perf(hnsw): deduplicate neighbor updates and fix error handling in insertHelper

Deduplicate addNeighbors calls by iterating over inboundEdgesAllLayersMap keys
instead of nnUidArray, which could contain duplicate UIDs when the same neighbor
appears across multiple layers. Also fix two bugs: (1) err from addNeighbors for
the inserted node was checked after the neighbor loop instead of immediately,
and (2) searchPersistentLayer errors were silently swallowed by returning the
uninitialized layerErr variable instead of err.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Author: shaunpatterson

tok/hnsw/persistent_hnsw.go

@@ -593,8 +593,6 @@ func (ph *persistentHNSW[T]) insertHelper(ctx context.Context, tc *TxnCache,
 	// var nns []persistentHeapElement[T]
 	visited := []persistentHeapElement[T]{}
 	inLevel := getInsertLayer(ph.maxLevels) // calculate layer to insert node at (randomized every time)
-	var layerErr error
-
 	for level := range inLevel {
 		// perform insertion for layers [level, max_level) only, when level < inLevel just find better start
 		err := ph.getVecFromUid(entry, tc, &startVec)
@@ -617,7 +615,6 @@ func (ph *persistentHNSW[T]) insertHelper(ctx context.Context, tc *TxnCache,
 
 	var outboundEdgesAllLayers = make([][]uint64, ph.maxLevels)
 	var inboundEdgesAllLayersMap = make(map[uint64][][]uint64)
-	nnUidArray := []uint64{}
 	for level := inLevel; level < ph.maxLevels; level++ {
 		err := ph.getVecFromUid(entry, tc, &startVec)
 		if err != nil {
@@ -626,38 +623,34 @@ func (ph *persistentHNSW[T]) insertHelper(ctx context.Context, tc *TxnCache,
 		layerResult, err := ph.searchPersistentLayer(tc, level, entry, startVec,
 			inVec, false, ph.efConstruction, index.AcceptAll[T])
 		if err != nil {
-			return []persistentHeapElement[T]{}, []*index.KeyValue{}, layerErr
+			return []persistentHeapElement[T]{}, []*index.KeyValue{}, err
 		}
 
 		entry = layerResult.bestNeighbor().index
 
 		nns := layerResult.neighbors
 		for i := range nns {
-			nnUidArray = append(nnUidArray, nns[i].index)
 			if inboundEdgesAllLayersMap[nns[i].index] == nil {
 				inboundEdgesAllLayersMap[nns[i].index] = make([][]uint64, ph.maxLevels)
 			}
 			inboundEdgesAllLayersMap[nns[i].index][level] =
 				append(inboundEdgesAllLayersMap[nns[i].index][level], inUuid)
-			// add nn to outboundEdges.
-			// These should already be correctly ordered.
 			outboundEdgesAllLayers[level] =
 				append(outboundEdgesAllLayers[level], nns[i].index)
 		}
 	}
 	edge, err := ph.addNeighbors(ctx, tc, inUuid, outboundEdgesAllLayers)
-	for i := range nnUidArray {
-		edge, err := ph.addNeighbors(
-			ctx, tc, nnUidArray[i], inboundEdgesAllLayersMap[nnUidArray[i]])
+	if err != nil {
+		return []persistentHeapElement[T]{}, []*index.KeyValue{}, err
+	}
+	edges = append(edges, edge)
+	for nnUid, inboundEdges := range inboundEdgesAllLayersMap {
+		edge, err := ph.addNeighbors(ctx, tc, nnUid, inboundEdges)
 		if err != nil {
 			return []persistentHeapElement[T]{}, []*index.KeyValue{}, err
 		}
 		edges = append(edges, edge)
 	}
-	if err != nil {
-		return []persistentHeapElement[T]{}, []*index.KeyValue{}, err
-	}
-	edges = append(edges, edge)
 
 	return visited, edges, nil
 }