Monitor.deleteById and Monitor.clear issue their IndexWriter delete outside the commit lock, so a delete or clear that runs while register is indexing a decomposed query leaves that query partly indexed: it stays registered but silently stops matching what its deleted parts match, also after a reopen

Finding LFV-501Area: monitor / query index updates (WritableQueryIndex.deleteQueries and clear versus commitWithoutNotify, reached through Monitor.deleteById, Monitor.clear and Monitor.register)Priority: lowImpact: silent wrong resultsNovelty: new

Finding page for LFV-501

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

What it shows

One thread registers query q = a OR b OR c, which the Monitor indexes as three parts, one per clause, while another thread calls monitor.deleteById("q") or monitor.clear(); you expect the two calls to take effect in some order, so that afterwards q is either fully registered or gone. Instead the delete goes to the IndexWriter without the commit lock that register holds, lands between register's per-part addDocument calls and removes only the parts already added. q is still reported as registered with its full query, getDisjunctCount is 2, and a document that contains only 'a' no longer matches, also after the Monitor is closed and reopened.

The test shows that Monitor.deleteById("q") running while register is indexing q = a OR b OR c (a hook pauses register while part f:b is indexed until deleteById is blocked) leaves q registered but only partly indexed.

Expected vs actual

Expected a document with only 'a' to match q exactly when q is still registered (1 match, or q gone). Actual: getQueryIds=[q] but 0 matches (expected:<1> but was:<0>).

Files

• src/org/apache/lucene/monitor/TestMonitorDeleteByIdDuringRegister.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=$CP:$LUCENE/lucene/analysis/common/build/libs/lucene-analysis-common-$V.jar
CP=$CP:$LUCENE/lucene/memory/build/libs/lucene-memory-$V.jar
CP=$CP:$LUCENE/lucene/monitor/build/libs/lucene-monitor-$V.jar
CP=$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.monitor.TestMonitorDeleteByIdDuringRegister

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: monitor-lifecycle-persistence-mbt

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.