Fix thread_pool shutdown race

[mvandeberg]

thread_pool::impl::stop() set the stop_ flag without holding the mutex, creating a window where notify_all() could fire before a worker thread entered cv_.wait(), causing a permanent hang. This was the root cause of async_event test timeouts on slower FreeBSD CI runners.

Fix by acquiring the mutex before setting stop_, per the C++ standard requirement that shared variables used in condition_variable predicates must be modified under the mutex.

Also demote stop_ from std::atomic<bool> to plain bool since all accesses are now mutex-protected.

Summary by CodeRabbit

• Refactor
  • Internal changes to thread-pool synchronization to improve reliability and consistency of shutdown and waiting behavior, reducing subtle concurrency issues without changing public interfaces.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 6abaec83-3f6f-4faf-9089-d7444b3fc9e4

📥 Commits

Reviewing files that changed from the base of the PR and between 0b568f0 and 04146ae.

📒 Files selected for processing (1)

• src/ex/thread_pool.cpp

🚧 Files skipped from review as they are similar to previous changes (1)

• src/ex/thread_pool.cpp

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📝 Walkthrough

Walkthrough

Replaces stop_ atomic with a mutex-guarded bool in src/ex/thread_pool.cpp. stop() now sets the flag under the mutex; the worker wait predicate and shutdown checks read the mutex-protected stop_. The <atomic> include was removed.

Changes

Cohort / File(s)	Summary
Thread Pool Synchronization src/ex/thread_pool.cpp	Replaced std::atomic<bool> stop_ with bool stop_ protected by a mutex; updated stop() to lock the mutex when setting the flag; changed condition-variable wait predicate and shutdown checks to read stop_ under mutex; removed <atomic> include.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• thread_pool improvements: jthread, stop, lazy #86: Modifies thread pool stopping logic and adds stop/jthread support, overlapping with changes to how the stop state is represented and signaled.

Poem

🐇 I guard the flag with cozy locks,
No atomic leaps or memory shocks,
Threads now wait with mutex song,
Quiet stops where they belong,
A carrot-coded sync — hop along! 🥕

🚥 Pre-merge checks | ✅ 2

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'Fix thread_pool shutdown race' directly and clearly describes the main change: fixing a race condition in the thread pool shutdown mechanism.

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[cppalliance-bot]

An automated preview of the documentation is available at https://199.capy.prtest3.cppalliance.org/index.html

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2026-03-04 16:26:47 UTC

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Build time: 2026-03-04 16:34:40 UTC

[cppalliance-bot]

GCOVR code coverage report https://199.capy.prtest3.cppalliance.org/gcovr/index.html
LCOV code coverage report https://199.capy.prtest3.cppalliance.org/genhtml/index.html
Coverage Diff Report https://199.capy.prtest3.cppalliance.org/diff-report/index.html

Build time: 2026-03-04 16:40:29 UTC