Monitor.match can throw NullPointerException while queries are being registered (or while a read-only Monitor refreshes), because QueryIndex keeps its per-reader term filters in a plain HashMap that the searcher refresh and reader-close listeners modify while match threads read it without a lock

Finding LFV-769Area: monitor / QueryIndex term-filter map (searcher refresh vs concurrent match, writable and read-only Monitor)Priority: lowImpact: crashNovelty: new

Finding page for LFV-769

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

What it shows

You share one Monitor between threads: some threads call Monitor.match on documents while another registers new queries, and each register commits and opens a new query-index searcher, which the Monitor's design for searches during commits allows. You expect every match of a valid document to return its matching queries. Instead, when the internal map from searcher to term filter (a plain java.util.HashMap) grows, a concurrent match can look up its own searcher's entry and get null, and Monitor.match throws NullPointerException from TermFilteredPresearcher.buildQuery; the read-only Monitor fails the same way while its refresh runs.

Expected vs actual

• Expected: No Monitor.match call throws while another thread registers queries.
• Actual: NullPointerException: ... "this.val$termAcceptor" is null at TermFilteredPresearcher.java:122, thrown out of Monitor.match, with assertions on and off.

Files

• src/org/apache/lucene/monitor/TestMonitorTermFilterMapRace.java (JUnit test)
• src/org/apache/lucene/monitor/TestReadonlyMonitorTermFilterMapRace.java (JUnit test)

Run it

• Tested on Lucene main e1faeba08bde (2026-11-02): still reproduces.
• Tested on Lucene main a9779ef3c193 (2026-10-26): reproduced.
• 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.TestMonitorTermFilterMapRace

# run 2: assertions on, seed DEADBEEF, default flags
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.TestReadonlyMonitorTermFilterMapRace

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).

Files shown on the finding page

• TestMonitorMatchDuringRegister.java

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.