Fix Windows server not accepting requests after returning from loop.create_server()

[Dreamsorcerer]

We have a flaky test where, very occasionally on Windows, a request to the server immediately after loop.create_server() gets rejected.

Comparing the code between selector and proactor loops, it appears that the proactor loop would need one more loop iteration than the selector loop, if I'm understanding the code correctly.

The proactor code is nested in a loop registered with call_soon(), so it won't start executing until one loop iteration later than the selector loop does:

cpython/Lib/asyncio/proactor_events.py

Lines 840 to 878 in 8d490b3

	def loop(f=None):
	try:
	if f is not None:
	conn, addr = f.result()
	if self._debug:
	logger.debug("%r got a new connection from %r: %r",
	server, addr, conn)
	protocol = protocol_factory()
	if sslcontext is not None:
	self._make_ssl_transport(
	conn, protocol, sslcontext, server_side=True,
	extra={'peername': addr}, server=server,
	ssl_handshake_timeout=ssl_handshake_timeout,
	ssl_shutdown_timeout=ssl_shutdown_timeout)
	else:
	self._make_socket_transport(
	conn, protocol,
	extra={'peername': addr}, server=server)
	if self.is_closed():
	return
	f = self._proactor.accept(sock)
	except OSError as exc:
	if sock.fileno() != -1:
	self.call_exception_handler({
	'message': 'Accept failed on a socket',
	'exception': exc,
	'socket': trsock.TransportSocket(sock),
	})
	sock.close()
	elif self._debug:
	logger.debug("Accept failed on socket %r",
	sock, exc_info=True)
	except exceptions.CancelledError:
	sock.close()
	else:
	self._accept_futures[sock.fileno()] = f
	f.add_done_callback(loop)
	self.call_soon(loop)

Compared to:

cpython/Lib/asyncio/selector_events.py

Lines 279 to 290 in 8d490b3

	self._check_closed()
	handle = events.Handle(callback, args, self, None)
	key = self._selector.get_map().get(fd)
	if key is None:
	self._selector.register(fd, selectors.EVENT_READ,
	(handle, None))
	else:
	mask, (reader, writer) = key.events, key.data
	self._selector.modify(fd, mask | selectors.EVENT_READ,
	(handle, writer))
	if reader is not None:
	reader.cancel()

i.e. selector.register() is called in loop iteration 0, whereas proactor.accept() is called in loop iteration 1.