The _xxsubinterpreters module was meant to only use public API. Some internal C-API usage snuck in over the last few years (e.g. gh-28969). This fixes that.
(cherry picked from commit e6373c0d8b)
Co-authored-by: Eric Snow <ericsnowcurrently@gmail.com>
Here we are doing no more than adding the value for Py_mod_multiple_interpreters and using it for stdlib modules. We will start checking for it in gh-104206 (once PyInterpreterState.ceval.own_gil is added in gh-104204).
The function is like Py_AtExit() but for a single interpreter. This is a companion to the atexit module's register() function, taking a C callback instead of a Python one.
We also update the _xxinterpchannels module to use _Py_AtExit(), which is the motivating case. (This is inspired by pain points felt while working on gh-101660.)
