MultiPassIndexSplitter builds every output part with a bare IndexWriterConfig and accepts no config, so it cannot split any index that uses soft deletes (7.4.0+) or has a parent field (9.10.0+): split() and its command line throw IllegalArgumentException before any part gets a document
Finding LFV-618Area: misc / index tools (MultiPassIndexSplitter.split, output IndexWriterConfig); IndexWriter.addIndexes field-role checksPriority: lowImpact: minorNovelty: 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 split an index with MultiPassIndexSplitter, for example one written with IndexWriterConfig#setSoftDeletesField("soft") and a single softUpdateDocument call, or one written with IndexWriterConfig#setParentField("_parent"), and expect it to split like any other index: the split() javadoc says the source 'can have deletions, can have multiple segments (or multiple readers)', and the only documented limitation is that document blocks may be broken up. Instead split() opens every part's IndexWriter with new IndexWriterConfig(null), which names no soft-deletes or parent field and cannot be replaced by the caller, so IndexWriter.addIndexes rejects the input's field infos with IllegalArgumentException ('this index has [soft] as soft-deletes already but soft-deletes field is not configured in IWC', or 'can't add field [_parent] as parent document field; this IndexWriter has no parent document field configured') and nothing is split.
The test shows that MultiPassIndexSplitter.split cannot split an index that uses soft deletes: each part's writer gets a bare IndexWriterConfig, so addIndexes rejects the input's soft-deletes field.
Expected vs actual
Expected the 2 live documents (one softUpdateDocument under setSoftDeletesField("soft")) split into 2 parts; actual: IllegalArgumentException: this index has [soft] as soft-deletes already but soft-deletes field is not configured in IWC, 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=$CP:$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.TestMultiPassIndexSplitterSoftDeletesGradle 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 2026-11-03. AI-assisted; not yet triaged by a human Lucene committer. Feedback: open an issue in lucene-ai-bug-hunt.