Root cause. The test sampled its flush baseline after a bare `Task.Delay(100)`
and then charged every later flush to the 128-event burst. Two facts make that
window unsound:
1. `RunHeartbeatLoopAsync` sends its first beat *immediately* on entering the
message loop — the far-off `HeartbeatInterval` only spaces later beats — so
the pre-burst frames are WorkerHello, WorkerReady, and a heartbeat, not the
two the test's comment assumed.
2. `WorkerFrameWriter.DrainQueuedFramesAsync` flushes *after* writing a drained
batch, so the bytes reach the pipe before the flush runs. Reading a frame off
the gateway side is therefore no evidence that its flush has been counted,
and no sleep makes it evidence.
Under load on the shared windows-x86 runner the first heartbeat's flush was
scheduled after the 100 ms sample, so it landed inside the measured window and
the assertion saw two flushes for the burst — exactly the observed
`Expected: 1 / Actual: 2` (Gitea run 675, job 2558, and the same failure since
a346d51). Nothing about the coalescing behavior was wrong; only the test's
timing assumption.
Fix. Replace the sleep with explicit synchronization, no widened timeouts.
`FlushCountingPassthroughStream` now records the flush *shape* — the number of
stream writes coalesced into each flush — and exposes
`WaitForAllWritesFlushedAsync`, a TaskCompletionSource signal released when the
next flush drains the pending writes. The test reads the first heartbeat (the
last pre-burst frame), waits for its flush, and only then samples the baseline;
after the burst it takes the same edge before asserting. The assertion is also
sharpened from a bare count to the shape: exactly one flush beyond the baseline
*and* that flush carried all 128 event frames, so a split batch fails even if
the reader observes it mid-split.
docs/WorkerFrameProtocol.md notes the peer-visible ordering the fix turns on:
frames reach the pipe before the flush that follows them, so an observer of the
flush must wait for it rather than infer it from frames arriving.
Joseph Doherty
50322bacb3