Add new option for the decoder - to disallow unknown fields (#959)

Fixes #448

runtime/marshal_jsonpb.go

@@ -172,7 +172,7 @@ func decodeJSONPb(d *json.Decoder, v interface{}) error {
 	if !ok {
 		return decodeNonProtoField(d, v)
 	}
-	unmarshaler := &jsonpb.Unmarshaler{AllowUnknownFields: true}
+	unmarshaler := &jsonpb.Unmarshaler{AllowUnknownFields: allowUnknownFields}
 	return unmarshaler.UnmarshalNext(d, p)
 }
 
@@ -186,7 +186,7 @@ func decodeNonProtoField(d *json.Decoder, v interface{}) error {
 			rv.Set(reflect.New(rv.Type().Elem()))
 		}
 		if rv.Type().ConvertibleTo(typeProtoMessage) {
-			unmarshaler := &jsonpb.Unmarshaler{AllowUnknownFields: true}
+			unmarshaler := &jsonpb.Unmarshaler{AllowUnknownFields: allowUnknownFields}
 			return unmarshaler.UnmarshalNext(d, rv.Interface().(proto.Message))
 		}
 		rv = rv.Elem()
@@ -248,3 +248,15 @@ var typeProtoMessage = reflect.TypeOf((*proto.Message)(nil)).Elem()
 func (j *JSONPb) Delimiter() []byte {
 	return []byte("\n")
 }
+
+// allowUnknownFields helps not to return an error when the destination
+// is a struct and the input contains object keys which do not match any
+// non-ignored, exported fields in the destination.
+var allowUnknownFields = true
+
+// DisallowUnknownFields enables option in decoder (unmarshaller) to
+// return an error when it finds an unknown field. This function must be
+// called before using the JSON marshaller.
+func DisallowUnknownFields() {
+	allowUnknownFields = false
+}

runtime/marshal_jsonpb_test.go

@@ -468,6 +468,25 @@ func TestJSONPbDecoderFields(t *testing.T) {
 	}
 }
 
+func TestJSONPbDecoderUnknownField(t *testing.T) {
+	var (
+		m   runtime.JSONPb
+		got examplepb.ABitOfEverything
+	)
+	data := `{
+		"uuid": "6EC2446F-7E89-4127-B3E6-5C05E6BECBA7",
+		"unknownField": "111"
+	}`
+
+	runtime.DisallowUnknownFields()
+
+	r := strings.NewReader(data)
+	dec := m.NewDecoder(r)
+	if err := dec.Decode(&got); err == nil {
+		t.Errorf("m.Unmarshal(&got) not failed; want `unknown field` error; data=%q", data)
+	}
+}
+
 var (
 	fieldFixtures = []struct {
 		data          interface{}