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

[cagney]

cagney <andrew.cagney at gmail.com> added the comment:

> acquiring locks before fork in the thread doing the forking and releasing them afterwards is always the safe thing to do.

It's also an easy way to cause a deadlock:

- register_at_fork() et.al. will cause per-logger locks to be acquired before the global lock (this isn't immediately obvious from the code)

If a thread were to grab its logging lock before the global lock then it would deadlock.  I'm guessing this isn't allowed - however I didn't see any comments to this effect?

Can I suggest documenting this, and also merging the two callbacks so that the ordering of these two acquires is made explicit.

- the per-logger locks are acquired in a random order

If a thread were to acquire two per-logger locks in a different order then things would deadlock.

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<https://bugs.python.org/issue6721>
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