QualityQuery.compareTo is not a total order when numeric and non-numeric topic ids are mixed (ints compared as numbers, everything else as strings), so TrecTopicsReader and Trec1MQReader.readQueries, and with them QueryDriver, can throw IllegalArgumentException 'Comparison method violates its general contract!'
Finding LFV-936Area: benchmark / search-quality package (QualityQuery, TREC topic readers)Priority: lowImpact: crashNovelty: 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 read a TREC topics file with TrecTopicsReader or Trec1MQReader (as QueryDriver does) whose ids are mostly numbers but include a few sub-topic ids such as 1a or 10.1, or a few digit-only ids larger than Integer.MAX_VALUE, and expect the topics back sorted by id, as the readers' '// sort result array (by ID)' comment says. Instead QualityQuery.compareTo compares two int ids as numbers and every other pair as strings, which forms cycles (2 < 100 as numbers, 100 < 1a and 1a < 2 as strings), so Arrays.sort can throw IllegalArgumentException 'Comparison method violates its general contract!' (the repro: ids 1..111 plus 1a, 20a, 64a and 99a, each listed after its parent); when the sort does not detect the cycle, the topics come back in an order that depends on the file order. Separately, the int branch returns n - nOther, which overflows for extreme ids such as 2147483647 vs -1 (this needs a negative id).
The test shows that QualityQuery.compareTo (the order TrecTopicsReader / Trec1MQReader sort topics with) is not transitive when int and non-int ids are mixed, with the three ids 2, 100, 1a.
Expected vs actual
Expected a total order (Comparable contract); actual 2 < 100 (compared as ints), 100 < 1a and 1a < 2 (compared as strings), a cycle, with -ea and -da.
Files
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=$CP:$LUCENE/lucene/benchmark/build/libs/lucene-benchmark-$V.jar
CP=$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.benchmark.quality.TestQualityQueryCompareToThe module(s) benchmark 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.