[FLUSS-1855][server] Add Null Safety for TimerTaskEntry Removal

[rionmonster]

Purpose

Linked Issue: Close #1855

Per Issue #1855, this pull request addresses a potential issue that could arise during the remove() operation for TimerTaskEntry that could result in a null pointer exception being thrown. This was possible as the underlying object to support the internal list was volatile and subject to operations from other threads.

Brief change log

Update the logic within the TimerTaskEntry.remove() operation to use a more concurrency-safe single-read snapshot pattern to avoid separate reads (e.g., reading during iteration and within the body as well). These changes were originally reproduced via a newly added unit test and retained to ensure the issue was resolved as expected.

Tests

Added a new TimerTaskEntryTest class along with an associated TimerTaskEntryTest.testRemoveEnsuresCurrentListNullSafety case that originally reproduced this issue (via concurrent, oscillating add/removals across separate threads) which was eventually updated after the fix was applied to confirm the exception will no longer be thrown.

API and Format

N/A

Documentation

N/A

[Copilot]

Pull request overview

This PR fixes a potential NullPointerException in the TimerTaskEntry.remove() method caused by a Time-Of-Check-Time-Of-Use (TOCTOU) race condition. The fix implements a single-read snapshot pattern to safely handle the volatile list field in concurrent scenarios.

Key Changes:

• Modified the remove() method to read the volatile list field once per loop iteration using an assignment within the while condition
• Added a comprehensive concurrency test (TimerTaskEntryTest) to verify the fix prevents NPEs during concurrent add/remove operations

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
fluss-server/src/main/java/org/apache/fluss/server/utils/timer/TimerTaskEntry.java	Updated remove() method to use single-read snapshot pattern, eliminating TOCTOU bug by assigning list directly in the while condition
fluss-server/src/test/java/org/apache/fluss/server/utils/timer/TimerTaskEntryTest.java	Added new test class with concurrency test that reproduces the race condition and verifies the fix prevents NullPointerException

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[wuchong]

Thanks @rionmonster for the awesome work and for providing a reproducible test case! The proposed fix looks solid to me.