[Dataflow Streaming] Fix race in StreamingEngineWorkComitter during stream shutdown

[arunpandianp]

The race is between commit() and stop().
Since it happens only during shutdown, it doesn't affect cloudpath where commiter is not shutdown till the worker shutdown.
directpath is affected since it can shutdown committers all the time.

[gemini-code-assist]

Summary of Changes

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This pull request resolves a critical race condition within the StreamingEngineWorkCommitter that could lead to uncommitted work items during stream shutdown, particularly affecting directpath environments. The changes ensure that all commits are properly processed, even when the system is concurrently shutting down, by adjusting the order of operations in the commit() method and explicitly draining the commit queue. A new, comprehensive test has been added to validate this fix under concurrent conditions, significantly improving the reliability of the streaming engine's commit mechanism.

Highlights

• Race Condition Fix: Addressed a race condition between the commit() and stop() methods within the StreamingEngineWorkCommitter during stream shutdown, specifically impacting directpath executions.
• Commit Handling during Shutdown: Modified the commit() method to ensure that work items are added to the commit queue before checking if the committer is shutting down. If shutdown is in progress, drainCommitQueue() is explicitly called to process any newly added commits, preventing them from being orphaned.
• New Concurrency Test: Introduced a new test case, testStop_drainsCommitQueue_concurrentCommit, which simulates concurrent commit() calls during a shutdown sequence to verify that the commit queue is correctly drained under these conditions.
• Test Robustness Improvement: Enhanced the waitForExpectedSetSize utility method in the test suite by adding a timeout, making tests more robust against indefinite waits.

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[arunpandianp]

R: @scwhittle @parveensania

[github-actions]

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