Fix timer fast path starvation when used as a yield point

[sgerbino]

The already-expired timer fast path used dispatch() which does symmetric transfer on the same thread, resuming the coroutine without yielding to the scheduler. This starved queued work (e.g. sub-tasks spawned via run_async) when a 0ns timer was used as a poll/yield mechanism.

Switch to post() + noop_coroutine() so the coroutine enters the scheduler queue, giving other pending work a chance to execute.

Summary by CodeRabbit

• Performance
  • Optimized timer handling for already-expired timers to yield to the scheduler, allowing other queued work to execute and improving overall responsiveness.

[coderabbitai]

📝 Walkthrough

Walkthrough

This change modifies the timer service's already-expired timer handling in timer_impl::wait. When a timer has expired (heap index at max and expiry <= now), the coroutine is now posted to the executor instead of dispatched, yielding to the scheduler. The comment describing this behavior is updated accordingly.

Changes

Cohort / File(s)	Summary
Timer Service Fast Path src/corosio/src/detail/timer_service.cpp	Modified already-expired timer handling to use post() instead of dispatch() for executor submission, allowing scheduler yields. Updated accompanying comment. Error output still set when applicable.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Possibly related PRs

• PR #112: Modifies timer_impl::wait to short-circuit already-expired timers by posting coroutines to the executor, implementing a similar optimization pattern.

Poem

🐰 A timer once eager to run,
Now learns patience—the scheduler's fun!
Post, not dispatch, let others play,
Fair scheduling wins the day! ✨

🚥 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 timer fast path starvation when used as a yield point' directly and clearly describes the main change: addressing a starvation issue in the timer fast path when used as a yield mechanism.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.15%. Comparing base (9ed1f7d) to head (c739b1e).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #135      +/-   ##
===========================================
+ Coverage    80.93%   81.15%   +0.21%     
===========================================
  Files           64       64              
  Lines         5666     5667       +1     
===========================================
+ Hits          4586     4599      +13     
+ Misses        1080     1068      -12     

Files with missing lines	Coverage Δ
src/corosio/src/detail/timer_service.cpp	87.22% <100.00%> (+0.03%)	⬆️

... and 1 file with indirect coverage changes

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[cppalliance-bot]

An automated preview of the documentation is available at https://135.corosio.prtest3.cppalliance.org/index.html

If more commits are pushed to the pull request, the docs will rebuild at the same URL.

2026-02-12 16:11:05 UTC

[cppalliance-bot]

GCOVR code coverage report https://135.corosio.prtest3.cppalliance.org/gcovr/index.html
LCOV code coverage report https://135.corosio.prtest3.cppalliance.org/genhtml/index.html
Coverage Diff Report https://135.corosio.prtest3.cppalliance.org/diff-report/index.html

Build time: 2026-02-12 16:19:33 UTC