Support custom JSON (de)serialization in jsonpb

Introduce jsonpb JSONPBMarshaler and JSONPBUnmarshaler interfaces for custom JSON serialization and deserialization of dynamic messages.  jsonpb will delegate to these interface methods passing in the default Marhaler/Unmarshaler in order to preserve the options as well as reuse these for regular protobuf values within the dynamic message.

jsonpb/jsonpb.go

@@ -74,6 +74,22 @@ type Marshaler struct {
 	OrigName bool
 }
 
+// JSONPBMarshaler is implemented by protobuf messages that customize the
+// way they are marshaled to JSON. Messages that implement this should
+// also implement JSONPBUnmarshaler so that the custom format can be
+// parsed.
+type JSONPBMarshaler interface {
+	MarshalJSONPB(*Marshaler) ([]byte, error)
+}
+
+// JSONPBUnmarshaler is implemented by protobuf messages that customize
+// the way they are unmarshaled from JSON. Messages that implement this
+// should also implement JSONPBMarshaler so that the custom format can be
+// produced.
+type JSONPBUnmarshaler interface {
+	UnmarshalJSONPB(*Unmarshaler, []byte) error
+}
+
 // Marshal marshals a protocol buffer into JSON.
 func (m *Marshaler) Marshal(out io.Writer, pb proto.Message) error {
 	writer := &errWriter{writer: out}
@@ -102,6 +118,15 @@ type wkt interface {
 
 // marshalObject writes a struct to the Writer.
 func (m *Marshaler) marshalObject(out *errWriter, v proto.Message, indent, typeURL string) error {
+	if jsm, ok := v.(JSONPBMarshaler); ok {
+		b, err := jsm.MarshalJSONPB(m)
+		if err != nil {
+			return err
+		}
+		out.write(string(b))
+		return out.err
+	}
+
 	s := reflect.ValueOf(v).Elem()
 
 	// Handle well-known types.
@@ -571,6 +596,10 @@ func (u *Unmarshaler) unmarshalValue(target reflect.Value, inputValue json.RawMe
 		return u.unmarshalValue(target.Elem(), inputValue, prop)
 	}
 
+	if jsu, ok := target.Addr().Interface().(JSONPBUnmarshaler); ok {
+		return jsu.UnmarshalJSONPB(u, []byte(inputValue))
+	}
+
 	// Handle well-known types.
 	type wkt interface {
 		XXX_WellKnownType() string

jsonpb/jsonpb_test.go

@@ -439,6 +439,18 @@ func TestMarshaling(t *testing.T) {
 	}
 }
 
+func TestMarshalingWithJSONPBMarshaler(t *testing.T) {
+	rawJson := `{ "foo": "bar", "baz": [0, 1, 2, 3] }`
+	msg := dynamicMessage{rawJson: rawJson}
+	str, err := new(Marshaler).MarshalToString(&msg)
+	if err != nil {
+		t.Errorf("an unexpected error occurred when marshalling JSONPBMarshaler: %v", err)
+	}
+	if str != rawJson {
+		t.Errorf("marshalling JSON produced incorrect output: got %s, wanted %s", str, rawJson)
+	}
+}
+
 var unmarshalingTests = []struct {
 	desc        string
 	unmarshaler Unmarshaler
@@ -635,3 +647,41 @@ func TestUnmarshalingBadInput(t *testing.T) {
 		}
 	}
 }
+
+func TestUnmarshalWithJSONPBUnmarshaler(t *testing.T) {
+	rawJson := `{ "foo": "bar", "baz": [0, 1, 2, 3] }`
+	var msg dynamicMessage
+	err := Unmarshal(strings.NewReader(rawJson), &msg)
+	if err != nil {
+		t.Errorf("an unexpected error occurred when parsing into JSONPBUnmarshaler: %v", err)
+	}
+	if msg.rawJson != rawJson {
+		t.Errorf("message contents not set correctly after unmarshalling JSON: got %s, wanted %s", msg.rawJson, rawJson)
+	}
+}
+
+// dynamicMessage implements protobuf.Message but is not a normal generated message type.
+// It provides implementations of JSONPBMarshaler and JSONPBUnmarshaler for JSON support.
+type dynamicMessage struct {
+	rawJson string
+}
+
+func (m *dynamicMessage) Reset() {
+	m.rawJson = "{}"
+}
+
+func (m *dynamicMessage) String() string {
+	return m.rawJson
+}
+
+func (m *dynamicMessage) ProtoMessage() {
+}
+
+func (m *dynamicMessage) MarshalJSONPB(jm *Marshaler) ([]byte, error) {
+	return []byte(m.rawJson), nil
+}
+
+func (m *dynamicMessage) UnmarshalJSONPB(jum *Unmarshaler, json []byte) error {
+	m.rawJson = string(json)
+	return nil
+}