Automaton.Builder.addEpsilon(s, s) never returns and exhausts the heap when state s already has a transition, because its loop re-reads nextTransition while it appends copies that leave s again

Finding LFV-1214Area: util / automaton (Automaton.Builder.addEpsilon)Priority: 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 build an automaton with Automaton.Builder, give a state s one accepting self-loop arc (s -a-> s), and call addEpsilon(s, s). You expect it to return at once: its javadoc says it copies dest's arcs over to source, an epsilon from a state to itself adds nothing, and the sibling Automaton.addEpsilon returns on the same call. Instead the call never returns: its loop re-reads the growing transition count, every copy it appends again leaves s and is copied again, and it runs until OutOfMemoryError: Java heap space.

Expected vs actual

Expected: addEpsilon(s, s) returns and the automaton accepts "aaa".
Actual: It never returns: java.lang.OutOfMemoryError: Java heap space from Builder.addTransition <- Builder.addEpsilon (Automaton.java:812), assertions on and off.

Files

src/org/apache/lucene/util/automaton/TestBuilderAddEpsilonToItself.java (JUnit test)

Run it

Tested on Lucene main 5adff78bb168 (2026-11-02): reproduced.
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 off (production-like), seed DEADBEEF
java -da -dsa -Dtests.asserts=false -Xmx512m --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.automaton.TestBuilderAddEpsilonToItself

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).

Files shown on the finding page

TestBuilderAddEpsilonToItself.java

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.