proto: fix self-merging

While odd, it is possible to merge a message into itself.
In such a situation, the material impact is that repeated
and unknown fields are duplicated. The previous logic would
inifinite loop since the list iteration logic uses the current
length, but since the current length is ever growing, this loop
will never terminate. Instead, record the list length once
and iterate exactly that many times.

Change-Id: Ief98afa1b20bd950a9c2422d4462b170dbe6fa11
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/196857
Reviewed-by: Damien Neil <dneil@google.com>

proto/merge.go

@@ -45,7 +45,7 @@ func mergeMessage(dst, src protoreflect.Message) {
 }
 
 func mergeList(dst, src protoreflect.List, fd protoreflect.FieldDescriptor) {
-	for i := 0; i < src.Len(); i++ {
+	for i, n := 0, src.Len(); i < n; i++ {
 		switch v := src.Get(i); {
 		case fd.Message() != nil:
 			dstv := dst.NewElement()

proto/merge_test.go

@@ -401,3 +401,41 @@ func TestMergeRace(t *testing.T) {
 		}(src)
 	}
 }
+
+func TestMergeSelf(t *testing.T) {
+	got := &testpb.TestAllTypes{
+		OptionalInt32:   proto.Int32(1),
+		OptionalString:  proto.String("hello"),
+		RepeatedInt32:   []int32{2, 3, 4},
+		RepeatedString:  []string{"goodbye"},
+		MapStringString: map[string]string{"key": "value"},
+		OptionalNestedMessage: &testpb.TestAllTypes_NestedMessage{
+			A: proto.Int32(5),
+		},
+	}
+	got.ProtoReflect().SetUnknown(pack.Message{
+		pack.Tag{Number: 50000, Type: pack.VarintType}, pack.Svarint(-5),
+	}.Marshal())
+	proto.Merge(got, got)
+
+	// The main impact of merging to self is that repeated fields and
+	// unknown fields are doubled.
+	want := &testpb.TestAllTypes{
+		OptionalInt32:   proto.Int32(1),
+		OptionalString:  proto.String("hello"),
+		RepeatedInt32:   []int32{2, 3, 4, 2, 3, 4},
+		RepeatedString:  []string{"goodbye", "goodbye"},
+		MapStringString: map[string]string{"key": "value"},
+		OptionalNestedMessage: &testpb.TestAllTypes_NestedMessage{
+			A: proto.Int32(5),
+		},
+	}
+	want.ProtoReflect().SetUnknown(pack.Message{
+		pack.Tag{Number: 50000, Type: pack.VarintType}, pack.Svarint(-5),
+		pack.Tag{Number: 50000, Type: pack.VarintType}, pack.Svarint(-5),
+	}.Marshal())
+
+	if !proto.Equal(got, want) {
+		t.Errorf("Equal mismatch:\ngot  %v\nwant %v", got, want)
+	}
+}