rework FetchGraph to be less of a memory hog

License: MIT
Signed-off-by: Jeromy <jeromyj@gmail.com>

merkledag/merkledag.go

@@ -149,6 +149,8 @@ func (ds *dagService) GetMany(ctx context.Context, keys []key.Key) (<-chan *Node
 	out := make(chan *Node)
 	errs := make(chan error, 1)
 	blocks := ds.Blocks.GetBlocks(ctx, keys)
+	var count int
+
 	go func() {
 		defer close(out)
 		defer close(errs)
@@ -156,6 +158,9 @@ func (ds *dagService) GetMany(ctx context.Context, keys []key.Key) (<-chan *Node
 			select {
 			case b, ok := <-blocks:
 				if !ok {
+					if count != len(keys) {
+						errs <- fmt.Errorf("failed to fetch all nodes")
+					}
 					return
 				}
 				nd, err := Decoded(b.Data)
@@ -165,6 +170,7 @@ func (ds *dagService) GetMany(ctx context.Context, keys []key.Key) (<-chan *Node
 				}
 				select {
 				case out <- nd:
+					count++
 				case <-ctx.Done():
 					return
 				}
@@ -404,28 +410,27 @@ func EnumerateChildrenAsync(ctx context.Context, ds DAGService, root *Node, set
 func fetchNodes(ctx context.Context, ds DAGService, in <-chan []key.Key, out chan<- *Node, errs chan<- error) {
 	defer close(out)
 
-	get := func(g NodeGetter) {
-		nd, err := g.Get(ctx)
-		if err != nil {
+	get := func(ks []key.Key) {
+		nodes, errch := ds.GetMany(ctx, ks)
+		for {
 			select {
-			case errs <- err:
-			case <-ctx.Done():
+			case nd, ok := <-nodes:
+				if !ok {
+					return
+				}
+				select {
+				case out <- nd:
+				case <-ctx.Done():
+					return
+				}
+			case err := <-errch:
+				errs <- err
+				return
 			}
-			return
-		}
-
-		select {
-		case out <- nd:
-		case <-ctx.Done():
-			return
 		}
 	}
 
 	for ks := range in {
-		ng := GetNodes(ctx, ds, ks)
-		for _, g := range ng {
-			go get(g)
-		}
+		go get(ks)
 	}
-
 }