Do not return a null name when the name cache is cleared concurrently

[softvisionfd]

Summary

ASAtom.getASAtom(String) looks up a dynamic name with containsKey followed by get. Since
#730, ASAtom.clearCache() can empty that cache at runtime. If it runs on another thread
between the two calls, getASAtom returns null for a non-null name, and the caller receives
a null atom. CodeRabbit pointed this out on #730.

Changes

• getASAtom reads the cache with a single get and returns the result only when it is
  non-null; otherwise it creates the atom as before.
• The predefined-name lookup is unchanged; that map is never cleared.

Verification

The race is timing-dependent, so this PR adds no unit test for it. A stress probe outside the
code base (19 threads calling getASAtom for 64 names while one thread calls clearCache() in
a loop, 4 seconds per run) returned null between 31,000 and 33,000 times per run before this
change and never after it, over three runs each.

Backward compatibility

No behaviour change when clearCache() is not called concurrently. Java 8 compatible; all 386
parser tests pass.

Summary by CodeRabbit

• Refactor
  • Streamlined PDF atom lookup while preserving existing behavior. This internal optimization does not change how atom names are resolved or affect user-facing functionality.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

• Configuration used: defaults
• Review profile: CHILL
• Plan: Advanced
• Run ID: 30d34817-4d37-4677-93f0-0637adb20921

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📥 Commits

Reviewing files that changed from the base of the PR and between a598f30 and 84093cf.

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📒 Files selected for processing (1)

• src/main/java/org/verapdf/as/ASAtom.java

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📝 Walkthrough

Walkthrough

getASAtom now uses one CACHED_PDF_NAMES lookup. It returns the cached atom when the result is non-null.

Changes

Atom Lookup

Layer / File(s)	Summary
Cached atom lookup src/main/java/org/verapdf/as/ASAtom.java	getASAtom replaces separate containsKey and get calls with one lookup. When the result is null, atom creation continues.

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~5 minutes

Change: Bug fix

Suggested reviewers: lonelymidoriya

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Merge Risk: ⚪ Minimal · up to 84093

The change addresses the reported null-return race, and the supplied context identifies no remaining actionable merge risk.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main change: preventing null results when the name cache is cleared concurrently.
Docstring Coverage		Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

• Commit to this branch
• Create a new PR

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🧪 Generate unit tests (beta)

• Create a new PR

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• Autofix · Keep fixing CodeRabbit findings and required CI, and resolving merge conflicts

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