SearchGroup.merge (the shard-merge helper of grouped search) with topN = Integer.MAX_VALUE ('all remaining groups') throws OutOfMemoryError because it pre-sizes its result list by topN (regression in 6.2.0, LUCENE-7356), and with offset >= 1 throws NullPointerException because offset + topN wraps to a negative queue bound (since 3.3.0)
Finding LFV-958Area: grouping / SearchGroup.merge (merging first-pass top groups across shards or slices; reduce step of FirstPassGroupingCollectorManager)Priority: lowImpact: crashNovelty: 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 merge the first-pass top groups of two shards (three groups in all) with SearchGroup.merge(shardGroups, 0, Integer.MAX_VALUE, groupSort) to get every group, and expect the three groups, the same as topN = 10 returns, as 6.1.0 and earlier returned for the same call, and as core TopDocs.merge(0, Integer.MAX_VALUE, ...) does for hits. Instead merge throws OutOfMemoryError 'Requested array size exceeds VM limit', because since 6.2.0 (LUCENE-7356) it pre-sizes its result list with new ArrayList<>(topN). With offset 1 it throws NullPointerException, because maxQueueSize = offset + topN wraps to a negative int and the pruning loop polls an empty queue (NoSuchElementException on 3.x).
The test shows that SearchGroup.merge of two one-group shard results with offset 0 and topN = Integer.MAX_VALUE ("all groups") throws, because the result list is pre-sized with new ArrayList<>(topN) (SearchGroup.java:294).
Expected vs actual
Expected: the 2 merged groups, as with topN = 10. Actual (-ea and -da): OutOfMemoryError "Requested array size exceeds VM limit".
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.TestSearchGroupMergeAllGroupsGradle 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: grouping-offset-plus-limit-overflow
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.