A negative cache limit (GroupingSearch.setCaching(-1, ..) / setCachingInMB(-1.0, ..), CachingCollector.create(.., -1), CachingCollectorManager) is accepted and caches nothing, yet isCached() stays true, so replay() delivers no documents and grouped search silently returns groups with no documents and totalHitCount 0 (since 9.8.0; from 5.0.0 to 9.7.0 replay threw instead)
Finding LFV-908Area: search / CachingCollector first-pass caching (CachingCollector, CachingCollectorManager) x grouping GroupingSearch two-pass cachingPriority: lowImpact: silent wrong resultsNovelty: new
AI-assisted finding, confirmed by automated checks, not yet triaged by a human Lucene committer. This site files nothing upstream.
What it shows
You enable caching for a grouped search (or wrap a first-pass collector in CachingCollector or CachingCollectorManager) with a negative limit, for example setCaching(-1, true) meant as 'no limit', and expect either an IllegalArgumentException or the documented fallback for a cache over its limit: isCached() false and the query re-run. Instead the limit is accepted and nothing is cached (a negative count is the collector's own internal 'purged' marker), but isCached() stays true, so replay() delivers none of the collected documents and GroupingSearch returns its top groups with no documents and totalHitCount 0 instead of 20 in the repro, with no error. From 5.0.0 to 9.7.0 the same input failed at replay with an AssertionError (-ea) or IndexOutOfBoundsException (-da).
The test shows that GroupingSearch.setCaching(-1, true) is accepted, caches nothing, still reports the cache as usable, and so replays no document into the second pass.
Expected vs actual
Expected the grouped search over one matching document to report totalHitCount 1 (the documented fallback: cache purged, query re-run); actual: 0 (expected:<1> but was:<0>), silently, with -ea and -da.
Files
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/grouping/build/libs/lucene-grouping-$V.jar
CP=$CP:$LUCENE/lucene/queries/build/libs/lucene-queries-$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.search.grouping.TestGroupingNegativeCacheLimitGradle 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.