AutomatonToTokenStream.toTokenStream throws ArrayIndexOutOfBoundsException ('Index 0 out of bounds for length 0') on a 0-state automaton, Lucene's standard empty-language value (Automata.makeEmpty(), or an empty Operations result such as intersection("a", "b"))

Finding LFV-865Area: core / analysis: AutomatonToTokenStream (automaton to token-graph conversion)Priority: lowImpact: crashNovelty: new

Finding page for LFV-865

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

What it shows

You convert an automaton whose language is empty, for example Automata.makeEmpty() or Operations.intersection(Automata.makeString("a"), Automata.makeString("b")), with AutomatonToTokenStream.toTokenStream, whose only stated precondition is a finite DAG. You expect a TokenStream with no tokens (what the same method already returns for Automata.makeEmptyString() and for the same empty language written with one dead state) or at least a clear IllegalArgumentException. Instead it throws ArrayIndexOutOfBoundsException 'Index 0 out of bounds for length 0', because these automata have 0 states and the method reads indegree[0], the in-degree of the start state, without checking that a state exists.

The test shows that AutomatonToTokenStream.toTokenStream(Automata.makeEmpty()) crashes on the 0-state empty-language automaton.

Expected vs actual

Expected: an empty TokenStream or an IllegalArgumentException; actual (-ea and -da): ArrayIndexOutOfBoundsException "Index 0 out of bounds for length 0" from the unguarded indegree[0] read (AutomatonToTokenStream.java:61).

Files

• src/org/apache/lucene/analysis/TestAutomatonToTokenStreamZeroStates.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=$CP:randomizedtesting-runner-2.9.2.jar:junit-4.13.2.jar:hamcrest-3.0.jar   # test-framework dependencies (Maven Central)
CP=$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.analysis.TestAutomatonToTokenStreamZeroStates

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.