async def wait_closed(self):
        """Coroutine to wait until all existing request handlers will exit
        properly.
        """
        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)
Coroutine to wait until all existing request handlers will exit
        properly.