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Fix Windows server not accepting requests after returning from loop.create_server()#134259

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Fix Windows server not accepting requests after returning from loop.create_server()#134259

Dreamsorcerer wants to merge2 commits intopython:mainfromDreamsorcerer:patch-8

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Dreamsorcerer commentedMay 19, 2025•
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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 in8d490b3

	defloop(f=None):
	try:
	iffisnotNone:
	conn,addr=f.result()
	ifself._debug:
	logger.debug("%r got a new connection from %r: %r",
	server,addr,conn)
	protocol=protocol_factory()
	ifsslcontextisnotNone:
	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)
	ifself.is_closed():
	return
	f=self._proactor.accept(sock)
	exceptOSErrorasexc:
	ifsock.fileno()!=-1:
	self.call_exception_handler({
	'message':'Accept failed on a socket',
	'exception':exc,
	'socket':trsock.TransportSocket(sock),
	})
	sock.close()
	elifself._debug:
	logger.debug("Accept failed on socket %r",
	sock,exc_info=True)
	exceptexceptions.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 in8d490b3

	self._check_closed()
	handle=events.Handle(callback,args,self,None)
	key=self._selector.get_map().get(fd)
	ifkeyisNone:
	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))
	ifreaderisnotNone:
	reader.cancel()

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

Fix Windows server not accepting requests when returning from loop.cr…

97afd26

…eate_server()

Dreamsorcerer requested review from1st1,asvetlov,kumaraditya303 andwillingc ascode owners

May 19, 2025 17:55

bedevere-appbot added the awaiting review label

May 19, 2025

Dreamsorcerer changed the title~~Fix Windows server not accepting requests when returning from loop.create_server()~~Fix Windows server not accepting requests after returning from loop.create_server()

May 19, 2025

Update base_events.py

d21dc15