[Fix-15886] Fix thread leak in NacosRegistry using Double-Checked Loc…

[QiuYucheng2003]

What is the purpose of the change

Fixes #15886

This PR fixes a thread leak issue in NacosRegistry.java.

The method scheduleServiceNamesLookup had a race condition where multiple threads could pass the if (scheduledExecutorService == null) check simultaneously, leading to the creation of multiple ScheduledExecutorService instances that were never shut down.

Brief changelog

• Applied Double-Checked Locking in scheduleServiceNamesLookup to ensure the executor service is instantiated safely and only once.

Verifying this change

• Verified via code logic analysis (fixing the race condition).
• Existing unit tests should pass.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.75%. Comparing base (a3a35b5) to head (502e794).

Files with missing lines	Patch %	Lines
...org/apache/dubbo/registry/nacos/NacosRegistry.java	0.00%	17 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                3.3   #15915      +/-   ##
============================================
+ Coverage     60.73%   60.75%   +0.01%     
- Complexity    11702    11704       +2     
============================================
  Files          1938     1938              
  Lines         88694    88697       +3     
  Branches      13387    13388       +1     
============================================
+ Hits          53869    53884      +15     
+ Misses        29294    29288       -6     
+ Partials       5531     5525       -6     

Flag	Coverage Δ
integration-tests-java21	32.36% <0.00%> (-0.01%)	⬇️
integration-tests-java8	32.53% <0.00%> (+<0.01%)	⬆️
samples-tests-java21	32.00% <0.00%> (-0.02%)	⬇️
samples-tests-java8	29.73% <0.00%> (+0.04%)	⬆️
unit-tests-java11	59.04% <0.00%> (-0.01%)	⬇️
unit-tests-java17	58.53% <0.00%> (?)
unit-tests-java21	58.53% <0.00%> (?)
unit-tests-java25	58.48% <0.00%> (+0.01%)	⬆️
unit-tests-java8	59.04% <0.00%> (+<0.01%)	⬆️

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[QiuYucheng2003]

Regarding the Codecov report: Since this PR fixes a race condition via Double-Checked Locking, it is difficult to reproduce the concurrency scenario deterministically in unit tests, hence the 0% coverage on the new lines

[Copilot]

Pull request overview

This PR fixes a thread leak issue in NacosRegistry caused by a race condition during lazy initialization of the scheduledExecutorService. The fix implements double-checked locking to ensure the executor service is created only once, even when multiple threads attempt concurrent initialization.

Key Changes:

• Applied double-checked locking pattern in scheduleServiceNamesLookup method to prevent multiple executor service instances from being created
• Added synchronized block around executor initialization to ensure thread-safe singleton instantiation

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[Copilot]

Potential race condition. This assignment to scheduledExecutorService is visible to other threads before the subsequent statements are executed.