The merged vector values of SlowCompositeCodecReaderWrapper throw NullPointerException from copy() when one merged segment has no vectors for the field, so merges through BPReorderingMergePolicy with BpVectorReorderer fail and close the IndexWriter, again after every reopen

Finding LFV-269Area: index / merged CodecReader view (SlowCompositeCodecReaderWrapper vector values); misc / BP merge-time reorderingPriority: mediumImpact: index unusableNovelty: new

Finding page for LFV-269

AI-assisted finding, confirmed by automated checks, not yet triaged by a human Lucene committer. This site files nothing upstream.

What it shows

You use BPReorderingMergePolicy with a BpVectorReorderer on a float vector field, and one segment holds only documents without that optional field, for example a batch of documents without embeddings that was flushed on its own. You expect merges to combine it with the other segments, as they do when every segment has vectors. Instead the merged view's FloatVectorValues.copy() throws NullPointerException ('Cannot invoke KnnVectorValues.copy() because this.sub is null'), IndexWriter treats the failed merge as a tragic event, closes itself and drops every change since the last commit, and the same merge fails again after each reopen.

The test shows that forceMerge(1) through BPReorderingMergePolicy with a BpVectorReorderer fails when one segment (here one empty document) has no vector for the field and the other has 64 vectors.

Expected vs actual

Expected: the two segments merge. Actual (-ea and -da): NullPointerException: Cannot invoke "KnnVectorValues.copy()" because "this.sub" is null from SlowCompositeCodecReaderWrapper$DocValuesSub.copy.

Files

• src/org/apache/lucene/misc/index/TestBpVectorReorderSegmentWithoutVectors.java (JUnit test)

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/misc/build/libs/lucene-misc-$V.jar
CP=$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.misc.index.TestBpVectorReorderSegmentWithoutVectors

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.