IndexWriter buffers the first doc-values update of each field twice in every buffered-updates packet (BufferedUpdates adds again the update that FieldUpdatesBuffer's constructor already recorded), so that update is RAM-accounted twice and, except on the all-same-value numeric path, applied twice to every matching document (since 7.7.0, LUCENE-8590)

Finding LFV-837Area: index / buffered doc-values updates (BufferedUpdates, FieldUpdatesBuffer, FrozenBufferedUpdates)Priority: lowImpact: minorNovelty: new

Finding page for LFV-837

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

What it shows

You update a doc-values field through IndexWriter (for example updateBinaryDocValue with a 1 MiB value) and expect each update to be buffered once, as every later update of the same field is. Instead BufferedUpdates creates the field's FieldUpdatesBuffer with the update, whose constructor already records it as entry 0, and then adds the same update again, so the first 1 MiB update adds 2,162,956 bytes to ramBytesUsed() while each later one adds 1,048,576, and IndexWriter's InfoStream reports '1 field updates; 20 new updates' for one binary update matching 10 documents. Final values stay correct because the duplicate is identical and adjacent; Lucene 7.6.0 buffered the update once, and the double insert arrived with LUCENE-8590 in 7.7.0.

Expected vs actual

Expected: Each 1 MiB update adds about 1,048,576 bytes to ramBytesUsed(), and the test passes.
Actual: AssertionError: first=2162956 second=1048576: the first update is counted twice.

Files

src/org/apache/lucene/index/TestFirstDocValuesUpdateBufferedTwice.java (JUnit test)
src/FirstDocValuesUpdateBufferedTwiceMain.java (main() class)

Run it

Tested on Lucene main e1faeba08bde (2026-11-02): still reproduces.
Tested on Lucene main a9779ef3c193 (2026-10-26): reproduced.
Run 1 runs the second method of the src/ test, testFirstBinaryUpdateIsAppliedOnce (one 1 MiB update matching 1,100 documents, the doubled apply): on main 5adff78bb168 its commit fails with ArrayIndexOutOfBoundsException (arraycopy: last destination index 2147483648 out of bounds for byte[2147483631]) and the writer closes. Run 2 prints first=2162956 second=1048576 and exits 1. The expected and actual lines above are those of the finding page's one-test copy at the top of this folder, TestFirstDocValuesUpdateBufferedTwice.java; put it in place of the src/ file of the same name and drop -Dtests.method from run 1 to run it.
How to read the outcome: a failing test is the finding reproducing (the failure message states expected against actual); a main() class that exits non-zero, with an exception or AssertionError, is the finding reproducing.
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 off (production-like), seed DEADBEEF
java -da -dsa -Dtests.asserts=false --add-modules jdk.incubator.vector --enable-native-access=ALL-UNNAMED -Dtests.seed=DEADBEEF -Dtests.method=testFirstBinaryUpdateIsAppliedOnce \
     -cp "classes:$CP" org.junit.platform.console.ConsoleLauncher \
     execute --select-class org.apache.lucene.index.TestFirstDocValuesUpdateBufferedTwice

# 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" FirstDocValuesUpdateBufferedTwiceMain

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).

Files shown on the finding page

TestFirstDocValuesUpdateBufferedTwice.java

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.