docs: def stopServing(self, exception=None):
          """
          Returns a deferred that will fire immediately if there are
          no pending requests, otherwise when the last request is removed
          from self.pending.
          """
          if exception is None:
              exception = ServiceUnavailableError
          self.serve_exception = exception
          if self.pending:
              d = self.out_of_service_deferred = defer.Deferred()
              return d
          return defer.succeed(None)