An NRT DirectoryTaxonomyReader reopened during DirectoryTaxonomyWriter.replaceTaxonomy keeps the replaced taxonomy's ordinal/label caches and parent arrays, because the epoch is incremented only after the content change (wrong labels that persist, or ArrayIndexOutOfBoundsException from openIfChanged)

Finding LFV-482Area: facet / taxonomy NRT reopen (DirectoryTaxonomyWriter.replaceTaxonomy and deleteAll epoch vs DirectoryTaxonomyReader.doOpenIfChanged)Priority: lowImpact: 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

You keep a near-real-time taxonomy reader open on a DirectoryTaxonomyWriter and refresh it with openIfChanged while the writer swaps in a rebuilt taxonomy with replaceTaxonomy, for example [a, b] replaced by [b, a]. You expect the refreshed reader to show either the old taxonomy or the new one, each with its own ordinals and labels, as the reader/writer concurrency contract and the reader's own 'was the taxonomy recreated?' check intend. Instead, a refresh that lands after the new content is visible but before replaceTaxonomy increments its epoch is treated as a plain append and reuses the replaced taxonomy's caches and parent arrays: it answers getOrdinal(a)=1 and getPath(1)=a, facet counts over the new index (three b documents, one a) come out labelled 'a (3), b (1)' instead of 'b (3), a (1)', and every later openIfChanged returns null, so the wrong reader stays current until the taxonomy changes again (with a smaller replacement the refresh throws ArrayIndexOutOfBoundsException instead).

Expected vs actual

Expected: You expect the refreshed reader to show either the old taxonomy or the new one, each with its own ordinals and labels, as the reader/writer concurrency contract and the reader's own 'was the taxonomy recreated?' check intend.
Actual: A refresh that lands after the new content is visible but before replaceTaxonomy increments its epoch is treated as a plain append and reuses the replaced taxonomy's caches and parent arrays: it answers getOrdinal(a)=1 and getPath(1)=a, facet counts over the new index (three b documents, one a) come out labelled 'a (3), b (1)' instead of 'b (3), a (1)', and every later openIfChanged returns null, so the wrong reader stays current until the taxonomy changes again (with a smaller replacement the refresh throws ArrayIndexOutOfBoundsException instead).

Files

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=$CP:$LUCENE/lucene/facet/build/libs/lucene-facet-$V.jar
CP=$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.facet.taxonomy.directory.TestNrtReopenDuringReplaceTaxonomy

# 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" org.junit.platform.console.ConsoleLauncher \
     execute --select-class org.apache.lucene.facet.taxonomy.directory.TestNrtReopenDuringReplaceTaxonomyPlain

The module(s) facet 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.