04: Channel Lock

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The channel lock enforces single-turn concurrency per Discord channel. It manages the Discord typing keepalive indicator, coordinates user interruptions and cancellations, and replays queued messages when the lock releases.

Flow and ownership

runWithChannelLock() in src/utils/chat/channelQueue.ts wraps turn planning and execution inside a per-channel mutex backed by the in-memory channelLocks map.

runWithChannelLock(admission, callback)
  │
  ├─► skipLock === true? ─────────────────► invoke callback(reuseOuterLock)
  │
  ├─► releaseStaleChannelLockIfExpired()
  ├─► acquireChannelLockForTurn()         ──► sets isLocked, initializes AbortController
  │
  ▼
try {
  callback(lockedTurn, startTyping)       ──► startDiscordTypingKeepalive() (8s cadence)
} finally {
  releaseChannelLockAndReplayQueue()      ──► stops typing, aborts turn controller
    │
    ├─► stopContext present?              ──► setImmediate(handleStopResponse)
    └─► messageQueue has items?           ──► setImmediate(processQueuedMessage)
}

The first turn starts typing alongside context building and waits for both before generation, so the initial typing indicator cannot arrive after the reply. typing_started records completion of the initial request. After that request settles, the keepalive starts only if the same abort controller still owns the lock; queued personas can share a trigger message ID across different lock acquisitions. A late interval callback clears only its own timer.

The lock entry also owns a bounded history window for the busy stretch: the active turn and its queued replays. Later personas and queued user or persona turns can reuse it. Release preserves the window only while handing off to a queued replay; a replay that returns before acquiring the lock drops it. Empty-queue release, stale recovery, queue clearing, and /kill free the window and its gateway change signals. A new trigger from an idle channel always starts with a REST read.

Mutex acquisition and re-entrancy

When an admitted turn arrives:

Re-entrant turns: calls passing skipLock=true (such as empty-response retries) reuse the existing lock entry and its active typing keepalive without acquiring a new lock or resetting timers.
Lock acquisition: acquireChannelLockForTurn() sets isLocked = true, records lockedAt and lastProgressAt, and creates a fresh AbortController (activeTurnAbortController). The signal is available to executing tools via ToolContext.abortSignal.
Stale lock recovery: releaseStaleChannelLockIfExpired() checks whether lastProgressAt is older than CHANNEL_LOCK_TIMEOUT_MS (180s). If expired, it aborts the active turn controller, fires the stream kill handle, clears queued messages, and resets the lock. Active operations using runUnderWatchdog (such as streaming chunks and tool calls) suppress stale-lock recovery because they enforce their own timeouts.

Interruption and cancellation ownership

Channel locks coordinate three forms of turn interruption:

Follow-up interrupts: queueFollowUpForLockedTurn() allows an inbound message from the same user to interrupt an active stream via StreamOrchestrator.requestFollowUp(), up to MAX_FOLLOW_UP_INTERRUPTS (3). If the active turn is executing a tool (isInToolCallChain), the message is enqueued as the latest follow-up without interrupting the stream, protecting tool completion.
Natural stops: requestNaturalStopForLockedTurn() captures the message and registers StreamOrchestrator.requestStop(). Active streaming halts gracefully and saves the stop context.
Hard cancellation via /kill: forceKillChannelStream() in src/commands/kill.ts immediately aborts activeTurnAbortController and invokes activeStreamKill(), rejecting the streaming race with an error. The /kill command also clears pending turns using clearChannelProcessingQueue(). Tools that generate media check abortSignal.aborted before sending to avoid posting discarded media.

Release and queue replay

When the turn completes or throws, releaseChannelLockAndReplayQueue() executes in a finally block:

Resets lock metadata, clears activeStreamKill, and aborts activeTurnAbortController.
Stops the Discord typing keepalive timer.
Checks StreamOrchestrator.getAndClearStopContext(). If a natural stop occurred, it schedules handleStopResponse() via setImmediate.
Pops the next message from the FIFO messageQueue and schedules processQueuedMessage() via setImmediate. The stop callback is scheduled first, but the callbacks are not awaited in sequence. Scheduling order alone does not guarantee that the stop confirmation finishes before queue replay.

Shard ownership and shutdown

The mutex and its live Discord messages and callbacks stay in the guild’s shard owner process. Admission, queue replay, persona queues, stop responses, and follow-up requests require configured ownership and safe live leases. DMs map to shard 0. Re-entry must still belong to the active outer turn.

Lease uncertainty aborts turns and clears pending replay. Captured owner signals stay aborted after lease recovery, so an old continuation cannot dispatch as a new owner. Delivery paths check again after awaited preparation and before sending; already dispatched requests cannot be recalled.

Shutdown rejects new turns and discards pending replays while allowing active turns, including valid re-entry, up to 25 seconds to finish. It then aborts remaining turns through the existing terminal stream-stop path, suppressing cancellation retries and timeout notices. Stats and connection cleanup follow the drain; the Discord client remains connected until those phases finish.

Constraints and rationale

Single active turn per channel: serializing turns prevents race conditions across message sends, webhook delivery, and conversational context ordering.
Asynchronous queue unrolling: invoking queued replays and stop responses through setImmediate prevents call stack growth under heavy traffic.
Tool progress protection: suppressing stream interrupts while isInToolCallChain is true ensures external tool effects finish cleanly before the user’s follow-up message processes.

Source pointers

src/utils/chat/channelQueue.ts: runWithChannelLock(), queue storage, typing keepalive, and release logic.
src/commands/kill.ts: /kill command driving stream termination and queue flushing.
src/utils/discord/streamOrchestrator.ts: stop and follow-up request state management.