Issue 40294: Use-after-free crash if multiple interpreters import asyncio module

Starting with Python 3.8 (GH-16598), the `_asyncio` module's C initialization is guarded behind a static variable. If the module is initialized a second time and this variable is set, the resources from the first initialization are used. However, when the module is freed and the corresponding resources released, the static variable is not cleared. If the module is subsequently initialized again, it will incorrectly believe it has already been initialized and use the previously freed resources, resulting in a crash.

This scenario is actually fairly easy to encounter in the presence of multiple interpreters whose lifetime is shorter than that of the whole program. Essentially, if any interpreter loads `asyncio` and then is freed with `Py_EndInterpreter`, any new interpreter that loads `asyncio` will crash. Since `asyncio` is a built-in module, it is loaded as a consequence of a wide variety of libraries.

I ran into this in my project because I use multiple interpreters to isolate user scripts, and I started to encounter crashes when switching to Python 3.8.

I've attached a simple reproduction program. I've personally tested that this runs without crashing in 3.6 and 3.7 (but I suspect it works down to 3.4 when `asyncio` was introduced).

_asyncio should be ported to multiphase initialization (PEP 489) and its types converted to PyType_FromSpec() rather than using statically allocated types. See bpo-1635741.

Would the simple fix (clearing the flag in `module_free`) be a candidate for a backport to 3.8? This seems to be a regression from the previous stable version that also limits the usability of subinterpreters -- `asyncio` is loaded by a wide variety of libraries and so in general it's not easy to know that a particular subinterpreter hasn't loaded `asyncio`.   However, I concede that subinterpreters with variable lifetimes isn't a common use case.

New changeset a75e730075cd25be1143e6183006f3b1d61bb80f by Jeffrey Quesnelle in branch 'master':
bpo-40294: Fix _asyncio when module is loaded/unloaded multiple times (GH-19542)
https://github.com/python/cpython/commit/a75e730075cd25be1143e6183006f3b1d61bb80f

> Would the simple fix (clearing the flag in `module_free`) be a candidate for a backport to 3.8?

Sure.

New changeset 6b0ca0aeab04d7b7b54086248ca9d5e70f770f2f by Miss Islington (bot) in branch '3.8':
bpo-40294: Fix _asyncio when module is loaded/unloaded multiple times (GH-19542)
https://github.com/python/cpython/commit/6b0ca0aeab04d7b7b54086248ca9d5e70f770f2f

Is Python 3.7 affected as well?

> Is Python 3.7 affected as well?

Nope, this was introduced in 3.8

Ok, the issue is now fixed: thanks Jeffrey Quesnelle!

> Nope, this was introduced in 3.8

I tested: attached main.c doesn't crash in 3.7. I guess that _asyncio leaks a few references, but it's not a big deal.

stage: patch review -> resolved