fix a fetch deadlock on error

fixes https://github.com/ipfs/go-ipfs/issues/5793

merkledag.go

@@ -397,7 +397,10 @@ func EnumerateChildrenAsyncDepth(ctx context.Context, getLinks GetLinks, c cid.C
 				if shouldVisit {
 					links, err := getLinks(ctx, ci)
 					if err != nil {
-						errChan <- err
+						select {
+						case errChan <- err:
+						case <-fetchersCtx.Done():
+						}
 						return
 					}
 

merkledag_test.go

@@ -677,6 +677,7 @@ func TestEnumerateAsyncFailsNotFound(t *testing.T) {
 	b := NodeWithData([]byte("foo2"))
 	c := NodeWithData([]byte("foo3"))
 	d := NodeWithData([]byte("foo4"))
+	e := NodeWithData([]byte("foo5"))
 
 	ds := dstest.Mock()
 	for _, n := range []ipld.Node{a, b, c} {
@@ -703,6 +704,10 @@ func TestEnumerateAsyncFailsNotFound(t *testing.T) {
 		t.Fatal(err)
 	}
 
+	if err := parent.AddNodeLink("e", e); err != nil {
+		t.Fatal(err)
+	}
+
 	err := ds.Add(ctx, parent)
 	if err != nil {
 		t.Fatal(err)