[FLINK-40475][runtime] Fix watermark loss and stall in StatusWatermarkValve when subpartitions realign after idleness

[MartijnVisser]

What is the purpose of the change

When a subpartition of StatusWatermarkValve reactivates after idleness and rejoins the aligned set, the min watermark across aligned subpartitions is not re-derived. This breaks the invariant that the aligned min equals the last output watermark, which two guards (subpartitionStatus.watermark == lastOutputWatermark) silently rely on, with two symptoms:

1. The all-idle flush-to-max from FLINK-7728 is skipped when the last subpartition to become idle is unaligned (went idle, resumed, only partially caught up), making the final watermark dependent on the order in which inputs became idle — the exact defect FLINK-7728 was created to fix. This also diverges from CombinedWatermarkStatus, which flushes unconditionally since FLINK-38454.
2. A subpartition that reactivates as the only aligned subpartition has its watermark stalled indefinitely, until an even larger watermark arrives on that subpartition — no all-idle transition involved.

Brief change log

• Re-derive (and possibly emit) the min watermark at the end of the idle→active branch, after the ACTIVE status is emitted so downstream inputs do not drop the watermark while they still consider the input idle
• Flush the max watermark unconditionally when all subpartitions become idle (the flush helper already only emits when the max advances past the last output watermark, so monotonicity is unaffected)
• Drop the equivalent guard on the partial-idle min re-derivation for the same reason
• Document the invariants of the aligned-subpartition set on alignedSubpartitionStatuses

Verifying this change

This change added tests and can be verified as follows:

• Added testMultipleInputFlushMaxWatermarkOnceAllInputsBecomeIdleWithUnalignedLastChannel, reproducing the order-dependent all-idle flush (fails on master: emits only WatermarkStatus(IDLE) instead of Watermark(100))
• Added testWatermarkAdvancesWhenReactivatedChannelBecomesOnlyAlignedChannel and testRealignmentAfterResumeActiveEmitsNewMinWatermark, reproducing the stalled watermark on realignment (fail on master: no watermark emitted)
• All pre-existing tests in StatusWatermarkValveTest are unaffected; also verified green: OneInputStreamTaskTest, TwoInputStreamTaskTest, WatermarkOutputMultiplexerTest, SourceOperatorSplitWatermarkAlignmentTest
• Each commit passes the valve test suite individually (red-green verified per commit)

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no (only watermark-status transition paths; the added re-derivation is O(1), the unconditional flush is O(#subpartitions) once per all-idle transition, next to an existing O(#subpartitions) scan on the same path)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

Note on behavior: jobs that previously ended with a lower watermark depending on idle-arrival order will now deterministically receive the flushed max watermark (timers/windows in that range fire); a release note is proposed on the JIRA ticket.

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Generated-by: Claude Code (claude-fable-5)

[flinkbot]

CI report:

• 2e39832 Azure: SUCCESS

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[rkhachatryan]

LGTM, thanks for the fix!