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branch-4.1: [fix](be) Fix waiter accounting in blocking queue #65827 - #65941
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### What problem does this PR solve? Issue Number: None Problem Summary: BlockingQueue split ownership of waiter counters between waiting threads and notifying threads. Around a timeout race, both sides could decrement the same registration, while a spurious wakeup could leave a stale registration behind. Once the counters drifted, a real waiter could be represented by zero and miss a notification, leaving routine load queue operations blocked until the one-hour fallback timeout. This is a missed wakeup and potentially long stall, rather than a permanent deadlock. In addition, try_put read the shutdown flag and queue size before acquiring the mutex, which introduced a data race. The following sequence shows how lock contention after a timeout corrupts the consumer waiter count: | Time | Consumer threads | Producer threads | get_waiting | Result | | --- | --- | --- | ---: | --- | | T0 | C increments the counter and calls wait_for, which releases the mutex. | - | 0 -> 1 | C is registered as a condition-variable waiter. | | T1 | C's timeout expires. C leaves the condition-variable wait set, but wait_for must reacquire the mutex before returning. | P acquires the mutex before C can reacquire it. | 1 | C's registration is still visible because C cannot update it without the mutex. | | T2 | C remains blocked while trying to reacquire the mutex. | P pushes an item, decrements the counter from 1 to 0, unlocks, and calls notify_one. | 1 -> 0 | The notification cannot wake C because its timed wait has already expired. | | T3 | C reacquires the mutex. wait_for returns timeout, so the old timeout branch decrements the same registration again. C then consumes the item. | - | 0 -> SIZE_MAX | The unsigned waiter count wraps around. | | T4 | After the queue becomes empty, a new consumer C2 increments the counter and starts waiting. | - | SIZE_MAX -> 0 | A real waiter is now represented by zero. | | T5 | Absent a spurious wakeup, shutdown, or another notification, C2 remains asleep until its next timeout, which is one hour by default. | P2 pushes an item, observes zero, and skips notify_one. | 0 | C2 suffers a missed wakeup and a potentially one-hour stall. | Make each waiting thread register immediately before wait_for and always unregister after wait_for returns, regardless of the wakeup reason. Notifiers now inspect waiter counters under the mutex without consuming registrations, then release the mutex before notifying. Remove the unsynchronized try_put fast path and add deterministic tests for both consumer and producer timeout races. ### Release note Fix potential long stalls in backend blocking queues under concurrent timeout and notification. ### Check List (For Author) - Test: Unit Test - Added BlockingQueueWaiterTest.TimedGetNotificationRace and BlockingQueueWaiterTest.TimedPutNotificationRace. - `./run-be-ut.sh --run --filter='BlockingQueueWaiterTest.*' -j 32` - Behavior changed: Yes. Waiter registrations are now released by the waiting thread, and try_put checks queue state only while holding the mutex. - Does this need documentation: No
hello-stephen
commented
Jul 23, 2026
Contributor
Thank you for your contribution to Apache Doris. Please clearly describe your PR:
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hello-stephen
commented
Jul 23, 2026
Contributor
run buildall |
yiguolei
approved these changes
Jul 23, 2026
ContributorAuthor
PR approved by at least one committer and no changes requested. |
ContributorAuthor
PR approved by anyone and no changes requested. |
hello-stephen
commented
Jul 23, 2026
Contributor
BE Regression && UT Coverage ReportIncrement line coverage Increment coverage report
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yiguolei
commented
Jul 23, 2026
Contributor
skip buildall |
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Cherry-picked from #65827