OfflineSorter.sort with an ExecutorService runs its failure cleanup before the partition-sort and merge tasks it already submitted have finished, so each task still queued or running leaves a temp file (prefix_sort_N.tmp) behind that nothing deletes (released since 6.6.0)

Finding LFV-357Area: util / OfflineSorter concurrent sort (temp-file cleanup on failure)Priority: lowImpact: minorNovelty: new

Finding page for LFV-357

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

What it shows

You give OfflineSorter your own ExecutorService so it sorts partitions in parallel, and when sort() fails (an I/O error, a truncated input file, or an interrupt of the sorting thread) you expect it to delete every temp file it created, as it does without an executor and as its own code comment says ('So we can remove any partially written temp files on exception'). Instead sort() deletes only the temp files that exist at that moment and rethrows, without waiting for or cancelling the partition-sort and merge tasks it already handed to the executor. Those tasks then run and each writes a new prefix_sort_N.tmp file that nothing ever deletes (executed: the directory holds no sorter file right after sort() throws, and foo_sort_1.tmp once the queued task has run; with exec == null the same failure leaves nothing).

The test shows that OfflineSorter.sort with an ExecutorService runs its failure cleanup without waiting for or cancelling the partition task it already submitted, so that task's temp file is left behind (single thread, deferred executor, truncated input).

Expected vs actual

Expected: only the input file is left after sort() throws EOFException. Actual: expected:<[input_tmp_0.tmp]> but was:<[foo_sort_1.tmp, input_tmp_0.tmp]> (main 645d4fe, -ea and -da). The candidate fix passes.

Files

• src/org/apache/lucene/util/TestOfflineSorterFailureCleanup.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.util.TestOfflineSorterFailureCleanup

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: offline-sorter

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.