UnifiedHighlighter cuts a stored value at maxLength UTF-16 units even between the two halves of a supplementary character (an emoji, a CJK Extension B ideograph), so the snippet can end in an unpaired high surrogate, for example '<b>Hello</b> \uD83D'

Finding LFV-775Area: highlighter / UnifiedHighlighter stored-value loading (LimitedStoredFieldVisitor maxLength cut)Priority: lowImpact: minorNovelty: new

Finding page for LFV-775

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

What it shows

You store the value 'Hello U+1F600 world' (the emoji takes UTF-16 indexes 6 and 7), search for hello and highlight the hit with UnifiedHighlighter.builder(searcher, analyzer).withMaxLength(7), and expect a snippet made of whole characters, '<b>Hello</b> ', just as maxLength 6 gives '<b>Hello</b> ' and maxLength 8 gives the whole emoji. Instead LimitedStoredFieldVisitor keeps exactly 7 UTF-16 units, so the content and the snippet end with the first half of the emoji, '<b>Hello</b> \uD83D', which String.getBytes(UTF_8) turns into '<b>Hello</b> ?' and a strict UTF-8 encoder rejects. The same happens at the default maxLength 10,000 when a value's 10,000th unit is the first half of a supplementary character, and for a later value of a multi-valued field at its per-value budget.

The test shows that with offsets in postings, UnifiedHighlighter.withMaxLength(7) cuts the stored value "Hello U+1F600 world" between the two UTF-16 halves of the emoji, so the snippet ends in an unpaired high surrogate.

Expected vs actual

Expected a well-formed snippet (whole characters only; maxLength 6 gives <b>Hello</b> ). Actual: <b>Hello</b> \uD83D.

Files

• src/org/apache/lucene/search/uhighlight/TestUnifiedHighlighterMaxLengthSurrogatePair.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/highlighter/build/libs/lucene-highlighter-$V.jar
CP=$CP:$LUCENE/lucene/memory/build/libs/lucene-memory-$V.jar
CP=$CP:$LUCENE/lucene/queries/build/libs/lucene-queries-$V.jar
CP=$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.search.uhighlight.TestUnifiedHighlighterMaxLengthSurrogatePair

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.