Bugfix/disable matches highlight knn (elastic#136563)

* Disable matches mode for all knn queries

matches highlight mode will utilize a MemoryIndex. Newer
knn queries utilize an optimization that makes assumptions about
the IndexSearch and reader utilized. This will end up throwing an
NPE.

While we have fixed this NPE in Apache Lucene, we just should not
utilize weight matches model for knn queries at all. Maybe
this restriction can be addressed later.

closes: elastic#136427

* adding comment

* formatting

* Update docs/changelog/136563.yaml

Author: benwtrent

docs/changelog/136563.yaml

@@ -0,0 +1,5 @@
+pr: 136563
+summary: Bugfix/disable matches highlight knn
+area: Vector Search
+type: bug
+issues: []

server/src/main/java/org/elasticsearch/lucene/search/uhighlight/CustomUnifiedHighlighter.java

@@ -35,7 +35,13 @@
 import org.elasticsearch.index.search.ESToParentBlockJoinQuery;
 import org.elasticsearch.search.retriever.rankdoc.RankDocsQuery;
 import org.elasticsearch.search.runtime.AbstractScriptFieldQuery;
+import org.elasticsearch.search.vectors.ESDiversifyingChildrenByteKnnVectorQuery;
+import org.elasticsearch.search.vectors.ESDiversifyingChildrenFloatKnnVectorQuery;
+import org.elasticsearch.search.vectors.ESKnnByteVectorQuery;
+import org.elasticsearch.search.vectors.ESKnnFloatVectorQuery;
 import org.elasticsearch.search.vectors.KnnScoreDocQuery;
+import org.elasticsearch.search.vectors.RescoreKnnVectorQuery;
+import org.elasticsearch.search.vectors.VectorSimilarityQuery;
 
 import java.io.IOException;
 import java.text.BreakIterator;
@@ -56,6 +62,20 @@
  * Supports both returning empty snippets and non highlighted snippets when no highlighting can be performed.
  */
 public final class CustomUnifiedHighlighter extends UnifiedHighlighter {
+    private static boolean isKnnQuery(Query q) {
+        // TODO: bug https://github.com/elastic/elasticsearch/issues/136427
+        // AbstractKnnVectorQuery is not public, so we need to list concrete classes
+        // currently there is a bug in Lucene that causes things that use
+        // AbstractKnnVectorQuery to throw NPEs in weight matches highlighting
+        return q instanceof KnnScoreDocQuery
+            || q instanceof RescoreKnnVectorQuery
+            || q instanceof VectorSimilarityQuery
+            || q instanceof ESKnnFloatVectorQuery
+            || q instanceof ESKnnByteVectorQuery
+            || q instanceof ESDiversifyingChildrenByteKnnVectorQuery
+            || q instanceof ESDiversifyingChildrenFloatKnnVectorQuery;
+    }
+
     public static final char MULTIVAL_SEP_CHAR = (char) 0;
     private static final Snippet[] EMPTY_SNIPPET = new Snippet[0];
 
@@ -259,8 +279,9 @@ public void visitLeaf(Query leafQuery) {
                 /**
                  * KnnScoreDocQuery and RankDocsQuery requires the same reader that built the docs
                  * When using {@link HighlightFlag#WEIGHT_MATCHES} different readers are used and isn't supported by this query
+                 * Additionally, kNN queries don't really work against MemoryIndex which is used in the matches API
                  */
-                if (leafQuery instanceof KnnScoreDocQuery || leafQuery instanceof RankDocsQuery.TopQuery) {
+                if (leafQuery instanceof RankDocsQuery.TopQuery || isKnnQuery(leafQuery)) {
                     hasUnknownLeaf[0] = true;
                 }
                 super.visitLeaf(query);