Since 9.4.0, IndexWriter.addIndexes(CodecReader...) blocks indefinitely (until another thread interrupts it or rolls the writer back) when the writer uses NoMergeScheduler, also together with NoMergePolicy as the NoMergeScheduler javadoc recommends: the reader merge is handed to the merge scheduler, which never runs it, and is then awaited with no time limit

Finding LFV-586Area: index / IndexWriter addIndexes(CodecReader...) (merge-scheduler hand-off, NoMergeScheduler)Priority: lowImpact: hangNovelty: new

Finding page for LFV-586

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

What it shows

You configure an IndexWriter with NoMergeScheduler (also with NoMergePolicy, the pairing the NoMergeScheduler javadoc recommends) and call addIndexes(CodecReader...). You expect the call to add the readers, as it did up to 9.3.0, or at least to return or throw. Instead, since 9.4.0 (LUCENE-10216) the call hands its reader merge to the merge scheduler and then waits for it with no time limit; NoMergeScheduler.merge does nothing, so the call blocks until another thread interrupts it or rolls the writer back.

The test shows that with NoMergeScheduler + NoMergePolicy, IndexWriter.addIndexes(CodecReader...) of one 1-document reader hands its merge to the scheduler, which never runs it, and parks in MergeSpecification.await() with no time limit.

Expected vs actual

Expected: the call returns or throws. Actual: addIndexes must return or throw, but was still blocked after 10 s (the test then calls rollback() to release it).

Files

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

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: iw-forcemerge-wait-protocol-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 2026-11-03. AI-assisted; not yet triaged by a human Lucene committer. Feedback: open an issue in lucene-ai-bug-hunt.