multiprocessing.Event.wait hangs when interrupted by signal that sets the event

[salvatoreingala]

BPO	41606
Files	event-test.py: Test code for multiprocessing.Event

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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']

[salvatoreingala]

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]

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:

1. In the interrupt handler, call threading.Thread(target=exit_event.set).start()
2. Change multiprocessing.Event to threading.Event

Solution for Linux with spawn start method

1. In the interrupt handler, call threading.Thread(target=exit_event.set).start()

Windows:

1. Instead of using signal.signal() to change the interrupt handler, the solution was to use:

import win32api
win32api.SetConsoleCtrlHandler(handler, add)

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).

2. (Not a great solution) In the interrupt handler, call threading.Thread(target=exit_event.set).start()
   • This solves the hang issue, but raises an InterruptedError exception

[gpshead]

Duplicate of #126434, lets deal with that there. This issue has great context though. (see also #95826).