HDDS-16275. OM checkpoint pause() stops flush thread but not apply executor.

[sadanand48]

What changes were proposed in this pull request?

• Added apply-path quiescing in OzoneManagerStateMachine:
  -pause() now blocks new apply entries and waits for in-flight apply work to drain.
  • After apply drain, pause() waits for double-buffer flush completion, then stops the buffer thread.
  • unpause() resumes apply in a finally block to ensure waiters are always released.
• Hardened applyTransaction() for executor scheduling failures:
  -if async scheduling fails (for example RejectedExecutionException), in-flight counters are released correctly to avoid pause hangs.
  -Added test coverage in TestOzoneManagerStateMachine:
  -verify apply is blocked while paused and resumes after unpause.
  -verify executor rejection does not leave pause blocked.

What is the link to the Apache JIRA

https://issues.apache.org/jira/browse/HDDS-16275

How was this patch tested?

Unit test

[Copilot]

🟡 Changes recommended

applyTransaction() can still leak in-flight counters (and unflushed permits) if supplyAsync rejects scheduling, which can deadlock pause(), and the described executor-rejection regression coverage is currently missing.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR updates OzoneManagerStateMachine checkpoint pause()/unpause() behavior to quiesce the apply path (not just the double-buffer flush thread), aiming to ensure checkpoint installation doesn’t race with in-flight apply work and to avoid pause hangs.

Changes:

• Added apply-transaction quiescing via a monitor/flag plus in-flight tracking around applyTransaction().
• Updated pause() to wait for apply drain and double-buffer flush before stopping the double-buffer thread; updated unpause() to resume apply in a finally block.
• Added a unit test asserting applyTransaction() blocks while paused and resumes after unpause().

File summaries

File	Description
hadoop-ozone/ozone-manager/src/main/java/org/apache/hadoop/ozone/om/ratis/OzoneManagerStateMachine.java	Introduces apply-path pause/resume and in-flight draining integrated into pause()/unpause() and applyTransaction().
hadoop-ozone/ozone-manager/src/test/java/org/apache/hadoop/ozone/om/ratis/TestOzoneManagerStateMachine.java	Adds a regression test for pause/unpause blocking behavior of applyTransaction().

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 2
• Review effort level: Lite

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