FSTMetadata.save always writes the current version, so re-saving a pre-9.0 BYTE2 FST breaks its lookups

Finding LFV-052Area: util / FSTPriority: 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 load an FST that was written before 9.0 with BYTE2 labels and save it again with Lucene 9.0 or later. You expect the re-saved FST to work like the original. Instead save() writes the header version VERSION_CURRENT, so readers decode the labels with the wrong byte order and the lookups fail (0 of 9 keys found in the repro); FSTs built with setVersion(VERSION_90) are also labelled 9 and rejected by 9.8.

The test shows that an FST saved by Lucene 8.11.4 (FST version 7, BYTE2 labels, one key "a" -> 1) loads correctly on main, but after FST#save and reloading, the copy no longer finds "a": FSTMetadata#save writes the header version VERSION_CURRENT, so the copied big-endian BYTE2 labels are read little-endian.

Expected vs actual

Expected Util.get(copy, "a") == 1 as in the loaded FST; actual null (expected:<1> but was:<null>).

Files

src/org/apache/lucene/util/fst/TestResavePre90Byte2Fst.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=$CP:randomizedtesting-runner-2.9.2.jar:junit-4.13.2.jar:hamcrest-3.0.jar   # test-framework dependencies (Maven Central)
CP=$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.util.fst.TestResavePre90Byte2Fst

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).

Formal models and scripts this finding's part of the hunt used: study-fst-verification

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.