Luke: Optimize and Add document open an IndexWriter without the index's sort, so on a sorted index they silently write unsorted segments and the application's IndexWriter configured with that sort then refuses to open the index

Finding LFV-1042Area: luke / IndexTools write operations (IndexToolsImpl.optimize and addDocument through IndexUtils.createWriter) behind Tools > Optimize and Documents > Add document; IndexWriter index-sort validationPriority: mediumImpact: index unusableNovelty: 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

You open a sorted index (written with IndexWriterConfig.setIndexSort) in Luke with the default options and run Tools > Optimize or Documents > Add document, and you expect a force merge or one more document, with the index's sort unchanged. Instead Luke opens its IndexWriter with a config that has no index sort, which IndexWriter accepts on a sorted index, so the merged or flushed segment records indexSort=null and Luke reports success. Afterwards the application's own IndexWriter, configured with the same sort, refuses to open the index: IllegalArgumentException 'cannot change previous indexSort=null (from segment=_2(11.0.0):c2) to new indexSort=<long: "ts">'.

Expected vs actual

• Expected: You expect a force merge or one more document, with the index's sort unchanged.
• Actual: Luke opens its IndexWriter with a config that has no index sort, which IndexWriter accepts on a sorted index, so the merged or flushed segment records indexSort=null and Luke reports success.

Files

• src/org/apache/lucene/luke/models/tools/TestIndexToolsKeepsIndexSort.java (JUnit test)

Run it

• Tested on Lucene main e1faeba08bde (2026-11-02): still reproduces.
• Tested on Lucene main 645d4fe14a20 (2026-10-28): 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=$CP:$LUCENE/lucene/luke/build/libs/lucene-luke-$V.jar
CP=$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.luke.models.tools.TestIndexToolsKeepsIndexSort

The module(s) luke also need their third-party runtime jars; ./gradlew -p lucene/<module> dependencies lists them.

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.