IndexWriter keeps a full-flush merge that registerMerge rejected: commit() does not throw the IllegalStateException that MergePolicy#findFullFlushMerges documents, the rejected merge's readers stay open after IndexWriter.close(), and commit/getReader wait the full maxFullFlushMergeWaitMillis for a merge that never runs

Finding LFV-502Area: index / IndexWriter point-in-time merges (merge-on-commit, merge-on-getReader: updatePendingMerges, registerMerge, preparePointInTimeMerge, MergeSpecification.await)Priority: lowImpact: minorNovelty: new

Finding page for LFV-502

AI-assisted finding, confirmed by automated checks, not yet triaged by a human Lucene committer. This site files nothing upstream.

What it shows

You install a merge policy whose findFullFlushMerges (merge-on-commit or merge-on-getReader) proposes a segment that another merge is already working on, a case whose outcome the javadoc spells out: commit() 'will throw a IllegalStateException'. Instead IndexWriter quietly refuses to register that merge but still opens its segment readers and waits for it, so commit() returns normally only after the full maxFullFlushMergeWaitMillis, the rejected merge's files stay open after IndexWriter.close(), and DirectoryReader.open(writer) keeps waiting even after another thread has called rollback(). The merges that did register are applied normally.

The test shows that a MergePolicy whose findFullFlushMerges returns two overlapping merges ({s0, s1} and {s1, s2}) does not make IndexWriter.commit() throw. IndexWriter silently rejects the second merge, waits the full 500 ms for it, and returns.

Expected vs actual

Expected the IllegalStateException that the MergePolicy#findFullFlushMerges javadoc documents; actual: no exception (Expected exception IllegalStateException but no exception was thrown), with -ea and -da.

Files

• src/org/apache/lucene/index/TestRejectedFullFlushMerge.java (JUnit test)

Run it

• Tested on Lucene main e1faeba08bde (2026-11-02): still reproduces.
• Tested on Lucene main 645d4fe14a20 (2026-10-28): reproduced.
• Recorded outcome of run 1 on Lucene main 645d4fe14a20: 1 test(s) run, 1 failed.
• How to read the outcome: a failing test is the finding reproducing (the failure message states expected against actual).
• Seeds: a rerun with the same seed replays the same random choices. Assertions off (-da -dsa -Dtests.asserts=false) is the production-like setting.

# Lucene checkout at the tested commit, built with: ./gradlew --no-daemon assemble   (JDK 25)
LUCENE=/path/to/lucene; V=12.0.0-SNAPSHOT
CP=$LUCENE/lucene/core/build/libs/lucene-core-$V.jar
CP=$CP:$LUCENE/lucene/codecs/build/libs/lucene-codecs-$V.jar
CP=$CP:$LUCENE/lucene/test-framework/build/libs/lucene-test-framework-$V.jar
CP=$CP:randomizedtesting-runner-2.9.2.jar:junit-4.13.2.jar:hamcrest-3.0.jar   # test-framework dependencies (Maven Central)
CP=$CP:junit-platform-console-standalone-6.1.3.jar   # JUnit Platform console launcher (Maven Central)

javac -d classes -cp "$CP" $(find src -name '*.java')

# run 1: assertions on, seed DEADBEEF
java -ea -esa --add-modules jdk.incubator.vector --enable-native-access=ALL-UNNAMED -Dtests.seed=DEADBEEF \
     -cp "classes:$CP" org.junit.platform.console.ConsoleLauncher \
     execute --select-class org.apache.lucene.index.TestRejectedFullFlushMerge

Gradle alternative: copy the test into the matching module's src/test/ tree of the checkout and run ./gradlew -p lucene/<module> test --tests <class> -Ptests.seed=<seed> (add -Ptests.asserts=false for a run without assertions).

Formal models and scripts this finding's part of the hunt used: merge-on-commit-refresh-tla

Feedback

Open an issue in lucene-ai-bug-hunt. Please do not open issues or pull requests in apache/lucene about this finding unless a Lucene committer agrees it should be filed.

Snapshot of 2026-11-04. AI-assisted; not yet triaged by a human Lucene committer. Feedback: open an issue in lucene-ai-bug-hunt.