ByteBuffersDataOutput.copyBytes narrows its long length to a signed int, so a copy of 2 GiB or more (for example Directory.copyFrom into a ByteBuffersDirectory) silently copies nothing or only (int) numBytes bytes

Finding LFV-107Area: store / ByteBuffersDataOutputPriority: lowImpact: data corruptionNovelty: 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 copy 2 GiB or more into an in-heap ByteBuffersDataOutput, for example with Directory.copyFrom or IndexWriter.addIndexes(Directory) into a ByteBuffersDirectory. You expect every byte to be copied, as DataOutput.copyBytes(DataInput, long numBytes) promises ('Copy numBytes bytes from input to ourself') and as Lucene 9.2.0 does. Instead the override stores the length in an int, copies max(0, (int) numBytes) bytes (nothing at all for any length from 2^31 to 2^32-1) and returns normally, so the destination file is empty or truncated with no exception; a copied index then fails to open with CorruptIndexException.

The test shows that ByteBuffersDataOutput.copyBytes(DataInput, long) narrows the length to an int: a 2^31-byte copy copies nothing and returns normally.

Expected vs actual

Expected: size() == 2147483648 after the copy. Actual: java.lang.AssertionError: expected:<2147483648> but was:<0> (main 645d4fe, -ea and -da). The 2-line long-length fix passes (needs about 2 GiB of heap, --xmx 6g).

Files

src/org/apache/lucene/store/TestByteBuffersDataOutputCopyBytes2GiB.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.store.TestByteBuffersDataOutputCopyBytes2GiB

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: datainput-impls-strings

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.