MultiPassIndexSplitter's round-robin split loses or duplicates the last few documents of an index at about IndexWriter.MAX_DOCS for most part counts of 145 or more, because its int test (j + numParts - i) % numParts overflows

Finding LFV-531Area: misc / index tools (MultiPassIndexSplitter.split, round-robin mode)Priority: lowImpact: data corruptionNovelty: new

Finding page for LFV-531

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

What it shows

You split an index of IndexWriter.MAX_DOCS (2,147,483,519) documents with MultiPassIndexSplitter in round-robin mode, the command-line default, into 145 parts, and expect every document in exactly one part (part i gets the documents whose id j has j % numParts == i), as the split javadoc says: ids are 'assigned in a deterministic round-robin fashion to one of the output splits'. Instead the parts hold 2,147,483,527 documents in total: documents 2,147,483,511..518 belong to parts 16..23 but are also copied into parts 0..7, and with 149 parts documents 2,147,483,509..518 end up in no part at all; nothing reports an error. The cause is the keep test (j + numParts - i) % numParts == 0 at MultiPassIndexSplitter.java:103, computed in int: for the top doc ids j + numParts - i wraps negative and Java's % keeps the sign, while the equivalent j % numParts == i cannot overflow.

Expected vs actual

Expected: Part 0 holds the 14,810,232 documents j with j % 145 == 0, and the test passes.
Actual: numDocs of round-robin part 0 of 145 expected:<14810232> but was:<14810233>, with assertions on and off: doc 2,147,483,511 of part 16 is in part 0 too.

Files

src/MultiPassSplitterRoundRobinOverflowMain.java (main() class)

Run it

Tested on Lucene main e1faeba08bde (2026-11-02): still reproduces.
Tested on Lucene main ea57360912ed (2026-10-24): reproduced.
How to read the outcome: a main() class that exits non-zero, with an exception or AssertionError, is the finding reproducing.
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, default flags
java -ea -esa --add-modules jdk.incubator.vector --enable-native-access=ALL-UNNAMED -Dtests.seed=DEADBEEF \
     -cp "classes:$CP" MultiPassSplitterRoundRobinOverflowMain

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).

Files shown on the finding page

TestMultiPassIndexSplitterRoundRobinOverflow.java

Formal models and scripts this finding's part of the hunt used: misc-reorder-split

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 2026-11-03. AI-assisted; not yet triaged by a human Lucene committer. Feedback: open an issue in lucene-ai-bug-hunt.