Use ImagingCore.ptr instead of ImagingCore.id

[homm]

Partially suppress #8340

The non-primary extensions in Pillow don't have direct access to Python's _imaging types, such as Imaging_Type and ImagingObject. However, they often still need to interact with objects passed from Python code. Currently, the raw memory pointer to an Imaging object (ImageCore.id) is passed as an integer and then cast to a memory pointer:

static PyObject *
cms_transform_apply(CmsTransformObject *self, PyObject *args) {
    Py_ssize_t idIn, idOut;
    Imaging im, imOut;

    if (!PyArg_ParseTuple(args, "nn:apply", &idIn, &idOut)) {
        return NULL;
    }

    im = (Imaging)idIn;
    imOut = (Imaging)idOut;

    return Py_BuildValue("i", pyCMSdoTransform(im, imOut, self->transform));
}

It's not safe and can lead to segmentation faults. Python has Capsule objects, which provide a safer way to pass pointers to C code. These are exposed through ImageCore.ptr.

This PR deprecates the raw pointer properties ImageCore.id and ImageCore.unsafe_ptrs, and migrates non-primary extensions to interact with Capsule objects.

[homm]

@python-pillow/pillow-team I need help here)

Could anyone debug pypy3.10 on windows? I can't get what's wrong, since as I can see, the capsule object is correctly restored from the pointer and the capsule object itself is alive.

https://github.com/python-pillow/Pillow/actions/runs/10664609198/job/29556218383?pr=8341#step:27:3342

[homm]

Get it! In pypy repr(capsule) holds the capsuled pointer itself, not the pointer to the PyObject. And even hex(id(capsule)) is not the pointer to PyObject. Lucky, we can reliably distinguish pypy capsule from cpython capsule by absence of trailing "<>".

https://github.com/pypy/pypy/blob/branches/py3.10/pypy/objspace/std/capsuleobject.py#L39

[radarhere]

So is the motivation here just to simplify the code?

Also, we don't necessarily need the deprecations - #4532 (comment)

Pillow makes a commitment to stable public interfaces, which are defined at the Python layer. The C interfaces are explicitly internal, and no effort is made to keep them stable even between minor releases, and no support or warning is given when they change. (IIRC, there are/were people using the internal interfaces, but they've always been explicitly on their own with that)

[homm]

So is the motivation here just to simplify the code?

Added description to PR.

Also, we don't necessarily need the deprecations

While ImageCore is internal Pillow object, it's still Python API. I believe ImageCore.id could be used by third-party extensions (Github search) and can't be removed in a one day.

[radarhere]

For the record, the last mention of this patch was removed in Pillow 4.1.0 by #2360

[radarhere]

In pypy repr(capsule) holds the capsuled pointer itself, not the pointer to the PyObject. And even hex(id(capsule)) is not the pointer to PyObject. Lucky, we can reliably distinguish pypy capsule from cpython capsule by absence of trailing "<>".

What's the advantage of using repr(capsule) in the first place? Why not just pass the capsule itself all the time?

[radarhere]

I imagine this change was just because you didn't like image.new_block(self.__mode, im.size)?

Rather than moving new_block in C, wouldn't it have been better for the C method to access the size on the image object instead of being passed it? Then this call could become image.new_block(self.__mode).

[homm]

I’d like to clarify that the reason for this change is not a matter of personal preference. The new_block method doesn't pertain to an existing image but instead creates a new block based on the passed parameters. As such, it makes more sense for it to reside in the Image.core module rather than being a method of the image object itself.

There is no intent to modify or enhance the behavior; I was simply correcting the method's improper placement.

[homm]

What's the advantage of using repr(capsule) in the first place? Why not just pass the capsule itself all the time?

I'm not aware of TK API, but it seems it converts any arguments to strings, at least PyImagingPhotoPut receives arguments as const char **argv. So looks like there is no way to pass the capsule object itself, instead repr(capsule) will be passed any way. I just made this conversion more explicit.