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https://issues.apache.org/jira/browse/KAFKA-19242?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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David Jacot resolved KAFKA-19242.
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Fix Version/s: 3.9.2
4.0.1
4.1.0
Resolution: Fixed
> Fix commit bugs caused by race condition during rebalancing.
> ------------------------------------------------------------
>
> Key: KAFKA-19242
> URL: https://issues.apache.org/jira/browse/KAFKA-19242
> Project: Kafka
> Issue Type: Bug
> Components: clients
> Reporter: sanghyeok An
> Assignee: sanghyeok An
> Priority: Major
> Fix For: 3.9.2, 4.0.1, 4.1.0
>
> Attachments: image-2025-05-05-19-19-06-147.png,
> image-2025-05-12-21-43-07-656.png
>
>
> *Motivation*
> While investigating “events skipped in group rebalancing”
> (spring‑projects/spring‑kafka#3703) I discovered a race condition between
> - the main poll/commit thread, and
> - the consumer‑coordinator heartbeat thread.
> If the main thread enters `ConsumerCoordinator.sendOffsetCommitRequest()`
> while the heartbeat thread is finishing a rebalance
> (`SyncGroupResponseHandler.handle()`), the group state transitions in the
> following order:
> !image-2025-05-12-21-43-07-656.png|width=761,height=448!
> Because we read the state twice without a lock:
> 1. `generationIfStable()` returns `null` (state still `COMPLETING_REBALANCE`),
> 2. the heartbeat thread flips the state to `STABLE`,
> 3. the main thread re‑checks with `rebalanceInProgress()` and wrongly decides
> that a rebalance is still active,
> 4. a spurious `CommitFailedException` is returned even though the commit
> could succeed.
>
> *Impact*
> - The exception is semantically wrong: the consumer is in a stable group,
> but reports failure.
> - Frameworks and applications that rely on the semantics of
> `CommitFailedException` and `RetryableCommitException` (for example `Spring
> Kafka`) take the wrong code path, which can ultimately skip the events and
> break “at‑most‑once” guarantees.
>
> *Fix*
> We enlarge the synchronized block in
> `ConsumerCoordinator.sendOffsetCommitRequest()` so that the consumer group
> state is examined atomically with respect to the heartbeat thread:
>
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