InitializedHnswGraphBuilder.fromGraph documents initializedNodes as 'can be null if not tracking', but the returned builder's addGraphNode(int) reads it without a null check, so a builder created with null throws NullPointerException on its first insertion (since 10.4.0)

Finding LFV-1197Area: core / util/hnsw: InitializedHnswGraphBuilder (fromGraph, addGraphNode; @lucene.experimental)Priority: lowImpact: minorNovelty: new

Finding page for LFV-1197

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

What it shows

You copy an existing HNSW graph into a new builder with InitializedHnswGraphBuilder.fromGraph, passing null for initializedNodes, which its javadoc allows ('can be null if not tracking'), and then add a new vector with addGraphNode. You expect the vector to be inserted (with a bit set marking the copied nodes, the graph grows from 2 to 3 nodes). Instead addGraphNode throws NullPointerException ('Cannot invoke BitSet.get(int) because this.initializedNodes is null'), because it checks the bit set for nodes to skip without first checking that there is one.

Expected vs actual

Expected: addGraphNode(1) inserts node 1 and the graph has 2 nodes.
Actual: NullPointerException: Cannot invoke "org.apache.lucene.util.BitSet.get(int)" because "this.initializedNodes" is null at InitializedHnswGraphBuilder.addGraphNode(InitializedHnswGraphBuilder.java:489).

Files

src/org/apache/lucene/util/hnsw/TestInitializedHnswGraphBuilderNullNodes.java (JUnit test)

Run it

Tested on Lucene main 5adff78bb168 (2026-11-02): 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=$CP:randomizedtesting-runner-2.9.2.jar:junit-4.13.2.jar:hamcrest-3.0.jar   # test-framework dependencies (Maven Central)
CP=$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 off (production-like), seed DEADBEEF
java -da -dsa -Dtests.asserts=false -Xmx1g --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.hnsw.TestInitializedHnswGraphBuilderNullNodes

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).

Files shown on the finding page

TestInitializedHnswGraphBuilderNullNodes.java

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 2026-11-03. AI-assisted; not yet triaged by a human Lucene committer. Feedback: open an issue in lucene-ai-bug-hunt.