UniformSplit postings formats fail to flush or merge a field when deltaNumLines > targetNumBlockLines/3, a setting validateSettings accepts

Finding LFV-166Area: codecs / UniformSplit postings formats (BlockWriter last-block split; UniformSplit and SharedTermsUniformSplit)Priority: lowImpact: crashNovelty: new

Finding page for LFV-166

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 UniformSplit (or SharedTermsUniformSplit) postings format with targetNumBlockLines=4 and deltaNumLines=2, which the constructor accepts because its documented rule is only 0 <= delta < target, and commit a field with 3 terms. You expect the commit to succeed and the blocks to hold target +- delta lines. Instead the flush throws (AssertionError 'blockLines size: 3' with assertions on, IndexOutOfBoundsException 'Index -1 out of bounds for length 3' with them off) and the IndexWriter closes, because the code that splits a field's last block assumes its look-back window of 2*delta lines starts at line 1 or later, which is false whenever 3*delta > target and the last block has between target-delta+1 and 2*delta lines.

The test shows that UniformSplitPostingsFormat(4, 2), which its constructor accepts (documented rule 0 <= deltaNumLines < targetNumBlockLines), cannot commit a field with 3 terms: the last-block split in BlockWriter starts its window at 3 - 2*2 = -1.

Expected vs actual

Expected the commit to succeed; actual: it throws (AssertionError: blockLines size: 3 with -ea, IndexOutOfBoundsException: Index -1 out of bounds for length 3 with -da), wrapped by the test as commit with UniformSplitPostingsFormat(4, 2) threw ....

Files

• src/org/apache/lucene/codecs/uniformsplit/TestUniformSplitLargeDeltaFlush.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=$CP:randomizedtesting-runner-2.9.2.jar:junit-4.13.2.jar:hamcrest-3.0.jar   # test-framework dependencies (Maven Central)
CP=$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.uniformsplit.TestUniformSplitLargeDeltaFlush

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.