Sorter.heapChild overflows int, so IntroSorter's heap-sort fallback compares a negative slot outside [from, to) when 2*to - from > 2^31 (ArrayIndexOutOfBoundsException from ArrayUtil.introSort on a legal array slice)
Finding LFV-372Area: util / sorters (Sorter heap sort, IntroSorter's depth fallback)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 sort a legal slice with IntroSorter, for example ArrayUtil.introSort on the last 1,000 elements of an array of length ArrayUtil.MAX_ARRAY_LENGTH (or a slice of more than 2^30 elements starting at 0), and the element order makes IntroSorter give up on quicksort and fall back to heap sort. You expect the slice [from, to) to be sorted using only slots inside it. Instead Sorter.heapChild computes 2*(i - from) + 1 + from in int, which wraps to a negative number for a node that has no child, siftDown's 'leftChild < to' check lets it through, and compare is called with a negative slot, so the array-backed sort throws ArrayIndexOutOfBoundsException ('Index -2147482757 out of bounds for length 2147483631').
The test shows that IntroSorter.sort on the last 60 slots of an ArrayUtil.MAX_ARRAY_LENGTH-long range, in an order that uses up its quicksort depth budget, falls back to heapSort, whose int heapChild wraps and asks the sorter for a negative slot.
Expected vs actual
Expected: only slots in [2147483571, 2147483631) are compared or swapped. Actual (-ea and -da): AssertionError: slot -2147483639 is outside [2147483571, 2147483631), from Sorter.siftDown(Sorter.java:279) <- heapSort <- IntroSorter.sort(IntroSorter.java:61).
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=$CP:randomizedtesting-runner-2.9.2.jar:junit-4.13.2.jar:hamcrest-3.0.jar # test-framework dependencies (Maven Central)
CP=$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.TestIntroSorterHeapSortAtArrayEndGradle 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: sorters-selectors
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.