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loop.sock_recv failure because of delayed callback handling #78976
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kyuupichan commented
on Sep 24, 2018 kyuupichanmannequinMannequinAuthorIn certain circumstances the coroutine loop.sock_recv() registers a callback internally, which is called on e.g. task cancellation. The callback assumes a file descriptor that was open and valid at the time the callback was registered is still open and valid when the callback is called, and this need not be the case. For example, when client code handling exceptions (in particular, cancellation) closes the socket itself.
I have reports of this error for Python 3.6 and 3.7 by a user of my library which uses asyncio, and that bad things happen on both Windows (the event loop dies) and Linux (a traceback happens but no loop death).
It likely happens in earlier versions of Python too but my library requires >= Python 3.6I hope the above description in addition to pointing out the problematic lines in asyncio make the bug clear. I tried to produce a simplified testcase but it doesn't trigger the issue on Linux. I am told it does trigger it on Windows (I don't have Windows to test on).
Here are the problematic lines in selector_events.py where the FD is registered; the callback above them assumes the DF remains valid (but the socket might have been closed in the meantime):
https://github.com/python/cpython/blob/master/Lib/asyncio/selector_events.py#L378-L380
The same problem is evident in sock_recv_into, and perhaps other calls.
Here is code I am told triggers the problem on Windows; it seems to require something a little more complex to trigger reliably on Unix platforms:
import asyncio import socket class MyProtocol(asyncio.Protocol): def connection_made(self, transport): transport.write(b'123') # just in case a write is needed port = 6666 async def connect_and_recv(loop, sock): try: await loop.sock_connect(sock, ('127.0.0.1', port)) while True: await loop.sock_recv(sock, 20) except asyncio.CancelledError: print("Cancelled") sock.close() async def main(loop): server = await loop.create_server(MyProtocol, '127.0.0.1', port) sock = socket.socket() sock.setblocking(False) task = loop.create_task(connect_and_recv(loop, sock)) await asyncio.sleep(0.1) task.cancel() await asyncio.sleep(0.1) loop = asyncio.get_event_loop() loop.run_until_complete(main(loop))
Here are backtraces from a client application doing things more complex but similar in spirit to the snippet above:
File "C:\Users\User\AppData\Local\Programs\Python\Python36-32\lib\asyncio\base_events.py", line 455, in run_until_complete
self.run_forever()
File "C:\Users\User\AppData\Local\Programs\Python\Python36-32\lib\asyncio\base_events.py", line 422, in run_forever
self._run_once()
File "C:\Users\User\AppData\Local\Programs\Python\Python36-32\lib\asyncio\base_events.py", line 1398, in _run_once
event_list = self._selector.select(timeout)
File "C:\Users\User\AppData\Local\Programs\Python\Python36-32\lib\selectors.py", line 323, in select
r, w, _ = self._select(self._readers, self._writers, [], timeout)
File "C:\Users\User\AppData\Local\Programs\Python\Python36-32\lib\selectors.py", line 314, in _select
r, w, x = select.select(r, w, w, timeout)
OSError: [WinError 10038] An operation was attempted on something that is not a socketand one for Linux:
Exception in callback BaseSelectorEventLoop._sock_recv(<Future cancelled>, True, <socket.socke...2049, proto=0>, 5) handle: <Handle BaseSelectorEventLoop._sock_recv(<Future cancelled>, True, <socket.socke...2049, proto=0>, 5)> Traceback (most recent call last): File "/usr/lib/python3.6/asyncio/events.py", line 127, in _run self._callback(*self._args) File "/usr/lib/python3.6/asyncio/selector_events.py", line 378, in _sock_recv self.remove_reader(fd) File "/usr/lib/python3.6/asyncio/selector_events.py", line 342, in remove_reader return self._remove_reader(fd) File "/usr/lib/python3.6/asyncio/selector_events.py", line 279, in _remove_reader key = self._selector.get_key(fd) File "/usr/lib/python3.6/selectors.py", line 189, in get_key return mapping[fileobj] File "/usr/lib/python3.6/selectors.py", line 70, in __getitem__ fd = self._selector._fileobj_lookup(fileobj) File "/usr/lib/python3.6/selectors.py", line 224, in _fileobj_lookup return _fileobj_to_fd(fileobj) File "/usr/lib/python3.6/selectors.py", line 41, in _fileobj_to_fd raise ValueError("Invalid file descriptor: {}".format(fd)) ValueError: Invalid file descriptor: -1
Can you reproduce this with uvloop?
Andrew: I think we need to "lock" sockets from closing in sock_recv and friends the same way we do it in uvloop.
kyuupichan commented
on Sep 24, 2018 kyuupichanmannequinMannequinAuthorThis seems related: https://bugs.python.org/issue30064
kyuupichan commented
on Sep 24, 2018 kyuupichanmannequinMannequinAuthorMy library user reports:
I can't reproduce the issue with uvloop on linux. (looks like uvloop does not work on windows atm)
Yuri, do you mean
socket._io_refsmanipulations?yes
Looks like a hack but we have no choice
Unable to reproduce on main or 3.10. I assume this was already fixed in #74250 so closing.
❯ ./python.bat main.py Running Debug|x64 interpreter... D:\python\main\main.py:34: DeprecationWarning: There is no current event loop loop = asyncio.get_event_loop() Cancelled
Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.
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