Issue 34680: [doc] asyncio event_loop fails when accessed from multiple threads (original) (raw)
Messages (8)
Author: Nic Watson (jnwatson)
Date: 2018-09-14 15:33
If a signal handler callback is registered on an event loop, and the event loop has close() called on it, the close will fail.
Exception: Exception in thread Thread-1: Traceback (most recent call last): File "/home/nic/.pyenv/versions/3.7.0/lib/python3.7/threading.py", line 917, in _bootstrap_inner self.run() File "/home/nic/.pyenv/versions/3.7.0/lib/python3.7/threading.py", line 865, in run self._target(*self._args, **self._kwargs) File "/home/nic/tmp/signal_event_loop_bug.py", line 9, in do_loop loop.close() File "/home/nic/.pyenv/versions/3.7.0/lib/python3.7/asyncio/unix_events.py", line 58, in close self.remove_signal_handler(sig) File "/home/nic/.pyenv/versions/3.7.0/lib/python3.7/asyncio/unix_events.py", line 147, in remove_signal_handler signal.signal(sig, handler) File "/home/nic/.pyenv/versions/3.7.0/lib/python3.7/signal.py", line 47, in signal handler = _signal.signal(_enum_to_int(signalnum), _enum_to_int(handler)) ValueError: signal only works in main thread
Code: import asyncio import signal import threading
def mysighandler(): pass
def do_loop(loop): loop.close()
loop = asyncio.new_event_loop() loop.add_signal_handler(signal.SIGINT, mysighandler) t = threading.Thread(target=do_loop, args=(loop,)) t.start() t.join()
Author: Irit Katriel (iritkatriel) * 
Date: 2022-01-24 18:31
The documentation should explicitly mention that an event loop should be accessed from only one thread (it hints at this by referring to "the loop of the current thread" but I don't see this stated clearly).
In the code, it would be good if multi-threaded access failed with a clearer error.
Author: Andrew Svetlov (asvetlov) *
Date: 2022-01-24 18:58
All asyncio loop methods except loop.call_soon_threadsafe() should be done from the same thread as been used for the loop creation.
Did you violate this rule?
Author: Irit Katriel (iritkatriel) *
Date: 2022-01-24 19:07
I think the script does violate the rule:
t = threading.Thread(target=do_loop, args=(loop,))
and then do_loop closes the loop that was created in the main thread.
Author: Andrew Svetlov (asvetlov) *
Date: 2022-01-26 10:41
BaseEventLoop has _check_loop() method that is closed in debug mode only. UnixEventLoop doesn't call this method for unix-specific API.
Adding the check to add_signal_handler()/remove_signal_handler() doesn't hurt, sure. But it doesn't help as the script is executed in non-debug mode.
Performing a check on every call_soon() call kills the performance, that's why debug mode is required.
Author: Irit Katriel (iritkatriel) *
Date: 2022-01-26 12:38
Perhaps this is just a doc issue - state explicitly that a loop should be used only in one thread, and mention that this is checked in debug mode.
Author: epiphyte (epiphyte)
Date: 2022-01-26 17:06
For additional context, this was encountered when porting an application from threading to asyncio, and at the time we were still running the ioloop in its own thread as opposed to the mainthread. We no longer do that :D
Updating the documentation to clarify that the loop won't work across threads, like what we ran into at the time, would resolve this.
Author: Andrew Svetlov (asvetlov) *
Date: 2022-01-26 17:31
Please feel free to propose pull request for documentation tuning.
History
Date
User
Action
Args
2022-04-11 14:59:05
admin
set
github: 78861
2022-03-21 11:39:31
iritkatriel
set
keywords: + easy
title: asyncio event_loop fails when accessed from multiple threads -> [doc] asyncio event_loop fails when accessed from multiple threads
2022-01-26 17:31:10
asvetlov
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messages: +
2022-01-26 17:06:14
epiphyte
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messages: +
2022-01-26 12:38:33
iritkatriel
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messages: +
2022-01-26 10:41:20
asvetlov
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messages: +
2022-01-24 19:07:46
iritkatriel
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messages: +
2022-01-24 18:58:50
asvetlov
set
messages: +
2022-01-24 18:32:49
iritkatriel
set
title: asyncio event_loop close fails off main thread if signal handler registered -> asyncio event_loop fails when accessed from multiple threads
2022-01-24 18:31:53
iritkatriel
set
nosy: + iritkatriel
messages: +
versions: + Python 3.9, Python 3.10, Python 3.11, - Python 3.6, Python 3.7
2018-09-14 15:37:34
epiphyte
set
nosy: + epiphyte
2018-09-14 15:33:28
jnwatson
create