[DiskBBQ] do not build empty priority queue

server/src/main/java/org/elasticsearch/index/codec/vectors/diskbbq/next/ESNextDiskBBQVectorsReader.java

@@ -324,6 +324,20 @@ private static CentroidIterator getCentroidIteratorWithParents(
         final NeighborQueue currentParentQueue = new NeighborQueue(maxChildrenSize, true);
         final int bufferSize = (int) Math.min(Math.max(centroidRatio * numCentroids, 1), numCentroids);
         final int numCentroidsFiltered = acceptCentroids == null ? numCentroids : acceptCentroids.cardinality();
+        if (numCentroidsFiltered == 0) {
+            // TODO maybe this makes CentroidIterator polymorphic?
+            return new CentroidIterator() {
+                @Override
+                public boolean hasNext() {
+                    return false;
+                }
+
+                @Override
+                public CentroidOffsetAndLength nextPostingListOffsetAndLength() {
+                    return null;
+                }
+            };
+        }
         final float[] scores = new float[ES92Int7VectorsScorer.BULK_SIZE];
         final NeighborQueue neighborQueue;
         if (acceptCentroids != null && numCentroidsFiltered <= bufferSize) {

server/src/test/java/org/elasticsearch/index/codec/vectors/diskbbq/next/ESNextDiskBBQVectorsFormatTests.java

@@ -29,6 +29,7 @@
 import org.apache.lucene.index.VectorEncoding;
 import org.apache.lucene.index.VectorSimilarityFunction;
 import org.apache.lucene.search.AcceptDocs;
+import org.apache.lucene.search.DocIdSetIterator;
 import org.apache.lucene.search.KnnCollector;
 import org.apache.lucene.search.TopDocs;
 import org.apache.lucene.search.TopKnnCollector;
@@ -382,6 +383,13 @@ private void doRestrictiveFilter(boolean dense) throws IOException {
                     uniqueDocIds.add(topDocs.scoreDocs[i].doc);
                 }
                 assertEquals(matchingDocs, uniqueDocIds.size());
+                // match no docs
+                leafReader.searchNearestVectors(
+                    "f",
+                    vector,
+                    new TopKnnCollector(2, Integer.MAX_VALUE),
+                    AcceptDocs.fromIteratorSupplier(DocIdSetIterator::empty, leafReader.getLiveDocs(), leafReader.maxDoc())
+                );
             }
         }
     }