Use `PyObject_GetTypeData` without the GIL/attached thread state

Feature or enhancement

Proposal:

In Cython we've historically let people do things like:

cdef class C:
    cdef int non_object_attribute

def f(C c):
    with nogil:
        value = c.non_object_attribute
        ...

i.e. we've allowed them to get direct access into extension types without the GIL (providing they aren't manipulating Python objects).

When using PEP 697 lookup this becomes a little trickier because access to the attribute has to go through PyObject_GetTypeData. Looking at the implementation of PyObject_GetTypeData, it doesn't appear to need the GIL (except in debug mode where it sanity-checks the type).

I think I can work around this by calculating the offset of the struct myself once at startup (using PyObject_GetTypeData) and then doing all the lookups manually via the offset. It also looks like nanobind use a similar trick in places so I wouldn't be alone in this abuse of the system.

However, it'd be nice not to have to do this and if PyObject_GetTypeData could somehow be exempted from the "C API needs the GIL" rule.

I do understand that people would probably prefer not to do this, so I'm happy to look at other options.

Has this already been discussed elsewhere?

I have already discussed this feature proposal on Discourse

Links to previous discussion of this feature:

I asked about this in https://discuss.python.org/t/does-pyobject-gettypedata-need-the-gil/104637/4 - it only generated a small amount of discussion.

Commencez par lire l’implémentation de PyObject_GetTypeData et la discussion Discourse liée concernant la nécessité du GIL. Déterminez si une approche sans GIL prise en charge est possible pour l’accès aux attributs de PEP 697, et documentez clairement le résultat ou une alternative pour les utilisateurs de Cython.