IndexWriter accepts doc-values updates of the reserved parent field; with index sorting and document blocks this leaves a segment that fails CheckIndex, or an index whose merges fail

Finding LFV-095Area: index / IndexWriter doc-values updatesPriority: 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 use index sorting with a parent field (IndexWriterConfig#setParentField) and document blocks, and call IndexWriter#updateNumericDocValue or #updateDocValues on that parent field, for example to give a child document a parent mark or to set a parent's mark to null. You expect the call to be rejected with IllegalArgumentException, as IndexWriter rejects doc-values updates of index-sort fields and any document that contains the parent field ('is a reserved field'), or else the index to stay valid. Instead the update is accepted: a child given a mark can put the segment out of index-sort order, so CheckIndex fails ('segment has indexSort=<long: "sort"> but docID=1 sorts after docID=2') as soon as the update is committed, and a removed mark makes the next merge throw EOFException in MultiSorter and close the writer when the unmarked block was the last of its segment and every document there has a sort value, and otherwise silently moves the orphaned documents into the next block.

The test shows that with an index sort and setParentField("_parent"), IndexWriter#updateNumericDocValue(id:child, "_parent", -1) on a committed block [child(sort=9), parent(sort=1)] is accepted and leaves a segment out of index-sort order (TestParentFieldDocValuesUpdate#testUpdateParentFieldOfChildKeepsIndexSorted).

Expected vs actual

Expected: the update is rejected with IllegalArgumentException or the segment stays sorted (CheckIndex.testSort error null). Actual: expected null, but was:<CheckIndexException: segment has indexSort=<long: "sort"> but docID=0 sorts after docID=1>, the same with -ea and -da.

Files

src/org/apache/lucene/index/TestParentFieldDocValuesUpdate.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.TestParentFieldDocValuesUpdate

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 2026-11-03. AI-assisted; not yet triaged by a human Lucene committer. Feedback: open an issue in lucene-ai-bug-hunt.