fix(agents): release initMutex after warming to restore pool concurrency

[drain-zine]

Description

ProcPool holds initMutex across proc.join(), which serialises the pool to effective concurrency 1 regardless of numIdleProcesses. Child procs spawned by JobProcExecutor are strictly one-shot:

• JobProcExecutor.launchJob throws "process already has a running job" if called twice (agents/src/ipc/job_proc_executor.ts:111).
• The child's message handler throws "job task already running" on a second startJobRequest (agents/src/ipc/job_proc_lazy_main.ts:264).
• After the single job completes, the child calls process.exit(0) (agents/src/ipc/job_proc_lazy_main.ts:303).
• The parent's SupervisedProc.#join Future only resolves on the child's 'exit' event (agents/src/ipc/supervised_proc.ts:163), so join() returns only after the child has exited.

Because initMutex was released in the outer finally after await proc.join(), the mutex was held for the entire lifetime of every running job. Since warming a replacement proc also requires initMutex, no replacement could initialise until the current job finished — making the pool effectively single-threaded.

We've been running this patch in production (via pnpm patch against @livekit/agents@1.2.3) for our voice agent. Before the patch: second concurrent sessions reliably timed out upstream regardless of numIdleProcesses, as the agent was blocked from dispatching workers to join the room. After the patch: concurrent sessions warm in parallel as expected at numIdleProcesses: 3.

Changes Made

• Release initMutex immediately after warmedProcQueue.put() succeeds, before await proc.join().
• Track release state with initReleased / procUnlockTransferred flags so the finally block doesn't double-unlock on the error path (e.g. if initialize() throws before enqueue).
• Add two regression tests in agents/src/ipc/proc_pool.test.ts:
  • releases initMutex after warming so concurrent procWatchTasks can initialise — asserts initMutex is released while proc.join() is still pending, and that the finally doesn't double-release.
  • releases initMutex in finally when initialization fails before enqueue — asserts both locks are still released on the error path.

Pre-Review Checklist

• Build passes: pnpm -w format:write, pnpm -w lint:fix, and pnpm --filter @livekit/agents exec vitest run src/ipc/proc_pool.test.ts all pass locally
• AI-generated code reviewed: Comments tightened, no dead code
• Changes explained: See above
• Scope appropriate: Only proc_pool.ts and proc_pool.test.ts touched
• Video demo: N/A — concurrency bug is not visually reproducible in Playground; covered by unit tests instead

Testing

• Automated tests added/updated — two new regression tests in proc_pool.test.ts
• All tests pass (5/5 in proc_pool.test.ts)
• Make sure both restaurant_agent.ts and realtime_agent.ts work properly — N/A, change is internal to ProcPool and doesn't affect agent APIs

Additional Notes

We've been running this patch in production (via pnpm patch against @livekit/agents@1.2.3) for our LiveKit agent handling Twilio inbound calls. Before the patch: second concurrent call reliably failed with no-answer. After the patch: concurrent calls work as expected with numIdleProcesses: 3.

Happy to adjust the flag-based double-unlock guard if you'd prefer a different structure (e.g. nulling out the unlock reference instead).

[changeset-bot]

🦋 Changeset detected

Latest commit: 1ffb239

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 22 packages

Name	Type
@livekit/agents	Patch
@livekit/agents-plugin-anam	Patch
@livekit/agents-plugin-baseten	Patch
@livekit/agents-plugin-bey	Patch
@livekit/agents-plugin-cartesia	Patch
@livekit/agents-plugin-deepgram	Patch
@livekit/agents-plugin-elevenlabs	Patch
@livekit/agents-plugin-google	Patch
@livekit/agents-plugin-hedra	Patch
@livekit/agents-plugin-inworld	Patch
@livekit/agents-plugin-lemonslice	Patch
@livekit/agents-plugin-livekit	Patch
@livekit/agents-plugin-neuphonic	Patch
@livekit/agents-plugin-openai	Patch
@livekit/agents-plugin-phonic	Patch
@livekit/agents-plugin-resemble	Patch
@livekit/agents-plugin-rime	Patch
@livekit/agents-plugin-sarvam	Patch
@livekit/agents-plugin-silero	Patch
@livekit/agents-plugins-test	Patch
@livekit/agents-plugin-trugen	Patch
@livekit/agents-plugin-xai	Patch

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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[toubatbrian]

@drain-zine Can you update the changeset plz?

[drain-zine]

@drain-zine Can you update the changeset plz?

@toubatbrian have done so! let me know how it looks -> 1ffb239