invalid shim warning");
        }
        let mut warnings = self.warnings.lock().unwrap();
        if !warnings.contains(&message) && warnings.len() < MAX_WARNINGS {
            eprintln!("mbx[warning]: {message}");
            self.emit(|| AgentEvent::Warning {
                message: message.clone(),
            });
            warnings.insert(message);
        }
        Ok(AgentResponse::WarningRecorded)
    }

    /// Answer file-digest lookups from the session ledger.
    ///
    /// A recorded digest is returned only when the requested identity matches
    /// the recorded one exactly. The caller statted the file to build the
    /// identity, so this is the [`VerifiedBlob`] freshness check with the
    /// stat moved to the side that was already making it.
    fn find_file_digests(
        &self,
        scope: FileDigestScope,
        files: Vec<FileIdentity>,
    ) -> Result<AgentResponse> {
        if files.len() > MAX_FILE_DIGEST_BATCH {
            bail!("too many file-digest lookups in one request");
        }
        let ledger = self.file_digests.lock().unwrap();
        let digests = files
            .into_iter()
            .map(|file| {
                let recorded = ledger.get(&(scope, file.path.clone()))?;
                (recorded.file == file).then(|| recorded.digest.clone())
            })
            .collect();
        Ok(AgentResponse::FileDigests { digests })
    }

    /// Record digests of files a shim read in full, for later reuse.
    ///
    /// Capped rather than evicted: a session that outgrows the cap loses the
    /// shortcut for the overflow and nothing else, and no workload observed so
    /// far comes near it.
    fn record_file_digests(
        &self,
        scope: FileDigestScope,
        entries: Vec<RecordedFileDigest>,
    ) -> Result<AgentResponse> {
        if entries.len() > MAX_FILE_DIGEST_BATCH {
            bail!("too many file-digest records in one request");
        }
        for entry in &entries {
            validate_file_digest_record(entry)?;
        }
        let mut ledger = self.file_digests.lock().unwrap();
        for entry in entries {
            if ledger.len() >= MAX_FILE_DIGEST_ENTRIES
                && !ledger.contains_key(&(scope, entry.file.path.clone()))
            {
                break;
            }
            ledger.insert((scope, entry.file.path.clone()), entry);
        }
        Ok(AgentResponse::FileDigestsRecorded)
    }

    /// Start the file-digest ledger from entries an earlier session left
    /// behind, so a file nothing has touched since is not read again by the
    /// first compilation of this session that names it.
    ///
    /// Each entry stands only while its recorded identity still matches the
    /// disk, the same rule a lookup applies to what this session recorded, so
    /// a stale seed costs a hash and nothing else. Entries this session has
    /// already recorded are kept over seeded ones. Returns how many were taken.
    pub fn seed_file_digests(&self, entries: Vec<(FileDigestScope, RecordedFileDigest)>) -> usize {
        self.seed_file_digests_with(|| entries)
    }

    /// Seed the ledger from entries produced under its lock.
    ///
    /// A lookup that arrives while `load` is still reading waits for it rather
    /// than missing, so a caller can start the read in the background and let
    /// whatever else the build is doing overlap it.
    pub fn seed_file_digests_with(
        &self,
        load: impl FnOnce() -> Vec<(FileDigestScope, RecordedFileDigest)>,
    ) -> usize {
        let mut ledger = self.file_digests.lock().unwrap();
        let entries = load();
        let mut seeded = 0;
        for (scope, entry) in entries {
            if ledger.len() >= MAX_FILE_DIGEST_ENTRIES {
                break;
            }
            if validate_file_digest_record(&entry).is_err() {
                continue;
            }
            if let std::collections::btree_map::Entry::Vacant(slot) =
                ledger.entry((scope, entry.file.path.clone()))
            {
                slot.insert(entry);
                seeded += 1;
            }
        }
        seeded
    }

