IndexWriter.deleteAll misses a segment flush that another thread is already running, so documents added before deleteAll survive it and are committed

Finding LFV-474Area: index / IndexWriter deleteAll versus a concurrent segment flush (DocumentsWriter.lockAndAbortAll, flush-ticket queue)Priority: mediumImpact: silent wrong resultsNovelty: 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

Several threads index into one IndexWriter, and one of them is in the middle of writing its full in-memory buffer out as a new segment when another thread calls deleteAll(). You expect deleteAll to drop every buffered document and every segment, like opening a new IndexWriter with OpenMode.CREATE, so the next commit is empty: every document was added with a smaller sequence number than deleteAll's, and some of those addDocument calls had already returned before deleteAll was called. Instead deleteAll returns without waiting for that segment to be published, the flushing thread then adds it to the freshly cleared index, and its documents survive deleteAll into the commit, still carrying the old field schema.

Expected vs actual

• Expected: You expect deleteAll to drop every buffered document and every segment, like opening a new IndexWriter with OpenMode.CREATE, so the next commit is empty: every document was added with a smaller sequence number than deleteAll's, and some of those addDocument calls had already returned before deleteAll was called.
• Actual: deleteAll returns without waiting for that segment to be published, the flushing thread then adds it to the freshly cleared index, and its documents survive deleteAll into the commit, still carrying the old field schema.

Files

• src/org/apache/lucene/index/TestDeleteAllMissesInFlightFlush.java (JUnit test)

Run it

• Tested on Lucene main e1faeba08bde (2026-11-02): still reproduces.
• Tested on Lucene main ea57360912ed (2026-10-24): reproduced.
• 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.index.TestDeleteAllMissesInFlightFlush

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: dw-flush-control-deletequeue-tla

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.