IndexSearcher#count hangs forever when Weight#count throws, for example on a query timeout, on a segment split into intra-segment partitions

Finding LFV-045Area: search / concurrencyPriority: mediumImpact: hangNovelty: new

Finding page for LFV-045

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

What it shows

You search with an executor and intra-segment partitions (IndexSearcher.slices(..., allowSegmentPartitions=true)) and call IndexSearcher#count, and the first partition of a segment to arrive throws from Weight#count, for example the ExitingReaderException of a stock ExitableDirectoryReader timeout. You expect the exception to propagate, as TaskExecutor#invokeAll promises. Instead TotalHitCountCollectorManager completes the segment's shared future only on success or CollectionTerminatedException, so the other partitions wait in an unbounded get() and the call never returns (with no test hooks, 124/200 and 176/200 calls hung on main, 44/50 on 10.5.2).

The test shows that IndexSearcher#count with an executor and intra-segment partitions never returns when the first partition's Weight#count throws (here a stock ExitableDirectoryReader timeout).

Expected vs actual

Expected: count() rethrows the exception (a bounded get sees ExecutionException); actual: the other partition parks in TotalHitCountCollectorManager (line 131) and the 10 s get sees TimeoutException.

Files

• src/org/apache/lucene/search/TestIndexSearcherCountPartitionHang.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.search.TestIndexSearcherCountPartitionHang

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: study-search-concurrency-model

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.