Add Merge to prototext.UnmarshalOptions

encoding/prototext/decode.go

@@ -31,6 +31,11 @@ func Unmarshal(b []byte, m proto.Message) error {
 type UnmarshalOptions struct {
 	pragma.NoUnkeyedLiterals
 
+	// Merge merges the input into the destination message.
+	// The default behavior is to always reset the message before unmarshaling,
+	// unless Merge is specified.
+	Merge bool
+
 	// AllowPartial accepts input for messages that will result in missing
 	// required fields. If AllowPartial is false (the default), Unmarshal will
 	// return error if there are any missing required fields.
@@ -62,7 +67,9 @@ func (o UnmarshalOptions) Unmarshal(b []byte, m proto.Message) error {
 // For profiling purposes, avoid changing the name of this function or
 // introducing other code paths for unmarshal that do not go through this.
 func (o UnmarshalOptions) unmarshal(b []byte, m proto.Message) error {
-	proto.Reset(m)
+	if !o.Merge {
+		proto.Reset(m)
+	}
 
 	if o.Resolver == nil {
 		o.Resolver = protoregistry.GlobalTypes

encoding/prototext/decode_test.go

@@ -1288,6 +1288,26 @@ str_to_nested: {
 		inputMessage: &pb2.Nests{},
 		inputText:    "reserved_field: 'ignore this'",
 		wantMessage:  &pb2.Nests{},
+	}, {
+		desc: "reset message when merge is not set",
+		inputMessage: &pb3.Scalars{
+			SBool: true,
+		},
+		inputText: `s_string: "abc"`,
+		wantMessage: &pb3.Scalars{
+			SString: "abc",
+		},
+	}, {
+		desc: "message not reset when merge is enabled",
+		umo:  prototext.UnmarshalOptions{Merge: true},
+		inputMessage: &pb3.Scalars{
+			SBool: true,
+		},
+		inputText: `s_string: "abc"`,
+		wantMessage: &pb3.Scalars{
+			SBool:   true,
+			SString: "abc",
+		},
 	}, {
 		desc:         "extensions of non-repeated fields",
 		inputMessage: &pb2.Extensions{},