gh-108014: Add Py_IsFinalizing() function

[vstinner]

• Issue: _Py_IsFinalizing has been removed, meaning C/C++ threads cannot avoid termination during shutdown #108014

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📚 Documentation preview 📚: https://cpython-previews--108032.org.readthedocs.build/

[vstinner]

@ericsnowcurrently: Oh, that's a "feature" PR, so I will need a formal approval to merge it :-)

[vstinner]

I prepared python/pythoncapi-compat#66 to add the function to the pythoncapi-compat project.

[ericsnowcurrently]

Is this referring to the runtime (AKA main interpreter) or to the current interpreter? I'd expect it to be the current interpreter.

Would it make sense to explicitly pass an interpreter and use NULL to mean the current one (or main)?

[vstinner]

I copied the documentation of the sys.is_finalizing() function. If you want a different feature (pass an interpreter), I would prefer to have a different function.

It's set by Py_Finalize() which finalizes the main interpreter: other sub-interpreters should be deleted before explicitly, no?

[vstinner]

I updated the doc to specify that we are talking about the main interpreter.

[serhiy-storchaka]

LGTM.

[vstinner]

Thanks for the review @ericsnowcurrently and @serhiy-storchaka!

[vstinner]

Follow-up: @wjakob asks to add this new function to the limited C API version 3.13.