bpo-38076: Fix crash on shutdown caused by PEP-384ification of _struct

[DinoV]

I think this is a partial fix for _struct crashing on shutdown after the PEP-384ification of it when using the multi-processing class with a finalizer:

from multiprocessing.pool import Pool
class A(object):
def init(self):
self.pool = Pool()
def del(self):
self.pool.close()
self.pool.join()
a = A()

Although I still see this hanging on Linux instead of cleaning shutting down.

There are still some other issues related to shutdown that this doesn't yet fix. Fixing other cases will likely require storing the module object off of the struct type object so that it can be retrieved later.

Feedback welcome, it might be better to fix this in a global manner by changing the finalization code to not clear the module list so early, but that seems like a much more significant change.

https://bugs.python.org/issue38076

[iritkatriel]

https://bugs.python.org/issue38076 is closed. What is the status of this PR?

[kumaraditya303]

There are merge conflicts, please create an issue to discuss the change first.