ProtectedTermFilterFactory does not pass its luceneMatchVersion to the filters it builds from wrappedFilters, so since 10.5 a wrapped truncate with prefixLength under a pinned pre-10.5 version truncates by code points instead of the legacy UTF-16 chars

Finding LFV-510Area: analysis / common miscellaneous (ProtectedTermFilterFactory#populateInnerFilters: wrappedFilters factory construction; symptom in TruncateTokenFilterFactory prefixLength)Priority: lowImpact: silent wrong resultsNovelty: new

Finding page for LFV-510

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

What it shows

You configure ProtectedTermFilterFactory with wrappedFilters=truncate and truncate.prefixLength=4 and pin luceneMatchVersion to 10.4.0 (in the factory args, or through CustomAnalyzer.withDefaultMatchVersion), on Lucene 10.5 or later. You expect the wrapped truncate to keep the legacy behaviour that the pinned version selects for back-compat, the first 4 UTF-16 chars, as an unwrapped truncate and the equivalent when()/endwhen() form do under the same version. Instead populateInnerFilters builds the wrapped factory from the 'truncate.*' arguments only, so it runs at Version.LATEST and keeps 4 code points: the token 'ab😀😀cd' becomes 'ab😀😀' instead of 'ab😀', with no error.

The test shows that ProtectedTermFilterFactory with luceneMatchVersion=10.4.0, wrappedFilters=truncate and truncate.prefixLength=4 builds the wrapped truncate at Version.LATEST, so it truncates by code points instead of the legacy UTF-16 chars.

Expected vs actual

Expected the token ab😀😀cd to become ab😀 (4 UTF-16 chars, the 10.4.0 behaviour); actual ab😀😀 (4 code points), with -ea and -da.

Files

src/org/apache/lucene/analysis/miscellaneous/TestProtectedTermWrappedFiltersMatchVersion.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: 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.TestProtectedTermWrappedFiltersMatchVersion

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 2026-11-04. AI-assisted; not yet triaged by a human Lucene committer. Feedback: open an issue in lucene-ai-bug-hunt.