    /// Everything the file-digest ledger holds, for a later session to start
    /// from.
    pub fn file_digests(&self) -> Vec<(FileDigestScope, RecordedFileDigest)> {
        self.file_digests
            .lock()
            .unwrap()
            .iter()
            .map(|((scope, _), entry)| (*scope, entry.clone()))
            .collect()
    }

    fn find_action_prediction(
        &self,
        task: &str,
        invocation: &CacheDigest,
    ) -> Result<AgentResponse> {
        validate_task_identity(task)?;
        invocation.validate()?;
        let (prediction, prefetch) = {
            let mut tasks = self.task_actions.lock().unwrap();
            let Some(state) = tasks.get_mut(task) else {
                return Ok(AgentResponse::ActionPrediction { prediction: None });
            };
            activate_prediction_adapter(state, invocation)
        };
        if let Some(prefetch) = prefetch {
            self.spawn_prefetch_predictions(prefetch);
        }
        Ok(AgentResponse::ActionPrediction { prediction })
    }

    fn record_action_prediction(
        &self,
        task: &str,
        prediction: ActionPrediction,
    ) -> Result<AgentResponse> {
        validate_task_identity(task)?;
        prediction.validate()?;
        let mut tasks = self.task_actions.lock().unwrap();
        let state = tasks.entry(task.to_string()).or_default();
        if !state.predictions.contains_key(&prediction.invocation)
            && state.predictions.len() >= MAX_TASK_ACTION_PREDICTIONS
        {
            bail!("task action manifest contains too many predictions");
        }
        state
            .predictions
            .insert(prediction.invocation.clone(), prediction.clone());
        state
            .pending_predictions
            .insert(prediction.invocation.clone(), prediction);
        Ok(AgentResponse::ActionPredictionRecorded)
    }

    fn executable_identity_key(
        &self,
        executable: PathBuf,
        environment: BTreeMap<String, Option<String>>,
    ) -> Result<ExecutableIdentityKey> {
        // Restricted to the variables that actually select what an identity
        // probe reports: the toolchain rustup resolves, the SDK a linker
        // driver builds against, and a `-fuse-ld` linker selection. Anything
        // else would let one key stand for two different compilers.
        if !environment.keys().all(|name| {
            matches!(
                name.as_str(),
                "MBX_FUSE_LD"
                    | "RUSTUP_HOME"
                    | "RUSTUP_TOOLCHAIN"
                    | "SDKROOT"
                    | "MACOSX_DEPLOYMENT_TARGET"
                    | "LIB"
                    | "UCRTVersion"
                    | "UniversalCRTSdkDir"
                    | "VCToolsInstallDir"
                    | "VCToolsVersion"
                    | "WindowsSdkDir"
                    | "WindowsSDKVersion"
            )
        }) {
            bail!("executable identity contains an unsupported environment variable");
        }
        Ok(ExecutableIdentityKey {
            executable,
            environment,
        })
    }

    fn find_executable_identity(
        &self,
        executable: PathBuf,
        environment: BTreeMap<String, Option<String>>,
    ) -> Result<AgentResponse> {
        let key = self.executable_identity_key(executable, environment)?;
        let stdout = self
            .executable_identities
            .lock()
            .unwrap()
            .get(&key)
            .cloned();
        Ok(AgentResponse::ExecutableIdentity { stdout })
    }

    fn store_executable_identity(
        &self,
        executable: PathBuf,
        environment: BTreeMap<String, Option<String>>,
        stdout: Vec<u8>,
    ) -> Result<AgentResponse> {
        if stdout.len() > MAX_EXECUTABLE_IDENTITY_SIZE {
            bail!("executable identity exceeds {MAX_EXECUTABLE_IDENTITY_SIZE} bytes");
        }
        let key = self.executable_identity_key(executable, environment)?;
        let mut identities = self.executable_identities.lock().unwrap();
        let is_new = !identities.contains_key(&key);
        let previous_size = identities.get(&key).map_or(0, Vec::len);
        if is_new && identities.len() >= MAX_EXECUTABLE_IDENTITIES {
            bail!("executable identity cache contains too many entries");
        }
        let retained_bytes = identities.values().map(Vec::len).sum::<usize>();
        if retained_bytes - previous_size + stdout.len() > MAX_EXECUTABLE_IDENTITY_BYTES {
            bail!("executable identity cache contains too many bytes");
        }
        identities.insert(key, stdout.clone());
        Ok(AgentResponse::ExecutableIdentity {
            stdout: Some(stdout),
        })
    }

