Please add getbufferproc and releasebufferproc to the limited API headers

[davidhewitt]

Feature or enhancement

Starting with Python 3.11 the Py_Buffer API has been made available in the limited API, including the bf_getbuffer and bf_releasebuffer type slots.

However the definitions for the function pointer types stored in those slots, getbufferproc and releasebufferproc are still only defined when including "cpython" headers.

This means on the limited API starting with Python 3.11 it is now possible to do PyType_GetSlot(Py_TYPE(obj), Py_bf_getbuffer), however the void * returned does not have a function pointer type available which it can be cast to (unless the user adds this typedef themselves in their own code).

Pitch

This is a relatively straightforward change to make use of these type slots more convenient on the limited API.

I wrote an initial implementation at #31158. A couple of tiny tweaks beyond moving the typedefs was necessary so that forward definitions continued to function. Please see that PR for the detail.

[encukou]

Thank you!

[vstinner]

I dislike the fact that most callback types used by PyTypeObject are not prefixed by Py_ prefix, nor use the same coding style (casing) than other names used by the Python API :-(

binaryfunc, ternaryfunc, ssizeobjargproc, etc.