SecondPassGroupingCollector and GroupReducer never call LeafCollector#finish on the per-group leaf collectors, so a custom GroupReducer whose collector does its per-segment work in finish (for example a stock FacetsCollector per group) silently sees no documents
Finding LFV-880Area: grouping / GroupReducer and SecondPassGroupingCollector (second-pass per-group collectors)Priority: 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 write a GroupReducer that gives each group its own FacetsCollector (or any collector that compiles a segment's results in LeafCollector#finish), run it through SecondPassGroupingCollector, and expect each group's collector to see the documents of its group, as GroupReducer's javadoc says and as a plain IndexSearcher search would give. Instead every group's FacetsCollector reports no documents, with no error: GroupReducer.setNextReader replaces each group's leaf collector on every segment without calling finish() on the previous one, GroupReducer has no finish hook, and SecondPassGroupingCollector does not override finish(), so IndexSearcher's per-leaf finish() reaches only the outer collector. In the repro 5 documents in 2 segments are routed to groups a (3) and b (2), yet both groups report 0 hits; with assertions enabled FacetsCollector throws AssertionError on the second segment instead.
The test shows that a custom GroupReducer whose per-group collector is a stock FacetsCollector sees no hits, because SecondPassGroupingCollector/GroupReducer never call LeafCollector#finish on the per-group leaf collectors.
Expected vs actual
Expected: group "a"'s FacetsCollector reports 1 hit (1 doc, 1 segment, 1 group). Actual: 0 (expected:<1> but was:<0>), 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.
- 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/facet/build/libs/lucene-facet-$V.jar
CP=$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.TestGroupReducerFinishThe module(s) facet also need their third-party runtime jars; ./gradlew -p lucene/<module> dependencies lists them.
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.