gh-98410: move getbufferproc and releasebufferproc to buffer.h

[davidhewitt]

After #29991 it is possible to set Py_bf_getbuffer and Py_bf_releasebuffer slots from the stable API.

This PR moves the corresponding typedefs for the function pointer types corresponding to these slots from Include/cpython/object.h to be alongside other typedefs in Include/pybuffer.h.

This may be useful for users who wish to cast to these function pointer types.

• Issue: Please add getbufferproc and releasebufferproc to the limited API headers #98410

[kumaraditya303]

Closing as the buffer protocol is available in 3.11 in Limited C API.

[davidhewitt]

@kumaraditya303 can I please push for you to consider reopening and reviewing this patch? The point that the buffer protocol is available in 3.11 is exactly why I think this should be merged.

As of current 3.11 release on the limited API I can call PyType_GetSlot(Py_TYPE(obj), Py_bf_getbuffer) to get a void * for the getbuffer slot but I can't cast this to a getbufferproc because the definition exists only on the unlimited API. This patch makes getbufferproc available on the limited API.

(And equivalent for releasebufferproc)

[kumaraditya303]

@davidhewitt Sorry for closing your PR. I reopened it and you are correct that the function typedefs are not exposed. I would suggest you to create issue for this issue to discuss this first, nobody looked at this because no issue was mentioned and there are conflicts and and this can't be fixed in 3.11 as the rc is out and final is about to be released.

There are conflicts in this PR, I would suggest you create a new PR with fixed conflicts but it's your choice if you want to continue with this one.

[davidhewitt]

@kumaraditya303 thanks very much - I have opened #98410 and I'm happy to spend the time to rebase this PR once someone has suggested that a change of this nature would potentially be suitable to merge.

[encukou]

Thanks for the catch!
Definitely suitable for merge. It won't make it to Python 3.11.0 (sorry!), but it can go in 3.12. (And possibly in 3.11.1, since it's only affecting API rather than ABI.)
Please move the definitions to Include/buffer.h, together with the other buffer API. That way you shouldn't need the forward definition.
Please add these to Misc/stable_abi.toml, as typedef entries added in 3.12, and re-run make regen-limited-abi.

Let me know if you want more info, or if you'd prefer to leave this to me.

[davidhewitt]

Thanks - I'll do my best to find a moment to tidy this up tonight!

[davidhewitt]

@encukou how does that look?

[davidhewitt]

rebased

[encukou]

Looks good, thank you!
The rules say these should be available for 3.12+ only, i.e. guarded with:

#if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 >= 0x030c0000 

but with typedefs, we can relax this rule a little: they'll be available to people building for 3.11 using 3.12.