fix(core): make Util.memoize thread-safe

[patientstreetlight]

Util.memoize stored its cached value and "computed" flag in plain
fields, so a thread racing the first call could observe the flag set
without the value and get null back, and concurrent callers could each
re-run the delegate. The memoizer backs function keys and anchors, the
ExtensionCollection lookup maps, and relation record types, so a shared
ExtensionCollection failed intermittently on first concurrent use with
"NullPointerException: key" or "Duplicate key null".

The flag is now volatile and initialization uses double-checked locking,
so the delegate runs exactly once and every caller sees the fully
computed value. The memoizer's delegate and AbstractRel's memoizer field
are now final so they are safely published along with their owner.

Closes #1389

[coderabbitai]

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 73232c3 and 9d0859c.

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📒 Files selected for processing (3)

• core/src/main/java/io/substrait/relation/AbstractRel.java
• core/src/main/java/io/substrait/util/Util.java
• core/src/test/java/io/substrait/util/UtilTest.java

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📝 Summary

Summary by CodeRabbit

• Bug Fixes
  • Memoized values are now initialized safely when accessed concurrently, preventing duplicate supplier calls and ensuring callers receive the same cached result.
  • If a supplier throws an exception, a later call can retry initialization. Successful null results are also cached.

Walkthrough

Util.memoize now synchronizes initialization and safely publishes the first successful result. Added tests cover repeated calls, null results, and concurrent first access. AbstractRel.recordType is now final.

Changes

Memoized supplier

Layer / File(s)	Summary
Memoizer initialization and validation core/src/main/java/io/substrait/util/Util.java, core/src/main/java/io/substrait/relation/AbstractRel.java, core/src/test/java/io/substrait/util/UtilTest.java	Util.memoize documents thread-safe caching after a successful delegate call. Initialization is synchronized and rechecks the retrieved flag. Exceptions propagate without setting the flag, so a later call can retry. Tests verify single invocation for repeated and null results, and one shared result across eight concurrent callers. AbstractRel.recordType is declared final.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~12 minutes

Change: Bug fix · Severity of issue fixed: Medium

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Merge Risk: ⚪ Minimal · up to 9d085

No actionable issue remains; the change is ready to merge after normal checks.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 5 functions across 3 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check		The title is concise, specific, and uses valid Conventional Commit format. It accurately identifies the main change: making Util.memoize thread-safe.
Description check		The description explains the concurrency failure, the affected memoizer behavior, the implemented fix, and the linked issue. It provides the rationale required by the template and is directly related …
Linked Issues check		Issue #1389 requires thread-safe memoization with safe publication and coordinated first initialization. Util.Memoizer now writes value before volatile retrieved, uses double-checked locking, an…
Out of Scope Changes check		The changes stay within issue #1389. The Util.memoize implementation and documentation address concurrent initialization and publication. The new UtilTest tests the required behavior. The `Abstrac…

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