[release/11.0] Fix ConcurrentQueue.TryDequeue spuriously reporting empty during a segment freeze - #132863
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[release/11.0] Fix ConcurrentQueue.TryDequeue spuriously reporting empty during a segment freeze#132863github-actions[bot] wants to merge 1 commit into
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…gment freeze (#132737) Fixes #132736 `ConcurrentQueueSegment.EnsureFrozenForEnqueues` published `_frozenForEnqueues = true` before bumping the Tail by `FreezeOffset`, while `TryDequeue`'s empty check reads the flag first and the Tail second. In the window between the two stores a dequeuer can pair `frozen == true` with the pre-freeze Tail and subtract `FreezeOffset` from a Tail that was never bumped. A segment holds fewer than `FreezeOffset` items, so `currentTail - FreezeOffset - currentHead <= 0` is then always true and `TryDequeue` reports the segment empty while it still holds committed items — a `false` return with no moment during the call at which the queue was empty. The `Interlocked.Add` full fence does not close the window; it only guarantees the stores become visible in exactly this order, and the freezing thread can stall between them (see the issue for the full interleaving and a reproducer that hits ~100 false-empties per 128M operations per round on current bits). The fix bumps the Tail before publishing the flag. The reader's three possible pairings become: 1. `frozen == true` — the bump is necessarily visible, so the Tail read afterwards includes `FreezeOffset` and the frozen clause reports empty only when the segment is genuinely drained. 2. `frozen == false` with a bumped Tail (freeze landed between the two reads) — `currentTail - currentHead` is a large positive value, so the check reports "not empty", spins, and retries; the next iteration takes pairing 1. This is the bounded benign retry the existing comment in `TryDequeue` already describes. 3. `frozen == false` with an un-bumped Tail — pre-freeze fast path, unchanged. Enqueuers never read the flag (a bumped Tail fails their sequence check and routes them to `EnqueueSlow`, unchanged), and `EnsureFrozenForEnqueues` only runs under the cross-segment lock, so the `if (!_frozenForEnqueues)` guard is unaffected by the reordering.
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This was referenced Aug 28, 2026
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Backport of #132737 to release/11.0
/cc @VSadov @kafka1991
Customer Impact
#132736
A race in
ConcurrentQueuemay result inTryDequeue/TryPeekassume the queue is empty when it actually has items.The race is rare because it can happen only when the circular segment is completely full and the queue allocates a larger segment, while marking the old one "frozen" for more enqueues.
Regression
Testing
A new test is included together with the fix.
Risk
Low.
The fix changes the order of publishing
_frozenForEnqueueswith respect to updating segment Tail position to ensure that the consuming side does not see the segment in inconsistent state.