Two field names that differ only in an unpaired surrogate are written to disk as the same U+FFFD name, so IndexWriter commits an index that DirectoryReader and a new IndexWriter refuse to open (on main also colliding field attribute keys and commit user-data keys)

Finding LFV-146Area: index / FieldInfos and segment metadata strings (DataOutput#writeString in .fnm, segments_N and .si)Priority: mediumImpact: index unusableNovelty: known upstream

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

What it shows

You add a document with two field names that differ only in an unpaired surrogate, for example 'f\uD800' and 'f\uFFFD' (or two different lone surrogates in the same position), and commit; addDocument and commit return normally, as IndexWriter documents that malformed UTF-16 is silently replaced with U+FFFD, and you expect the committed index to open. Instead DataOutput#writeString writes both names as the same bytes ('f' + U+FFFD), so DirectoryReader.open and a new IndexWriter throw IllegalArgumentException 'duplicate field names: 0 and 1 have: f\uFFFD' for the whole index, and CheckIndex can only recover it by dropping the whole segment. On main the same happens for two colliding field attribute keys or commit user-data keys ('duplicate key'), and a lone-surrogate index-sort field name is stored in its U+FFFD form, so a new IndexWriter with the same sort is refused.

Expected vs actual

Files

Run it

# 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.index.TestUnpairedSurrogateNameCollision

# run 2: assertions on, seed DEADBEEF, default flags
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.index.TestUnpairedSurrogateNameCollisionWriter

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.