// if validate is true, an error is returned if the user does not exist.
func getUserWithProvider(ctx context.Context, userProvider, user string, validate bool) (string, error) {
	switch userProvider {
	case madmin.BuiltinProvider:
		if validate {
			if _, ok := globalIAMSys.GetUser(ctx, user); !ok {
				return "", errNoSuchUser
			}
		}
		return user, nil