multiprocessing should log a warning when forking multithreaded process

[trcarden]

BPO	24512
Nosy	@applio

^{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 2015-06-25.18:19:54.174>
labels = ['type-feature', 'library']
title = 'multiprocessing should log a warning when forking multithreaded process'
updated_at = <Date 2015-06-28.01:08:09.370>
user = 'https://bugs.python.org/trcarden'

bugs.python.org fields:

activity = <Date 2015-06-28.01:08:09.370>
actor = 'ned.deily'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2015-06-25.18:19:54.174>
creator = 'trcarden'
dependencies = []
files = []
hgrepos = []
issue_num = 24512
keywords = []
message_count = 1.0
messages = ['245825']
nosy_count = 3.0
nosy_names = ['sbt', 'davin', 'trcarden']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue24512'
versions = ['Python 2.7', 'Python 3.4', 'Python 3.5', 'Python 3.6']

[trcarden]

We were tracking down a bug the other day that was rather hard to find involving a forking a process that had both a primary thread and a logging thread. The docs clearly state that forking a multithreaded process is "problematic" https://docs.python.org/3.4/library/multiprocessing.html#contexts-and-start-methods

However given that it is very simple to check if the process currently has multiple threads at the time of the forking operation and that its almost never safe to do so with multiple running threads I was wondering if it made sense to add a message that warned that a unsafe operation (forking a multithreaded process without the forkserver option) was occurring.