Remove usage of merkledag.Link.Node pointer outside of merkledag

This prepares for inclusion of IPLD where the Node pointer won't be there.

License: MIT
Signed-off-by: Mildred Ki'Lya <mildred-pub.git@mildred.fr>

node.go

@@ -2,6 +2,7 @@ package merkledag
 
 import (
 	"fmt"
+	"time"
 
 	"gx/ipfs/QmZy2y8t9zQH2a1b8q2ZSLKp17ATuJoCNxxyMFG5qFExpt/go-net/context"
 
@@ -85,6 +86,26 @@ func (l *Link) GetNode(ctx context.Context, serv DAGService) (*Node, error) {
 	return serv.Get(ctx, key.Key(l.Hash))
 }
 
+// GetNodeAndCache return the MDAG Node that the link points to and store a
+// pointer to that node along with the link to speed up further retrivals. A
+// timeout is to be specified to avoid taking too much time.
+func (l *Link) GetNodeAndCache(ctx context.Context, serv DAGService, timeout time.Duration) (*Node, error) {
+	if l.Node == nil {
+		if timeout != 0 {
+			var cancel context.CancelFunc
+			ctx, cancel = context.WithTimeout(ctx, time.Minute)
+			defer cancel()
+		}
+		nd, err := serv.Get(ctx, key.Key(l.Hash))
+		if err != nil {
+			return nil, err
+		}
+		l.Node = nd
+	}
+
+	return l.Node, nil
+}
+
 // AddNodeLink adds a link to another node.
 func (n *Node) AddNodeLink(name string, that *Node) error {
 	n.encoded = nil