The file-format javadoc of the default compound format (Lucene90CompoundFormat) gives an incomplete .cfe entry-table grammar: it leaves out the codec footer the writer appends (the .cfs line beside it lists one), and it says each entry holds the file's name although the writer stores the name with the segment name stripped (documentation defect; the writer and reader agree)

Finding LFV-971Area: codecs / compound files (Lucene90CompoundFormat .cfe file-format javadoc vs the Lucene90CompoundFormat writer / Lucene90CompoundReader)Priority: lowImpact: minorNovelty: new

Finding page for LFV-971

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

What it shows

You decode the compound entry table (.cfe) of a default-codec segment by following the Lucene90CompoundFormat javadoc, which says the file is Header, FileCount and then FileCount entries of FileName, DataOffset, DataLength, each holding 'a String with that file's name'. The writer also appends a 16-byte codec footer (the .cfs line just above lists its Footer, the .cfe line does not) and stores each name with the segment name stripped, for example '.nvd' for _0.nvd, so a parser built from the text ends 16 bytes early (byte 134 of 150 in the minimal repro, byte 502 of 518 in the hunt's 20-document test) and cannot find the sub-files by the names it read. Lucene's own reader checks the footer and puts the segment name back, so nothing breaks at runtime; only the documentation is wrong (the type line also lists an unused 'Checksum', which belongs to the linked CodecFooter grammar, so dropping it is cleanup).

The test shows that the .cfe grammar in the Lucene90CompoundFormat class javadoc does not match what the writer writes: it leaves out the trailing Footer, and FileName is stored without the segment name (a documentation defect; the writer and reader agree).

Expected vs actual

Expected (javadoc): nothing after the last entry, and FileName = the file's name. Actual: bytes after the documented grammar expected:<0> but was:<16> and FileName values expected:<[_0.fdm, _0.fdt, _0.fdx, _0.fnm]> but was:<[.fdm, .fdt, .fdx, .fnm]> (-ea and -da).

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.codecs.lucene90.TestLucene90CompoundEntriesJavadoc

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: codec-format-javadoc-grammar-overflow-liv-cfe-extension-table

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.