Merge pull request #52 from ipfs/fix/empty-link

error when trying to encode an empty link

node.go

@@ -534,5 +534,8 @@ func castBytesToCid(x []byte) (cid.Cid, error) {
 }
 
 func castCidToBytes(link cid.Cid) ([]byte, error) {
+	if !link.Defined() {
+		return nil, ErrEmptyLink
+	}
 	return append([]byte{0}, link.Bytes()...), nil
 }

node_test.go

@@ -88,7 +88,6 @@ func TestBasicMarshal(t *testing.T) {
 		"name": "foo",
 		"bar":  c,
 	}
-	fmt.Printf("cid: %s\n", c.String())
 	nd, err := WrapObject(obj, mh.SHA2_256, -1)
 	if err != nil {
 		t.Fatal(err)
@@ -103,9 +102,6 @@ func TestBasicMarshal(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	fmt.Printf("before %v\n", nd.RawData())
-	fmt.Printf("after %v\n", back.RawData())
-
 	if err := assertCid(back.Cid(), "zdpuApUZEHofKXuTs2Yv2CLBeiASQrc9FojFLSZWcyZq6dZhb"); err != nil {
 		t.Fatal(err)
 	}
@@ -529,6 +525,27 @@ func TestCidAndBigInt(t *testing.T) {
 	}
 }
 
+func TestEmptyCid(t *testing.T) {
+	type Foo struct {
+		A cid.Cid
+	}
+	type Bar struct {
+		A cid.Cid `refmt:",omitempty"`
+	}
+	RegisterCborType(Foo{})
+	RegisterCborType(Bar{})
+
+	_, err := WrapObject(&Foo{}, mh.SHA2_256, -1)
+	if err == nil {
+		t.Fatal("should have failed to encode an object with an empty but non-omitted CID")
+	}
+
+	_, err = WrapObject(&Bar{}, mh.SHA2_256, -1)
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
 func TestCanonicalStructEncoding(t *testing.T) {
 	type Foo struct {
 		Zebra string