Add json unmarshal code and fix panic

A panic would occur when a link was created with a nil cid, this should
be allowable, just catch the potential problem and skip marshaling the
cid.

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

coding.go

@@ -60,7 +60,9 @@ func (n *ProtoNode) getPBNode() *pb.PBNode {
 		pbn.Links[i] = &pb.PBLink{}
 		pbn.Links[i].Name = &l.Name
 		pbn.Links[i].Tsize = &l.Size
-		pbn.Links[i].Hash = l.Cid.Bytes()
+		if l.Cid != nil {
+			pbn.Links[i].Hash = l.Cid.Bytes()
+		}
 	}
 
 	if len(n.data) > 0 {

node.go

@@ -229,6 +229,22 @@ func (n *ProtoNode) Loggable() map[string]interface{} {
 	}
 }
 
+func (n *ProtoNode) UnmarshalJSON(b []byte) error {
+	s := struct {
+		Data  []byte       `json:"data"`
+		Links []*node.Link `json:"links"`
+	}{}
+
+	err := json.Unmarshal(b, &s)
+	if err != nil {
+		return err
+	}
+
+	n.data = s.Data
+	n.links = s.Links
+	return nil
+}
+
 func (n *ProtoNode) MarshalJSON() ([]byte, error) {
 	out := map[string]interface{}{
 		"data":  n.data,

node_test.go

 package merkledag_test
 
 import (
+	"bytes"
 	"context"
 	"testing"
 
@@ -128,3 +129,32 @@ func TestNodeCopy(t *testing.T) {
 		t.Fatal("should be different objects")
 	}
 }
+
+func TestJsonRoundtrip(t *testing.T) {
+	nd := new(ProtoNode)
+	nd.SetLinks([]*node.Link{
+		{Name: "a"},
+		{Name: "c"},
+		{Name: "b"},
+	})
+	nd.SetData([]byte("testing"))
+
+	jb, err := nd.MarshalJSON()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	nn := new(ProtoNode)
+	err = nn.UnmarshalJSON(jb)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if !bytes.Equal(nn.Data(), nd.Data()) {
+		t.Fatal("data wasnt the same")
+	}
+
+	if !nn.Cid().Equals(nd.Cid()) {
+		t.Fatal("objects differed after marshaling")
+	}
+}