The HNSW vectors formats (Lucene99HnswVectorsFormat, Lucene104HnswScalarQuantizedVectorsFormat, HnswBitVectorsFormat and, since 10.6.0, the sandbox DedupHnswVectorsFormat and DedupHnswScalarQuantizedVectorsFormat) accept numMergeWorkers <= 0 together with a merge ExecutorService, and then every merge that builds an HNSW graph throws ArrayIndexOutOfBoundsException (0 workers) or NegativeArraySizeException (negative) and closes the IndexWriter as a tragic event (since 9.9.0)
Finding LFV-589Area: codecs / HNSW vectors formats (constructor parameter validation; concurrent HNSW merge)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 build a codec with, for example, new Lucene99HnswVectorsFormat(16, 100, 0, executor), that is zero merge workers plus a merge executor. The constructor already rejects maxConn or beamWidth <= 0 and one worker with an executor, so you expect it to reject 0 as well (or to merge serially, as it does without an executor). Instead it accepts the value, indexing works, and the first merge that builds an HNSW graph throws ArrayIndexOutOfBoundsException (NegativeArraySizeException for -1) inside HnswConcurrentMergeBuilder, whose worker array is sized by that count, and the IndexWriter closes itself with that exception.
The test shows that new Lucene99HnswVectorsFormat(16, 100, 0, executor, 0) (zero merge workers plus an executor) is accepted, and then merging two vector segments fails.
Expected vs actual
Expected an IllegalArgumentException at construction or a working forceMerge(1); actual: ArrayIndexOutOfBoundsException: Index 0 out of bounds for length 0 at HnswConcurrentMergeBuilder.runInBatches, 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=$CP:randomizedtesting-runner-2.9.2.jar:junit-4.13.2.jar:hamcrest-3.0.jar # test-framework dependencies (Maven Central)
CP=$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.codecs.lucene99.TestHnswZeroMergeWorkersGradle 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.