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

[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 60.72%. Comparing base (405bd9f) to head (cb26c94).

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     60.71%   60.72%           
- Complexity    11769    11775    +6     
=========================================
  Files          1948     1948           
  Lines         88732    88735    +3     
  Branches      13379    13380    +1     
=========================================
+ Hits          53877    53886    +9     
+ Misses        29325    29321    -4     
+ Partials       5530     5528    -2     

Flag	Coverage Δ
integration-tests-java21	32.26% <0.00%> (+0.05%)	⬆️
integration-tests-java8	32.29% <0.00%> (-0.05%)	⬇️
samples-tests-java21	34.86% <0.00%> (-0.03%)	⬇️
samples-tests-java8	32.65% <0.00%> (+0.10%)	⬆️
unit-tests-java11	58.94% <0.00%> (-0.05%)	⬇️
unit-tests-java17	58.45% <0.00%> (-0.01%)	⬇️
unit-tests-java21	58.44% <0.00%> (-0.02%)	⬇️
unit-tests-java25	58.40% <0.00%> (-0.03%)	⬇️
unit-tests-java8	58.98% <0.00%> (+0.02%)	⬆️

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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.

[QiuYucheng2003]

Verified that scheduledExecutorService is declared as volatile, so this DCL pattern is thread-safe.