With index sorting and a parent field, deleting only a block's parent makes every later merge fail

Finding LFV-011Area: index / IndexWriter mergePriority: highImpact: index unusableNovelty: new

Finding page for LFV-011

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

What it shows

You use index sorting with a parent field and document blocks, and delete only the parent document of a block. You expect later merges to work. Instead MultiSorter finds no parent for the orphaned child (nextSetBit returns NO_MORE_DOCS), every later merge of that segment throws (an EOFException with the default codec on a file-system directory, IndexOutOfBoundsException on ByteBuffersDirectory), the writer closes tragically and uncommitted documents are lost.

The test shows that with an index sort and setParentField, deleting only the parent of a block [c1, p1] leaves the merged segment [x, c1] ending with a parentless child, and the next forceMerge(1) throws.

Expected vs actual

Expected: the second merge succeeds; actual (-ea and -da): IndexOutOfBoundsException from MultiSorter.setComparableValues (MultiSorter.java:166-168) reading the sort value at parents.nextSetBit(docID) = NO_MORE_DOCS.

Files

• src/org/apache/lucene/index/TestIndexSortDeletedParentMerge.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.index.TestIndexSortDeletedParentMerge

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.