// updateForBatch updates the first argument (the header of a request contained
// in a batch) from the second one (the batch header), returning an error when
// inconsistencies are found.
// It is checked that the individual call does not have a User, UserPriority
// or Txn set that differs from the batch's.
func updateForBatch(aHeader *proto.RequestHeader, bHeader proto.RequestHeader) error {
	// Disallow transaction, user and priority on individual calls, unless
	// equal.
	if aHeader.User != "" && aHeader.User != bHeader.User {
		return util.Error("conflicting user on call in batch")
	}
	if aPrio := aHeader.GetUserPriority(); aPrio != proto.Default_RequestHeader_UserPriority && aPrio != bHeader.GetUserPriority() {
		return util.Error("conflicting user priority on call in batch")
	}
	if aHeader.Txn != nil && !aHeader.Txn.Equal(bHeader.Txn) {
		return util.Error("conflicting transaction in transactional batch")
	}

	aHeader.User = bHeader.User
	aHeader.UserPriority = bHeader.UserPriority
	aHeader.Txn = bHeader.Txn
	return nil
}