IndexWriter.forceMergeDeletes returns a MergeObserver that also tracks merges IndexWriter refused to register, so the observer never completes and await() blocks forever

Finding LFV-207Area: index / IndexWriter forceMergeDeletes MergeObserver (registerMerge, MergePolicy.MergeObserver)Priority: lowImpact: hangNovelty: new

Finding page for LFV-207

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

What it shows

You use LogDocMergePolicy or LogByteSizeMergePolicy (or a custom merge policy), call IndexWriter.forceMergeDeletes(false) while another merge is already running on one of the segments with deletes, and wait on the returned MergeObserver, whose javadoc says it lets you 'monitor merge progress and wait for completion'. IndexWriter refuses to register the conflicting forced-deletes merge (it only logs 'reject merge'), so that merge never runs, but it still puts the merge into the observer. The observer then reports 0 of 1 merges complete forever: await() blocks forever even after the writer is closed, await(timeout) returns false, and the blocking forceMergeDeletes() returns at once with an incomplete observer although its javadoc says 'merges will already be complete when it returns'.

The test shows that IndexWriter.forceMergeDeletes() (blocking) returns a MergeObserver that includes a merge IndexWriter refused to register because its segment is already being merged (LogDocMergePolicy, a held background forceMerge(1, false)), so that merge never runs or completes.

Expected vs actual

Expected numCompletedMerges() == numMerges() on return (javadoc: merges will already be complete); actual: completed merges expected:<1> but was:<0>, with -ea and -da.

Files

• src/org/apache/lucene/index/TestForceMergeDeletesRejectedMerge.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.TestForceMergeDeletesRejectedMerge

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).

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.