feat: add allowWorkerIdleExit option to suppress idle-worker error events

[trafgals]

What

Tinypool currently emits an error event on the public interface whenever a worker exits while it has no in-flight task. The intent is to surface post-task uncaught exceptions (e.g. an async setTimeout(() => { throw ... }, 0) that lands after pool.run() has returned), and that behavior is locked in by test/uncaught-exception-from-handler.test.ts.

The same code path also fires for workers that die from external causes that the consuming pool already has visibility into — most concretely, workerd subprocesses that segfault during teardown (see cloudflare/workerd#6763, also discussed in this comment thread on the upstream workerd issue tracker). When the workerd child dies after a vitest test file has reported its results, cloudflare/vitest-pool-workers is unable to suppress the resulting Worker exited unexpectedly event, and the whole vitest run exits with code 1 even though every test body was already reported as passing.

This PR adds an opt-in allowWorkerIdleExit: boolean option (default false) that lets consuming pools opt out of the public error emission for the idle-worker case. Workers that exit mid-task still surface the error to the in-flight task's callback, so genuine task failures continue to fail the affected task.

How

• Options.allowWorkerIdleExit?: boolean (default false)
• FilledOptions.allowWorkerIdleExit: boolean (the runtime-resolved value)
• kDefaultOptions.allowWorkerIdleExit = false (preserves current behavior)
• The existing worker 'error' handler is now a 3-branch conditional:
  • task in flight → reject the task's callback (unchanged)
  • task not in flight, opt-out enabled → silent (no error event)
  • task not in flight, opt-out disabled → emit error event (unchanged)

Tests

• New test/allow-worker-idle-exit.test.ts runs the same fixture twice — once with the option unset (asserting the existing error event) and once with allowWorkerIdleExit: true (asserting the error event is suppressed).
• The existing test/uncaught-exception-from-handler.test.ts continues to lock in the default behavior.

Cross-references

• cloudflare/workerd#6763 — upstream workerd SIGSEGV
• cloudflare/vitest-pool-workers PR #14793 — companion change that handles the same scenario at the pool layer for the bundled 0.16.x version.

Why not just patch vitest-pool-workers?

That pool does include a mitigation (PR #14793), but it has to live in their vendored copy of tinypool (dist/worker/lib/tinypool.mjs) until/unless this option exists upstream. Surfacing it here lets the fix land once and apply to every pool that wants to suppress idle-worker noise.

🤖 Generated with Claude Code

Co-Authored-By: Claude noreply@anthropic.com

[AriPerkkio]

Vitest no longer uses tinypool. Please try to reproduce the issue with vitest@4, and open a bug report on Vitest repository with a fix.