darinspivey opened a new pull request, #25572:
URL: https://github.com/apache/pulsar/pull/25572
Topics with compaction enabled — most visibly `__change_events` system
topics — accumulate an unbounded `__compaction` subscription backlog that is
never drained, polluting backlog metrics across the cluster.
The broker disconnects the compaction consumer as part of
`PersistentSubscription.resetCursorInternal`, then resets the cursor and sends
the success response. This races with the client: `channelInactive` fires on
the consumer's `ClientCnx` and fails the in-flight seek future with
`ConnectException` before the broker's success response arrives.
`AbstractTwoPhaseCompactor.phaseTwoSeekThenLoop` has no retry for this
transient failure, so every compaction attempt aborts and the `__compaction`
subscription backlog grows without bound. This is most visible on
`__change_events` system topics, where compaction is triggered on any non-zero
backlog.
The seek is idempotent and the cursor is already repositioned server-side,
so retry the seek on `ConnectException` with a bounded backoff (100ms -> 1s,
10s budget). Non-transient failures propagate immediately. A unit test
reproduces the race via an injection hook and asserts the compactor recovers.
Fixes #25279
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