FacetsConfig writes the bare dimension term of a flat multiValued + requireDimCount SortedSetDocValues dimension without escaping it, so a dimension name containing U+001F or U+001E is read back as a different path: the dimension loses one label and reports that label's count as its own, a phantom dimension appears, a name with two U+001F makes facet-state construction throw for the whole field, and (-ea) a name ending in U+001E throws AssertionError
Finding LFV-597Area: facet / SortedSetDocValues faceting (FacetsConfig.processSSDVFacetFields path encoding with DELIM_CHAR U+001F / ESCAPE_CHAR U+001E, decoded by DefaultSortedSetDocValuesReaderState)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 three documents under a flat SortedSetDocValues facet dimension named 'color<U+001F>code', configured multiValued and requireDimCount, with labels {x, y}, {x} and {z}, and expect getTopChildren to return value=3 with x=2, y=1, z=1, as taxonomy faceting does for the same name. Instead it returns value=2 with only y=1 and z=1, and getAllDims also shows a phantom dimension 'color' with label 'code'. FacetsConfig escapes U+001F/U+001E in every facet term except this one bare dimension term (FacetsConfig.java:494), while the reader decodes every term as if it were escaped, so the raw term is read back as a different path.
The test shows that for a flat SortedSetDocValues facet dimension configured multiValued + requireDimCount whose name contains U+001F ("c\u001Fd"), FacetsConfig writes the bare dimension term unescaped, so the reader loses one label.
Expected vs actual
One document with labels x and y; expected getTopChildren labels [x (1), y (1)]; actual [y (1)], 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/facet/build/libs/lucene-facet-$V.jar
CP=$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.facet.sortedset.TestSortedSetDocValuesDimCountReservedCharThe module(s) facet also need their third-party runtime jars; ./gradlew -p lucene/<module> dependencies lists them.
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 2026-11-03. AI-assisted; not yet triaged by a human Lucene committer. Feedback: open an issue in lucene-ai-bug-hunt.