code: func GetBuilderID(c context.Context, id int64) (builder *buildbucketpb.BuilderID, number int32, err error) {
  	host, err := getHost(c)
  	if err != nil {
  		return
  	}
  	client, err := buildbucketClient(c, host, auth.AsUser)
  	if err != nil {
  		return
  	}
  	br, err := client.GetBuild(c, &buildbucketpb.GetBuildRequest{
  		Id:     id,
  		Fields: builderIDMask,
  	})
  	switch grpcutil.Code(err) {
  	case codes.OK:
  		builder = br.Builder
  		number = br.Number
  	case codes.NotFound:
  		if auth.CurrentIdentity(c) == identity.AnonymousIdentity {
  			err = ErrNotLoggedIn
  			return
  		}
  		fallthrough
  	case codes.PermissionDenied:
  		err = ErrNotFound
  	}
  	return
  }
docs: // GetBuilderID returns the builder, and maybe the build number, for a build id.