[chrisuthe]

SyncTask::wait_for_start_acknowledgement() could erase a stale start acknowledgement without noting idle for it, leaving the main loop's parked STREAM_END waiting forever.

Summary

• The race. The loop read flags once at the top, then cleared COMMAND_START and decided whether an acknowledgement had arrived from that earlier snapshot. A stale acknowledgement (the start of a stream whose end overtook it) landing between the read and the clear was cleared but never seen, so the sync task skipped the note_sync_idle() the main loop's held STREAM_END waits for. Playback then deadlocked.
• The fix. Decide from the bits event_flags_.clear() returns. EventFlags::clear() returns the pre-clear bits on both platforms, so whatever the clear took is what gets judged.

Notes

• Not platform-specific. It showed up as a rare CI hang of ClientLifecycle.StreamEventsDrainedTogetherStartEachStreamOnItsOwnStart, and was split out of Fix what running on Windows turns up #211.
• No new test. The window sits between two statements inside one function, and a deterministic test would need a hook in production code to stop between them, which the test standards rule out. The existing lifecycle test covers the behaviour; the reproduction below shows it catches the race once the window is widened.
• docs/internals.md "Stream End and Start" already describes the intended behaviour (a stale acknowledgement notes idle), so it is unchanged.

Testing

• Reproduced by adding a 2 ms sleep before the clear: without the fix the lifecycle test hung 3/3 runs; with the fix it passed 20/20 with the sleep in place (and 40/40 earlier, in Fix what running on Windows turns up #211).
• Linux, GCC 16: host build clean; sendspin_tests 705/705. The ASan/UBSan build could not link on this machine (no sanitizer runtime installed), so the sanitizer and TSan runs are this PR's CI.

[Copilot]

🟢 Approval recommended

No active review issues remain for the change addressing the audio playback startup race.

No open issues 🎉

What changed in this PR

Fixes a race in audio playback startup where a stale start acknowledgement could be cleared without notifying the main loop that the sync task is idle, leaving a pending STREAM_END blocked indefinitely.

Changes:

• Uses the pre-clear bits returned by EventFlags::clear() to detect start acknowledgements.
• Updates the inline comment to explain why the earlier flag snapshot is insufficient.

File	Description
src/​sync_task.cpp	Detects acknowledgements arriving between the flag read and clear so stale starts receive the required idle notification.

_{Review effort: Balanced}

[kahrendt]

LGTM