    /// Serve newline-delimited protocol requests on an authenticated session stream.
    pub async fn handle_connection<S>(&self, stream: S) -> Result<()>
    where
        S: AsyncRead + AsyncWrite + Unpin,
    {
        let (reader, mut writer) = tokio::io::split(stream);
        let mut reader = BufReader::new(reader);
        let hello = read_request(&mut reader)
            .await?
            .ok_or_else(|| eyre::eyre!("connection closed before the agent handshake"))?;
        let request: AgentRequest = serde_json::from_str(&hello)?;
        match request {
            AgentRequest::Hello {
                protocol,
                client_version,
            } if protocol == AGENT_PROTOCOL_VERSION && client_version == self.version.as_ref() => {}
            AgentRequest::Hello { protocol, .. } if protocol != AGENT_PROTOCOL_VERSION => {
                send_response(
                    &mut writer,
                    &AgentResponse::Error {
                        message: format!(
                            "unsupported agent protocol {protocol}; expected {AGENT_PROTOCOL_VERSION}"
                        ),
                    },
                )
                .await?;
                return Ok(());
            }
            AgentRequest::Hello { client_version, .. } => {
                send_response(
                    &mut writer,
                    &AgentResponse::Error {
                        message: format!(
                            "cache client {client_version} does not match agent {}",
                            self.version
                        ),
                    },
                )
                .await?;
                return Ok(());
            }
            _ => bail!("the first agent request must be hello"),
        }
        send_response(
            &mut writer,
            &AgentResponse::Hello {
                protocol: AGENT_PROTOCOL_VERSION,
                agent_version: self.version.to_string(),
            },
        )
        .await?;

        // Tickets accumulate for the life of the connection because a shim
        // publishes a compilation's blobs and the action result naming them over
        // one connection, in that order.
        let mut connection = ConnectionUploads::default();
        while let Some(line) = read_request(&mut reader).await? {
            let response = match serde_json::from_str(&line) {
                Ok(request) => self.respond_on(request, &mut connection).await,
                Err(error) => AgentResponse::Error {
                    message: format!("invalid agent request: {error}"),
                },
            };
            send_response(&mut writer, &response).await?;
        }
        Ok(())
    }
}

/// Read one newline-delimited request, refusing one that grows past the cap.
///
/// Any process running as this user can open the session socket, so a request
/// that never terminates its line must not be able to grow the agent's memory
/// without bound.
async fn read_request<R>(reader: &mut R) -> Result<Option<String>>
where
    R: AsyncBufRead + Unpin,
{
    let mut line = Vec::new();
    loop {
        let available = reader.fill_buf().await?;
        if available.is_empty() {
            break;
        }
        let (consumed, complete) = match available.iter().position(|byte| *byte == b'\n') {
            Some(index) => (index, true),
            None => (available.len(), false),
        };
        if line.len() + consumed > MAX_REQUEST_BYTES {
            bail!("agent request exceeded {MAX_REQUEST_BYTES} bytes");
        }
        line.extend_from_slice(&available[..consumed]);
        // The newline itself is consumed but never kept.
        reader.consume(consumed + usize::from(complete));
        if complete {
            return Ok(Some(String::from_utf8(line)?));
        }
    }
    if line.is_empty() {
        Ok(None)
    } else {
        Ok(Some(String::from_utf8(line)?))
    }
}

async fn send_response(
    writer: &mut (impl AsyncWrite + Unpin),
    response: &AgentResponse,
) -> Result<()> {
    let mut encoded = serde_json::to_vec(response)?;
    encoded.push(b'\n');
    writer.write_all(&encoded).await?;
    writer.flush().await?;
    Ok(())
}

#[cfg(test)]
#[path = "agent_tests.rs"]
mod tests;