TSTLookup.get('') and TSTLookup.lookup('') throw StringIndexOutOfBoundsException instead of returning null or an empty list: TSTAutocomplete.prefixCompletion reads key.charAt(0) before it checks the key length (every release since 3.3.0)
Finding LFV-251Area: suggest / ternary search tree suggester (TSTLookup.lookup and get, TSTAutocomplete.prefixCompletion)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 build a TSTLookup suggester and call get('') or lookup('', false, 5), for example because a user cleared the search box. You expect null from get(''), whose javadoc says it returns null if the key does not exist (the empty key is never stored), and a list, possibly empty, from lookup(''). Instead both throw StringIndexOutOfBoundsException 'Index 0 out of bounds for length 0', because TSTAutocomplete.prefixCompletion reads key.charAt(0) on its first loop step before it checks the key length, and the tree's root node is never null; non-empty keys work normally on the same lookup.
The test shows that new TSTLookup().get("") (an empty key on a TSTLookup) throws instead of answering.
Expected vs actual
Expected null (get javadoc: "or null if the key does not exist"; the empty key is never stored); actual: StringIndexOutOfBoundsException: Index 0 out of bounds for length 0 from TSTAutocomplete.prefixCompletion, with -ea and -da.
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=$CP:$LUCENE/lucene/analysis/common/build/libs/lucene-analysis-common-$V.jar
CP=$CP:$LUCENE/lucene/suggest/build/libs/lucene-suggest-$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.search.suggest.tst.TestTSTLookupEmptyKeyGradle 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.