Add only forward links when repairing HNSW merge (fixes #15467)

lucene/core/src/java/org/apache/lucene/util/hnsw/HnswGraphBuilder.java

@@ -351,15 +351,7 @@ void addDiverseNeighbors(
       UpdateableRandomVectorScorer scorer,
       boolean outOfOrderInsertion)
       throws IOException {
-    /* For each of the beamWidth nearest candidates (going from best to worst), select it only if it
-     * is closer to target than it is to any of the already-selected neighbors (ie selected in this method,
-     * since the node is new and has no prior neighbors).
-     */
-    NeighborArray neighbors = hnsw.getNeighbors(level, node);
-    int maxConnOnLevel = level == 0 ? M * 2 : M;
-    boolean[] mask =
-        selectAndLinkDiverse(
-            node, neighbors, candidates, maxConnOnLevel, scorer, outOfOrderInsertion);
+    boolean[] mask = addDiverseForward(level, node, candidates, scorer, outOfOrderInsertion);
 
     // Link the selected nodes to the new node, and the new node to the selected nodes (again
     // applying diversity heuristic)
@@ -386,6 +378,25 @@ void addDiverseNeighbors(
     }
   }
 
+  boolean[] addDiverseForward(
+      int level,
+      int node,
+      NeighborArray candidates,
+      UpdateableRandomVectorScorer scorer,
+      boolean outOfOrderInsertion)
+      throws IOException {
+    /* For each of the beamWidth nearest candidates (going from best to worst), select it only if it
+     * is closer to target than it is to any of the already-selected neighbors (ie selected in this method,
+     * since the node is new and has no prior neighbors).
+     */
+    NeighborArray neighbors = hnsw.getNeighbors(level, node);
+    int maxConnOnLevel = level == 0 ? M * 2 : M;
+    boolean[] mask =
+        selectAndLinkDiverse(
+            node, neighbors, candidates, maxConnOnLevel, scorer, outOfOrderInsertion);
+    return mask;
+  }
+
   /**
    * This method will select neighbors to add and return a mask telling the caller which candidates
    * are selected

lucene/core/src/java/org/apache/lucene/util/hnsw/InitializedHnswGraphBuilder.java

@@ -325,9 +325,9 @@ private void fixDisconnectedNodes(
         popToScratch(candidates, scratchArray);
 
         // Add diverse neighbors using HNSW heuristic (prunes similar neighbors)
-        addDiverseNeighbors(level, node, scratchArray, scorer, true);
+        addDiverseForward(level, node, scratchArray, scorer, true);
       } else {
-        // Node has no nighbors, add connections from scratch
+        // Node has no neighbors, add connections from scratch
         addConnections(node, level, scorer);
       }