Fix: make ProjectIndex thread-safe for parallel builds (backport #12832) - #12928
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Backport of #12832 to maven-3.9.x. Switch ProjectIndex backing maps from HashMap to ConcurrentHashMap to fix root cause of index corruption under parallel builds (-T). Add null guards for both index and project lookups with enriched error messages. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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gnodet
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Aug 30, 2026
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Clean backport of #12832 to maven-3.9.x. The adaptation is correct — on master the MojoExecutor already had a basic null check that #12832 enhanced, whereas on maven-3.9.x there was no null check at all (raw auto-unboxing), so the backport correctly introduces the full null guard pattern.
Observations (informational):
- The
ConcurrentHashMapswitch inProjectIndexis identical to the master change and is particularly important on 3.9.x whereprojectIndexis passed as a method parameter and shared across threads - The path difference (
maven-core/vsimpl/maven-core/on master) reflects the expected directory reorganization between branches - All three maintenance branches are covered: #12926 (maven-4.0.x), #12927 (maven-3.10.x), #12928 (maven-3.9.x)
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Claude Code on behalf of Guillaume Nodet
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Summary
Backport of #12832 to
maven-3.9.x.ProjectIndexbacking maps fromHashMaptoConcurrentHashMapto fix root cause of index corruption under parallel builds (-T)indicesandprojectslookups with enriched error messages mentioning parallel builds and extensions as possible causesTest plan
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