Fix flaky CompactionWorkerTest by replacing fixed sleep with CountDownLatch

[shuwenwei]

Problem

CompactionWorkerTest.testFailedToAllocateFileNumInCrossTask and testFailedToCheckValidInCrossTask intermittently fail with:

expected:<NORMAL> but was:<COMPACTION_CANDIDATE>

The tests use thread.join(2s) to wait for the compaction task to be dropped. However, in these failure cases queue.take() never returns: the task is dropped by CompactionTaskQueue.prepareTask -> dropCompactionTask, which calls resetCompactionCandidateStatusForAllSourceFiles() to change the source files from COMPACTION_CANDIDATE back to NORMAL. The worker thread then loops back and waits on the empty queue.

The fixed 2-second join only waits for elapsed time, not for the actual state transition. Under load or scheduling delay, the main thread can assert before the worker thread performs the status reset, producing the flaky failure.

Fix

Replace the timing-based wait with a CountDownLatch triggered inside the worker thread at the exact state-transition point. A Mockito doAnswer hook calls countDown() right after resetCompactionCandidateStatusForAllSourceFiles() completes:

CountDownLatch statusResetLatch = new CountDownLatch(1);
Mockito.doAnswer(invocation -> {
  invocation.callRealMethod();
  statusResetLatch.countDown();
  return null;
}).when(taskMock).resetCompactionCandidateStatusForAllSourceFiles();

thread.start();
Assert.assertTrue(statusResetLatch.await(5, TimeUnit.SECONDS));

This waits for the exact condition (source files back to NORMAL) with a 5-second timeout, making the tests deterministic instead of relying on arbitrary sleep.

Applied to all four task-drop tests in CompactionWorkerTest that used the same pattern.

[sonarqubecloud]

Quality Gate failed

Failed conditions
B Reliability Rating on New Code (required ≥ A)

See analysis details on SonarQube Cloud

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.95%. Comparing base (ada62cb) to head (03920be).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #18498      +/-   ##
============================================
- Coverage     43.95%   43.95%   -0.01%     
  Complexity      374      374              
============================================
  Files          5402     5402              
  Lines        388198   388198              
  Branches      50694    50694              
============================================
- Hits         170633   170623      -10     
- Misses       217565   217575      +10     

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