jsonpb: avoid unexported fields in hand-crafted message (#671)

The purego implementation is unable to handle unexported fields since it is
impossible for purego code to mutate such fields without using unsafe.

Also, modify the Makefile to test the purego code paths.

Fixes #670

Makefile

@@ -36,6 +36,7 @@ install:
 
 test:
 	go test ./... ./protoc-gen-go/testdata
+	go test -tags purego ./... ./protoc-gen-go/testdata
 	go build ./protoc-gen-go/testdata/grpc/grpc.pb.go
 	make -C conformance test
 

jsonpb/jsonpb_test.go

@@ -572,7 +572,7 @@ func TestMarshalIllegalTime(t *testing.T) {
 
 func TestMarshalJSONPBMarshaler(t *testing.T) {
 	rawJson := `{ "foo": "bar", "baz": [0, 1, 2, 3] }`
-	msg := dynamicMessage{rawJson: rawJson}
+	msg := dynamicMessage{RawJson: rawJson}
 	str, err := new(Marshaler).MarshalToString(&msg)
 	if err != nil {
 		t.Errorf("an unexpected error occurred when marshalling JSONPBMarshaler: %v", err)
@@ -583,7 +583,7 @@ func TestMarshalJSONPBMarshaler(t *testing.T) {
 }
 
 func TestMarshalAnyJSONPBMarshaler(t *testing.T) {
-	msg := dynamicMessage{rawJson: `{ "foo": "bar", "baz": [0, 1, 2, 3] }`}
+	msg := dynamicMessage{RawJson: `{ "foo": "bar", "baz": [0, 1, 2, 3] }`}
 	a, err := ptypes.MarshalAny(&msg)
 	if err != nil {
 		t.Errorf("an unexpected error occurred when marshalling to Any: %v", err)
@@ -601,7 +601,7 @@ func TestMarshalAnyJSONPBMarshaler(t *testing.T) {
 }
 
 func TestMarshalWithCustomValidation(t *testing.T) {
-	msg := dynamicMessage{rawJson: `{ "foo": "bar", "baz": [0, 1, 2, 3] }`, dummy: &dynamicMessage{}}
+	msg := dynamicMessage{RawJson: `{ "foo": "bar", "baz": [0, 1, 2, 3] }`, Dummy: &dynamicMessage{}}
 
 	js, err := new(Marshaler).MarshalToString(&msg)
 	if err != nil {
@@ -1011,8 +1011,8 @@ func TestUnmarshalJSONPBUnmarshaler(t *testing.T) {
 	if err := Unmarshal(strings.NewReader(rawJson), &msg); 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)
+	if msg.RawJson != rawJson {
+		t.Errorf("message contents not set correctly after unmarshalling JSON: got %s, wanted %s", msg.RawJson, rawJson)
 	}
 }
 
@@ -1036,7 +1036,7 @@ func TestUnmarshalAnyJSONPBUnmarshaler(t *testing.T) {
 		t.Errorf("an unexpected error occurred when parsing into JSONPBUnmarshaler: %v", err)
 	}
 
-	dm := &dynamicMessage{rawJson: `{"baz":[0,1,2,3],"foo":"bar"}`}
+	dm := &dynamicMessage{RawJson: `{"baz":[0,1,2,3],"foo":"bar"}`}
 	var want anypb.Any
 	if b, err := proto.Marshal(dm); err != nil {
 		t.Errorf("an unexpected error occurred when marshaling message: %v", err)
@@ -1097,30 +1097,30 @@ func (s *stringField) UnmarshalJSONPB(jum *Unmarshaler, js []byte) error {
 // 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 `protobuf:"bytes,1,opt,name=rawJson"`
+	RawJson string `protobuf:"bytes,1,opt,name=rawJson"`
 
 	// an unexported nested message is present just to ensure that it
 	// won't result in a panic (see issue #509)
-	dummy *dynamicMessage `protobuf:"bytes,2,opt,name=dummy"`
+	Dummy *dynamicMessage `protobuf:"bytes,2,opt,name=dummy"`
 }
 
 func (m *dynamicMessage) Reset() {
-	m.rawJson = "{}"
+	m.RawJson = "{}"
 }
 
 func (m *dynamicMessage) String() string {
-	return m.rawJson
+	return m.RawJson
 }
 
 func (m *dynamicMessage) ProtoMessage() {
 }
 
 func (m *dynamicMessage) MarshalJSONPB(jm *Marshaler) ([]byte, error) {
-	return []byte(m.rawJson), nil
+	return []byte(m.RawJson), nil
 }
 
 func (m *dynamicMessage) UnmarshalJSONPB(jum *Unmarshaler, js []byte) error {
-	m.rawJson = string(js)
+	m.RawJson = string(js)
 	return nil
 }