GROOVY-12163: fix concurrent modification of ExpandoMetaClass mixinClasses

[paulk-asert]

Provides ticket number and improves test for #2698

[Copilot]

Pull request overview

This PR addresses a concurrency bug in ExpandoMetaClass mixin handling where concurrent mixin additions could throw ConcurrentModificationException during method dispatch, by switching the underlying mixin set to a thread-safe implementation and adding a regression test.

Changes:

• Replace ExpandoMetaClass’s mixinClasses backing set with CopyOnWriteArraySet to support concurrent iteration + structural modification.
• Add a new concurrency regression test that exercises concurrent mixin addition while dispatch scans mixins.
• Clarify MixinInMetaClass#hashCode() documentation in relation to mixin deduplication behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/main/java/groovy/lang/ExpandoMetaClass.java	Switch mixinClasses from LinkedHashSet to CopyOnWriteArraySet to avoid concurrent modification during dispatch.
src/main/java/org/codehaus/groovy/reflection/MixinInMetaClass.java	Expand Javadoc around hashCode() in the context of mixin deduplication.
src/test/groovy/groovy/lang/ExpandoMetaClassMixinConcurrencyTest.groovy	Add a concurrency test intended to reproduce/guard against concurrent mixin-add vs dispatch iteration issues.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 69.1108%. Comparing base (86c6648) to head (b94cb8c).

Files with missing lines	Patch %	Lines
...g/codehaus/groovy/reflection/MixinInMetaClass.java	0.0000%	1 Missing ⚠️

Additional details and impacted files

@@                Coverage Diff                 @@
##               master      #2708        +/-   ##
==================================================
+ Coverage     69.1077%   69.1108%   +0.0031%     
  Complexity      34235      34235                
==================================================
  Files            1537       1537                
  Lines          129356     129356                
  Branches        23503      23503                
==================================================
+ Hits            89395      89399         +4     
+ Misses          31941      31937         -4     
  Partials         8020       8020                

Files with missing lines	Coverage Δ
src/main/java/groovy/lang/ExpandoMetaClass.java	77.1376% <100.0000%> (+0.3718%)	⬆️
...g/codehaus/groovy/reflection/MixinInMetaClass.java	81.3333% <0.0000%> (-1.3333%)	⬇️

... and 7 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[testlens-app]

✅ All tests passed ✅

🏷️ Commit: b94cb8c
▶️ Tests: 103507 executed
⚪️ Checks: 31/31 completed

---

Learn more about TestLens at testlens.app.