addIndexes(CodecReader...) with the default config rejects inputs whose maxDoc() sum, deleted documents included, exceeds MAX_DOCS although only their live documents are added, because it always builds a merged view of the inputs for OneMerge#reorder (regression since 9.9.0)
Finding LFV-285Area: index / IndexWriter addIndexes(CodecReader...) (addIndexesReaderMerge merged view: SlowCompositeCodecReaderWrapper over a MultiReader, BaseCompositeReader doc limit)Priority: lowImpact: wrong errorNovelty: 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 call IndexWriter.addIndexes(CodecReader...) with readers that have many deleted or filtered-out documents, so their live documents fit under IndexWriter.MAX_DOCS but their maxDoc() values (deleted documents included) add up to more. You expect the call to succeed, since addIndexes counts and reserves only live documents, its javadoc throws IllegalArgumentException only if the index would exceed MAX_DOCS, and 9.8.0 accepted the same call. Instead, with the default configuration it throws IllegalArgumentException 'Too many documents: composite IndexReaders cannot exceed ...' and adds nothing, because since 9.9.0 addIndexesReaderMerge always wraps the inputs in a SlowCompositeCodecReaderWrapper (a MultiReader over the uncompacted inputs) for OneMerge#reorder, whose default returns null and discards the view.
The test shows that IndexWriter.addIndexes(CodecReader...) without an index sort rejects inputs whose live docs fit the doc limit when their maxDoc sum (deleted docs included) does not, because it always builds a merged view for OneMerge#reorder (limit lowered to 2 with setIndexWriterMaxDocs).
Expected vs actual
Expected the 2 live docs (inputs with maxDoc 2 and 1) to be added; actual IllegalArgumentException: Too many documents: composite IndexReaders cannot exceed 2 but readers have total maxDoc=3, 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=$CP:randomizedtesting-runner-2.9.2.jar:junit-4.13.2.jar:hamcrest-3.0.jar # test-framework dependencies (Maven Central)
CP=$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.index.TestAddIndexesDeletedDocsLimitGradle 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).
Formal models and scripts this finding's part of the hunt used: codecreader-wrappers
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.