BPIndexReorderer assumes every segment ends with a parent document: once a segment ends with child documents that have no parent after them, each BP reordering merge in which that segment comes last throws and closes the IndexWriter
Finding LFV-417Area: misc / BP merge-time reordering (BPIndexReorderer with document blocks)Priority: mediumImpact: index unusableNovelty: 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 index document blocks with IndexWriterConfig#setParentField, merge through BPReorderingMergePolicy with a BPIndexReorderer, and a segment ends up ending with child documents that have no parent after them, for example because you deleted or updated only the parent of its last block and a merge dropped it, or because IndexWriter rejected a block at one of its children (with the documented IllegalArgumentException) just before a flush. You expect later merges to work: IndexWriter accepted every call, CheckIndex calls the index clean and a plain merge of the same segment succeeds. Instead BPIndexReorderer, which assumes the merged view ends with a parent, runs past the end of its arrays in the next reordering merge in which that segment comes last (ArrayIndexOutOfBoundsException with assertions off, AssertionError with them on), and IndexWriter closes with a tragic exception, loses uncommitted changes and fails the same way after every reopen.
The test shows that with setParentField and BPReorderingMergePolicy(BPIndexReorderer), a block that IndexWriter rejects at a child (immense term) as the last block of a segment makes the next forceMerge(1) fail.
Expected vs actual
Expected: forceMerge(1) succeeds; actual: it throws (AssertionError at BPIndexReorderer.java:329 with -ea, ArrayIndexOutOfBoundsException "Index 65 out of bounds for length 65" at :377 with -da) and closes the writer.
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/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" $(find src -name '*.java')
# run 1: assertions on, seed 41700001
java -ea -esa --add-modules jdk.incubator.vector --enable-native-access=ALL-UNNAMED -Dtests.seed=41700001 \
-cp "classes:$CP" org.junit.platform.console.ConsoleLauncher \
execute --select-class org.apache.lucene.misc.index.TestBPReorderAfterRejectedChildGradle 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.