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Using a lock after fork() is unsafe and can crash.
Example of a crash in logging after a fork on AIX:
https://bugs.python.org/issue40068#msg365028
This problem is explained in length in bpo-6721: "Locks in the standard library should be sanitized on fork".
The threading module registers an "at fork" callback: Thread._reset_internal_locks() is called to reset self._started (threading.Event) and self._tstate_lock. The current implementation creates new Python lock objects and forgets about the old ones.
I propose to add a new _at_fork_reinit() method to Python lock objects which reinitializes the native lock internally without having to create a new Python object.
Currently, my implementation basically creates a new native lock object and forgets about the old new (don't call PyThread_free_lock()).
Tomorrow, we can imagine a more efficient impementation using platform specific function to handle this case without having to forget about the old lock.