merkledag: add a concurrency limit to merkledag fetch graph

License: MIT
Signed-off-by: Jeromy <why@ipfs.io>

merkledag/merkledag.go

@@ -449,6 +449,10 @@ func EnumerateChildrenAsync(ctx context.Context, ds DAGService, c *cid.Cid, visi
 	}
 }
 
+// FetchGraphConcurrency is total number of concurrenct fetches that
+// 'fetchNodes' will start at a time
+var FetchGraphConcurrency = 8
+
 func fetchNodes(ctx context.Context, ds DAGService, in <-chan []*cid.Cid, out chan<- *NodeOption) {
 	var wg sync.WaitGroup
 	defer func() {
@@ -458,8 +462,13 @@ func fetchNodes(ctx context.Context, ds DAGService, in <-chan []*cid.Cid, out ch
 		close(out)
 	}()
 
+	rateLimit := make(chan struct{}, FetchGraphConcurrency)
+
 	get := func(ks []*cid.Cid) {
 		defer wg.Done()
+		defer func() {
+			<-rateLimit
+		}()
 		nodes := ds.GetMany(ctx, ks)
 		for opt := range nodes {
 			select {
@@ -471,6 +480,11 @@ func fetchNodes(ctx context.Context, ds DAGService, in <-chan []*cid.Cid, out ch
 	}
 
 	for ks := range in {
+		select {
+		case rateLimit <- struct{}{}:
+		case <-ctx.Done():
+			return
+		}
 		wg.Add(1)
 		go get(ks)
 	}