A zero-length token makes TokenStreamToAutomaton drop every path through it, so ConcatenateGraphFilter emits nothing and the document suggester, AnalyzingSuggester and FuzzySuggester silently lose that suggestion (stock trigger: a standalone tatweel U+0640 with ArabicAnalyzer, PersianAnalyzer or SoraniAnalyzer)
Finding LFV-523Area: analysis / TokenStreamToAutomaton (core), reached through ConcatenateGraphFilter (analysis-common) and the completion suggesters (suggest: NRT document suggester, AnalyzingSuggester, FuzzySuggester)Priority: lowImpact: silent wrong resultsNovelty: 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 index the suggestion 'foo ـ bar' (a standalone Arabic tatweel between two words) with ArabicAnalyzer into a SuggestField, or build an AnalyzingSuggester or FuzzySuggester from it, and expect a lookup for 'foo' to return it next to 'foo bar', because ArabicAnalyzer turns the tatweel into an ordinary empty token and ConcatenateGraphFilter promises one output token for every path through the token graph. Instead TokenStreamToAutomaton adds a token's transitions only inside its per-byte loop, which never runs for a zero-length token, so the token's end state is unreachable: ConcatenateGraphFilter emits nothing, the document suggester indexes no entry for the value and the analyzing suggesters skip the input, so the suggestion never comes back. A stacked empty token loses only its own alternative path; nothing throws, with assertions on or off.
The test shows that a zero-length token (ArabicAnalyzer turns the standalone tatweel in foo ـ bar into foo, "", bar) makes TokenStreamToAutomaton drop the path through it, so ConcatenateGraphFilter emits nothing.
Expected vs actual
Expected one output token for the one path; actual none (expected:<1> but was:<0>), 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: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.miscellaneous.TestConcatenateGraphFilterEmptyTokenGradle 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.