OSError occurs when workers > 1

[mnghiap]

Checklist

• The bug is reproducible against the latest release or master.
• There are no similar issues or pull requests to fix it yet.

Describe the bug

This bug outputs an error similar to #529 but occurs on different parameters of uvicorn. It occurs when I run uvicorn with the following settings:
uvicorn.run("main:app", host="0.0.0.0", port=8000, workers=2, reload=False, loop="asyncio")
I received the following exceptions:

INFO: Uvicorn running on http://0.0.0.0:8000 (Press CTRL+C to quit)
INFO: Started parent process [16448]
INFO: Started server process [17848]
INFO: Waiting for application startup.
INFO: Application startup complete.
INFO: Started server process [8104]
INFO: Waiting for application startup.
INFO: Application startup complete.
Accept failed on a socket
socket: <asyncio.TransportSocket fd=3552, family=AddressFamily.AF_INET, type=SocketKind.SOCK_STREAM, proto=6, laddr=('0.0.0.0', 8000)>
Traceback (most recent call last):
File "C:\Users\phanm\AppData\Local\Programs\Python\Python38\lib\asyncio\proactor_events.py", line 834, in loop
f = self._proactor.accept(sock)
File "C:\Users\phanm\AppData\Local\Programs\Python\Python38\lib\asyncio\windows_events.py", line 549, in accept
self._register_with_iocp(listener)
File "C:\Users\phanm\AppData\Local\Programs\Python\Python38\lib\asyncio\windows_events.py", line 718, in _register_with_iocp
_overlapped.CreateIoCompletionPort(obj.fileno(), self._iocp, 0, 0)
OSError: [WinError 87] The parameter is incorrect

Note that the app is still working completely fine. Setting either workers=1 or reload=True makes the exceptions go away, while the app still works perfectly fine. Setting workers=n to any number n>1 makes the exception appear n-1 times. For example, with uvicorn.run("main:app", host="0.0.0.0", port=8000, workers=4, reload=False, loop="asyncio"):

INFO: Uvicorn running on http://0.0.0.0:8000 (Press CTRL+C to quit)
INFO: Started parent process [14188]
INFO: Started server process [18744]
INFO: Waiting for application startup.
INFO: Application startup complete.
INFO: Started server process [25288]
INFO: Waiting for application startup.
INFO: Application startup complete.
Accept failed on a socket
socket: <asyncio.TransportSocket fd=880, family=AddressFamily.AF_INET, type=SocketKind.SOCK_STREAM, proto=6, laddr=('0.0.0.0', 8000)>
Traceback (most recent call last):
File "C:\Users\phanm\AppData\Local\Programs\Python\Python38\lib\asyncio\proactor_events.py", line 834, in loop
f = self._proactor.accept(sock)
File "C:\Users\phanm\AppData\Local\Programs\Python\Python38\lib\asyncio\windows_events.py", line 549, in accept
self._register_with_iocp(listener)
File "C:\Users\phanm\AppData\Local\Programs\Python\Python38\lib\asyncio\windows_events.py", line 718, in _register_with_iocp
_overlapped.CreateIoCompletionPort(obj.fileno(), self._iocp, 0, 0)
OSError: [WinError 87] The parameter is incorrect
INFO: Started server process [25760]
INFO: Waiting for application startup.
INFO: Application startup complete.
Accept failed on a socket
socket: <asyncio.TransportSocket fd=1772, family=AddressFamily.AF_INET, type=SocketKind.SOCK_STREAM, proto=6, laddr=('0.0.0.0', 8000)>
Traceback (most recent call last):
File "C:\Users\phanm\AppData\Local\Programs\Python\Python38\lib\asyncio\proactor_events.py", line 834, in loop
f = self._proactor.accept(sock)
File "C:\Users\phanm\AppData\Local\Programs\Python\Python38\lib\asyncio\windows_events.py", line 549, in accept
self._register_with_iocp(listener)
File "C:\Users\phanm\AppData\Local\Programs\Python\Python38\lib\asyncio\windows_events.py", line 718, in _register_with_iocp
_overlapped.CreateIoCompletionPort(obj.fileno(), self._iocp, 0, 0)
OSError: [WinError 87] The parameter is incorrect
INFO: Started server process [21280]
INFO: Waiting for application startup.
INFO: Application startup complete.
Accept failed on a socket
socket: <asyncio.TransportSocket fd=3872, family=AddressFamily.AF_INET, type=SocketKind.SOCK_STREAM, proto=6, laddr=('0.0.0.0', 8000)>
Traceback (most recent call last):
File "C:\Users\phanm\AppData\Local\Programs\Python\Python38\lib\asyncio\proactor_events.py", line 834, in loop
f = self._proactor.accept(sock)
File "C:\Users\phanm\AppData\Local\Programs\Python\Python38\lib\asyncio\windows_events.py", line 549, in accept
self._register_with_iocp(listener)
File "C:\Users\phanm\AppData\Local\Programs\Python\Python38\lib\asyncio\windows_events.py", line 718, in _register_with_iocp
_overlapped.CreateIoCompletionPort(obj.fileno(), self._iocp, 0, 0)
OSError: [WinError 87] The parameter is incorrect

Steps to reproduce the bug

No response

Expected behavior

No response

Actual behavior

No response

Debugging material

No response

Environment

• OS: Windows-10-10.0.19042-SP0
• Python: 3.8.10
• uvicorn: 0.16.0
• uvicorn.run("main:app", host="0.0.0.0", port=8000, workers=2, reload=False, loop="asyncio")

Additional context

The exceptions do not occur with the following settings:
uvicorn.run("main:app", host="0.0.0.0", port=8000, workers=2, reload=True, loop="asyncio")
uvicorn.run("main:app", host="0.0.0.0", port=8000, workers=1, reload=False, loop="asyncio")
uvicorn.run("main:app", host="0.0.0.0", port=8000, workers=1, reload=True, loop="asyncio")

[grahamrjuk]

I have this problem as well - reload=True with workers >= 2 removes the error.
reload=False causes the error.

Presumably it works with reload=True since that ignores the workers request and uses only one.

I see a log message:
asyncio proactor_events DEBUG Using proactor: IocpProactor

Windows 10 Pro
Python 3.9.4
Uvicorn 0.16.0

This does not happen with Uvicorn 0.15.0
The equivalent log message is:
asyncio selector_events DEBUG Using selector: SelectSelector

[euri10]

can confirm I boote a win box, the app seems functional though just with this error message
note that running from the CLI you dont have the issue, and it seems tied to we using iocp again
PR welcome if you find the root of it !

[euri10]

maybe the issue is that in the multiprocess case we dont call Server.startup but Server.serve directly, and iirc in startup method we have something to share sockets on windows so maybe we got the flow wrong here ?

[smitchell6879]

This seemed to fix this issue for me Pull Request #1443