encoding/proto: do not panic when types do not match (grpc#4218)

encoding/proto/proto.go

@@ -21,6 +21,8 @@
 package proto
 
 import (
+	"fmt"
+
 	"github.com/golang/protobuf/proto"
 	"google.golang.org/grpc/encoding"
 )
@@ -36,11 +38,19 @@ func init() {
 type codec struct{}
 
 func (codec) Marshal(v interface{}) ([]byte, error) {
-	return proto.Marshal(v.(proto.Message))
+	vv, ok := v.(proto.Message)
+	if !ok {
+		return nil, fmt.Errorf("failed to marshal, message is %T, want proto.Message", v)
+	}
+	return proto.Marshal(vv)
 }
 
 func (codec) Unmarshal(data []byte, v interface{}) error {
-	return proto.Unmarshal(data, v.(proto.Message))
+	vv, ok := v.(proto.Message)
+	if !ok {
+		return fmt.Errorf("failed to unmarshal, message is %T, want proto.Message", v)
+	}
+	return proto.Unmarshal(data, vv)
 }
 
 func (codec) Name() string {