code: async def wait_closed(self):
          
          if self._server is None:
              raise RuntimeError('Server is not started')
          await self._server.wait_closed()
          if self._handlers:
              await asyncio.wait({h.wait_closed() for h in self._handlers},
                                 loop=self._loop)
docs: Coroutine to wait until all existing request handlers will exit
          properly.