Hunspell COMPLEXPREFIXES: after removing the second prefix the Stemmer keeps removing prefixes, so words with three prefixes are accepted and a zero-length prefix recurses until StackOverflowError (also in HunspellStemFilter, where it closes the IndexWriter)

Finding LFV-375Area: analysis / hunspell (Stemmer affix removal)Priority: lowImpact: crashNovelty: 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 load a hunspell dictionary that sets COMPLEXPREFIXES (twofold prefix stripping, meant for morphologically complex right-to-left languages) and chains prefixes through continuation classes, and expect at most two prefixes per word, as hunspell(5), C Hunspell and Lucene up to 9.4.2 do. Instead, after removing the second prefix the Stemmer goes on removing prefixes: with prefixes a, b, c chained on root x, the three-prefix word cbax is accepted by spell() and stemmed to x, and with a zero-length prefix that only passes on a flag (PFX A 0 0/B), the same prefix is removed again and again without shortening the word, so spell("bbx"), getRoots("bx") and HunspellStemFilter throw StackOverflowError. Inside IndexWriter.addDocument that Error closes the writer, and every later addDocument throws AlreadyClosedException.

The test shows that with COMPLEXPREFIXES the hunspell Stemmer keeps stripping prefixes after the second one: with chained prefixes a, b, c on root x/A, spell("cbax") is true, and with a zero-length chained prefix (PFX A 0 0/B) spell("bbx") recurses until StackOverflowError (TestComplexPrefixesAtMostTwo, 2 methods, one per symptom).

Expected vs actual

Expected: both words rejected (false), as hunspell(5) twofold prefix stripping, C Hunspell and Lucene <= 9.4.2 do; actual: expected spell("cbax") == false, got true and expected spell("bbx") == false, got StackOverflowError, with -ea and -da.

Files

src/org/apache/lucene/analysis/hunspell/TestComplexPrefixesAtMostTwo.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: 2 test(s) run, 2 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/analysis/common/build/libs/lucene-analysis-common-$V.jar
CP=$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.hunspell.TestComplexPrefixesAtMostTwo

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 2026-11-03. AI-assisted; not yet triaged by a human Lucene committer. Feedback: open an issue in lucene-ai-bug-hunt.