FacetSetsField.create sizes its buffer from the first FacetSet's sizePackedBytes(), so a list of custom variable-size facet sets (e.g. compressed, which the FacetSet javadoc and FacetSets.adoc allow) throws ArrayIndexOutOfBoundsException when a later set packs to more bytes than the first, and whether a document can be built depends only on the order of its sets

Finding LFV-1009Area: facet / facet sets (FacetSetsField packing of custom FacetSet encodings)Priority: lowImpact: crashNovelty: new

Finding page for LFV-1009

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

What it shows

You write a FacetSet subclass that compresses its value (one length byte plus the value's significant bytes, so sizePackedBytes() is 2 for the value 7 and 7 for 2^40) with a matching FacetSetDecoder, and you expect FacetSetsField.create("f", set(7), set(2^40)) to work just like the same call with the two sets swapped. Instead create allocates 4 + the first set's sizePackedBytes() times the number of sets (8 bytes) and then writes each set's own packed bytes (13 needed), so the second set's packValues throws ArrayIndexOutOfBoundsException 'Index 8 out of bounds for length 8'. With the larger set first the same two sets pack, index and count correctly.

The test shows that FacetSetsField.create sizes its buffer from the first set's sizePackedBytes(), so a custom variable-size FacetSet that packs larger than the first set overflows it.

Expected vs actual

Expected create("f", set(7), set(2^40)) (2 and 7 packed bytes) to return a field; actual: ArrayIndexOutOfBoundsException: Index 8 out of bounds for length 8 from FacetSetsField.toPackedValues, with -ea and -da.

Files

• src/org/apache/lucene/facet/facetset/TestFacetSetsFieldVariableSizeSets.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=$CP:$LUCENE/lucene/facet/build/libs/lucene-facet-$V.jar
CP=$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.facet.facetset.TestFacetSetsFieldVariableSizeSets

The module(s) facet also need their third-party runtime jars; ./gradlew -p lucene/<module> dependencies lists them.

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.