docs: def single_user_mode(self, command=None, options=None):
          """run a given command in a single-user mode. If the command is empty - then just start and stop"""
          cmd = [self._pgcommand('postgres'), '--single', '-D', self._data_dir]
          for opt, val in sorted((options or {}).items()):
              cmd.extend(['-c', '{0}={1}'.format(opt, val)])
          # need a database name to connect
          cmd.append(self._database)
          return self.cancellable_subprocess_call(cmd, communicate_input=command)