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multiprocessing.Event.wait hangs when interrupted by signal that sets the event #85772
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Description
| BPO | 41606 |
|---|---|
| Files |
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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GitHub fields:
assignee = None
closed_at = None
created_at = <Date 2020-08-21.08:01:43.342>
labels = ['type-bug', '3.8']
title = 'multiprocessing.Event.wait hangs when interrupted by signal that sets the event'
updated_at = <Date 2020-08-21.08:01:43.342>
user = 'https://bugs.python.org/salvatoreingala'bugs.python.org fields:
activity = <Date 2020-08-21.08:01:43.342>
actor = 'salvatore.ingala'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2020-08-21.08:01:43.342>
creator = 'salvatore.ingala'
dependencies = []
files = ['49419']
hgrepos = []
issue_num = 41606
keywords = []
message_count = 1.0
messages = ['375745']
nosy_count = 1.0
nosy_names = ['salvatore.ingala']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue41606'
versions = ['Python 3.8']
Activity
salvatoreingala commentedon Aug 21, 2020
According to the docs, multiprocessing.Event is a clone of threading.Event. Yet, there is a strange behavior that is observed only on multiprocessing.Event. If an event.wait() is interrupted by a SIGINT and the signal handler sets the event, then the call to event.set() hangs if event is an instance of multiprocessing.Event, in what looks like a deadlock; instead, it works as expected if it's an instance of threading.Event.
See the file attached for a reproduction.
It seems to have been so for a long time, see: https://stackoverflow.com/questions/24422154/multiprocessing-event-wait-hangs-when-interrupted-by-a-signal/30831867
marcos4758 commentedon Aug 13, 2022
I just spent a lot of time with this problem. I have a multiprocessing.Event that sleeps the MainThread of a child process and an interrupt handler that sets that event. As described here, when the interrupt triggers the event, the process hangs. Doing some research on the web, including on the cited stackoverflow topic, I got the following solution:
Solutions for Linux with fork start method:
threading.Thread(target=exit_event.set).start()Solution for Linux with spawn start method
threading.Thread(target=exit_event.set).start()Windows:
Reference: https://docs.microsoft.com/en-us/windows/console/setconsolectrlhandler
This works like a charm. The key difference is that it runs the interrupt funcition on a thread (named Dummy-1).
threading.Thread(target=exit_event.set).start()gpshead commentedon Nov 14, 2024
Duplicate of #126434, lets deal with that there. This issue has great context though. (see also #95826).