FSTCompletion.lookup returns one shared, mutable static ArrayList for an empty key or an instance with no entries, so modifying that result leaks into every other FSTCompletion in the JVM

Finding LFV-1089Area: suggest / FST completion (FSTCompletion.lookup empty result)Priority: lowImpact: minorNovelty: new

Finding page for LFV-1089

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 two FSTCompletion instances from different inputs ('apple' and 'zebra'), call lookup("", 5) on the first and add a fallback suggestion to the empty list it returns, and expect the second instance's lookup("", 5) to still return no suggestions. Instead it returns [apple/1], with no error: lookup hands out the same private static mutable ArrayList (EMPTY_RESULT) for every empty key and for every instance with no entries, so the change shows up in every FSTCompletion in the JVM. Since 10.0.0 non-empty results are unmodifiable, so only the empty result accepts the change silently.

Expected vs actual

Expected: A different, empty FSTCompletion answers lookup("", 1) with [].
Actual: It answers [apple/1]: AssertionError: expected:<[]> but was:<[apple/1]>.

Files

src/org/apache/lucene/search/suggest/fst/TestFSTCompletionEmptyResult.java (JUnit test)

Run it

Tested on Lucene main e1faeba08bde (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=$CP:$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 off (production-like), seed DEADBEEF
java -da -dsa -Dtests.asserts=false --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.fst.TestFSTCompletionEmptyResult

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

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