A read-only Monitor's background refresh stops for good after one run throws an unchecked exception, and no error is reported

Finding LFV-552Area: monitor / read-only query index (ReadonlyQueryIndex background refresh)Priority: lowImpact: minorNovelty: 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

You open a read-only Monitor over a query index that a writable Monitor updates, and count on its background thread to pick up the writer's changes at the configured purge frequency, which the upstream test TestMonitorReadonly.testReadonlyMonitorGetsRefreshed asserts. You expect a failed refresh run to be reported through MonitorUpdateListener.onPurgeError and the next run to try again, as the writable Monitor's purge task does. Instead the read-only task catches only IOException, so one unchecked exception (for example from an onPurge listener that throws once, or a ConcurrentModificationException when a listener is added while a run is calling the listeners) escapes the scheduled task, the executor silently cancels every later run, and the Monitor keeps matching its old query set with no error.

Expected vs actual

• Expected: You expect a failed refresh run to be reported through MonitorUpdateListener.onPurgeError and the next run to try again, as the writable Monitor's purge task does.
• Actual: The read-only task catches only IOException, so one unchecked exception (for example from an onPurge listener that throws once, or a ConcurrentModificationException when a listener is added while a run is calling the listeners) escapes the scheduled task, the executor silently cancels every later run, and the Monitor keeps matching its old query set with no error.

Files

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

Run it

• Tested on Lucene main e1faeba08bde (2026-11-02): still reproduces.
• Tested on Lucene main ea57360912ed (2026-10-24): 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.TestReadonlyRefreshSurvivesListenerFailure

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.