Data race on SegmentInfos.version between newSegmentName and checkpoint

Finding LFV-017Area: index / IndexWriter NRTPriority: lowImpact: silent wrong resultsNovelty: known upstream

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

What it shows

Several indexing threads allocate new segment names while the writer checkpoints. You expect every SegmentInfos.version increment to be kept, so the version check of a reader is reliable. Instead newSegmentName increments the version under synchronized(segmentInfos) while other increments hold the IndexWriter monitor, so an increment can be lost or go backwards and a stale reader can look current.

Expected vs actual

• Expected: You expect every SegmentInfos.version increment to be kept, so the version check of a reader is reliable.
• Actual: newSegmentName increments the version under synchronized(segmentInfos) while other increments hold the IndexWriter monitor, so an increment can be lost or go backwards and a stale reader can look current.

Files

• src/R6Hook.java
• src/R6Count.java
• src/TestR6SegmentInfosVersionRace.java (JUnit test)

Run it

• Tested on Lucene main e1faeba08bde (2026-11-02): still reproduces.
• Tested on Lucene main b83101cdd2 (2026-10-23): reproduced.
• Recorded outcome of run 1 on Lucene main a9779ef3c193: 2 test(s) run, 2 failed (testAddIndexesRaceLowersVersionAndHidesChanges, testDwptCreationRaceLosesIncrements).
• 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.
• Not runnable from this site as is: this repro also needs extra JVM setup (a system property or an instrumentation hook, for example) that is not exported here; see the comments at the top of the source.
• This repro reads private Lucene state by reflection (setAccessible), so it depends on internals that may change.

# 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 -Xmx4g --add-modules jdk.incubator.vector --enable-native-access=ALL-UNNAMED -Dtests.seed=DEADBEEF \
     -cp "classes:$CP" org.junit.platform.console.ConsoleLauncher \
     execute --select-class TestR6SegmentInfosVersionRace

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.