FixedBitSet.ensureCapacity computes the grown length as int arr.length << 6, which wraps negative once the requested bit reaches about 1.91 billion, so DocsWithFieldSet iterates no documents and a flushed segment silently drops every value of its sparse doc-values and norms fields
Finding LFV-137Area: util / FixedBitSet growth (ensureCapacity) and index / DocsWithFieldSet (doc-values and norms flush)Priority: lowImpact: data corruptionNovelty: 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 into one segment until doc IDs pass about 1.91 billion (legal: IndexWriter.MAX_DOCS is 2^31-129) with a doc-values or norms field that only some documents have, then commit; you expect every indexed value to be in the segment. Instead, when the field's DocsWithFieldSet grows its FixedBitSet to the new doc ID, FixedBitSet.ensureCapacity computes the new length as arr.length << 6 in int arithmetic, which wraps negative once the backing array has 2^25 words, so the set's iterator returns no document although cardinality() counts them, and the flushed segment has no document with a value for that field (with -da; with -ea addDocument throws ArrayIndexOutOfBoundsException for that document). The same call on a small set, ensureCapacity(new FixedBitSet(64), 2_000_000_000), returns a set with length() = -2,044,967,232; the twin LongBitSet.ensureCapacity had the identical overflow and was fixed in 2015 (LUCENE-6409), but FixedBitSet was not.
The test shows that FixedBitSet.ensureCapacity(new FixedBitSet(64), 2_000_000_000) returns a set that cannot hold bit 2,000,000,000, because the new length is computed as arr.length << 6 in int arithmetic and wraps negative once the grown array has 2^25 words.
Expected vs actual
Expected: length() > 2,000,000,000. Actual: length() = -2,044,967,232 with -da; with -ea the FixedBitSet constructor's ghost-bits assertion throws ArrayIndexOutOfBoundsException: Index -31952613 out of bounds for length 35156251.
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.util.TestFixedBitSetEnsureCapacityOverflowGradle 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: bitsets-livedocs-acceptdocs
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.