ctxtags TagBasedRequestFieldExtractor does not extract tags from fields in a oneof

[nvx]

With the following message and interceptor configuration:

message FooRequest {
  oneof identifier {
    string Bar = 1 [(gogoproto.moretags) = 'log_field:"bar"'];
    string Baz = 2 [(gogoproto.moretags) = 'log_field:"baz"'];
  }
}

grpc_ctxtags.UnaryServerInterceptor(grpc_ctxtags.WithFieldExtractorForInitialReq(grpc_ctxtags.TagBasedRequestFieldExtractor("log_field")))

On messages that has either Bar or Baz set, they do not appear in the ctxtags. Other fields outside of oneof's works as expected.

[nvx]

It looks like there's two parts of this, one is that gogoproto.moretags doesn't support oneof currently either, so outside the scope of this repo.

It looks like what's missing is

go-grpc-middleware/tags/fieldextractor.go

Line 67 in b4e7e96

if kind == reflect.Ptr && field.CanInterface() {

does not match as kind==reflect.Interface for oneof fields.

Changing it to also check for reflect.Interface seems to do the trick there, for example:

if (kind == reflect.Ptr || kind == reflect.Interface) && field.CanInterface() {

Note that noting that once field.Interface() is called on a field where kind==reflect.Interface, the returned value when passed to reflect.ValueOf().Kind() becomes reflect.Ptr again. So the earlier check here is doesn't need updating:

go-grpc-middleware/tags/fieldextractor.go

Line 57 in b4e7e96

if v.Kind() != reflect.Ptr || v.Elem().Kind() != reflect.Struct {

While gogoproto.moretags can't support setting tags on fields in a oneof directly, making the above change would at least allow it to work where the oneof field is a message type rather than a string/etc and future proof for when gogoproto.moretags (or another tagging option) adds support.

Edit: Looks like I had an old version of gogoproto, the current version does support setting tags on oneof fields.