code: func fetchOp(c context.Context, wrappedOpID string) (*upload.Operation, error) {
  	opID, err := upload.UnwrapOpID(c, wrappedOpID, auth.CurrentIdentity(c))
  	if err != nil {
  		if transient.Tag.In(err) {
  			return nil, errors.Annotate(err, "failed to check HMAC on upload_operation_id").Err()
  		}
  		return nil, errors.Reason("no such upload operation").
  			InternalReason("HMAC check failed - %s", err).
  			Tag(grpcutil.NotFoundTag).Err()
  	}
  
  	op := &upload.Operation{ID: opID}
  	switch err := datastore.Get(c, op); {
  	case err == datastore.ErrNoSuchEntity:
  		return nil, errors.Reason("no such upload operation").
  			Tag(grpcutil.NotFoundTag).Err()
  	case err != nil:
  		return nil, errors.Annotate(err, "failed to fetch the upload operation").
  			Tag(grpcutil.InternalTag).Err()
  	}
  
  	return op, nil
  }
docs: // fethcOp unwraps upload operation ID and fetches upload.Operation entity.
  //
  // Returns an grpc-tagged error on failure that can be returned to the RPC
  // caller right away.