ConcatenateGraphFilter without a token separator (and so CompletionAnalyzer with preserveSep=false) silently drops a concatenated path around a position hole, because Automaton.addEpsilon copies from a dest state that its own first addTransition sorts and reduces in place (AssertionError with -ea)

Finding LFV-1118Area: analysis-common / ConcatenateGraphFilter no-separator mode, and core / util.automaton (Automaton.addEpsilon); reached from suggest (CompletionAnalyzer preserveSep=false, document suggester)Priority: lowImpact: silent wrong resultsNovelty: new

Finding page for LFV-1118

AI-assisted finding, confirmed by automated checks, not yet triaged by a human Lucene committer. This site files nothing upstream.

What it shows

You concatenate the token graph play | thewho (spanning two positions) | who, which a multi-word synonym ('the who' => 'thewho') followed by a stop filter produces for 'play the who', with ConcatenateGraphFilter and no separator (a null tokenSeparator, or CompletionAnalyzer with preserveSep=false), and expect one token per path, 'playthewho' and 'playwho', as the class javadoc promises. With assertions disabled you get only 'playwho', so a suggestion reached through the synonym (prefix 'playth') is silently missing; with -ea the filter throws an AssertionError in Automaton.getNextTransition. The cause is Automaton.addEpsilon: it copies the transitions of a state that is still being built, and its own first addTransition to the other state sorts and shrinks that state in place mid-copy, so some transitions are copied twice and others never.

Expected vs actual

Expected: two tokens: playthewho, then playwho.
Actual: assertions off: one token, playwho (term 0 expected:<play[the]who> but was:<play[]who>); assertions on: AssertionError at Automaton.getNextTransition(Automaton.java:493).

Files

src/org/apache/lucene/analysis/miscellaneous/TestConcatenateGraphFilterHoleWithoutSeparator.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=$CP:$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 -Xmx2g --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.TestConcatenateGraphFilterHoleWithoutSeparator

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

TestConcatenateGraphFilterHoleWithoutSeparator.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.