[issue6721] Locks in the standard library should be sanitized on fork

[Connor Wolf]

Connor Wolf added the comment:

> IMNSHO, working on "fixes" for this issue while ignoring the larger application design flaw elephant in the room doesn't make a lot of sense.

I understand the desire for a canonically "correct" fix, but it seems the issue with fixing it "correctly" has lead to the /actual/ implementation being broken for at least 6 years now.

As it is, my options are:
A. Rewrite the many, many libraries I use that internally spawn threads.
B. Not use multiprocessing.

(A) is prohibitive from a time perspective (I don't even know how many libraries I'd have to rewrite!), and (B) means I'd get 1/24-th of my VMs performance, so it's somewhat prohibitive.

At the moment, I've thrown together a horrible, horrible fix where I reach into the logging library (which is where I'm seeing deadlocks), and manually iterate over all attached log managers, resetting the locks in each immediately when each process spawns. 
This is, I think it can be agreed, a horrible, horrible hack, but in my particular case it works (the worst case result is garbled console output for a line or two). 

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If a canonical fix is not possible, at least add a facility to the threading fork() call that lets the user decide what to do. In my case, my program is wedging in the logging system, and I am entirely OK with having transiently garbled logs, if it means I don't wind up deadlocking and having to force kill the interpreter (which is, I think, far /more/ destructive an action).

If I could basically do `multiprocessing.Process(*args, *kwargs, _clear_locks=True)`, that would be entirely sufficient, and not change existing behaviour at all.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue6721>
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