Fix pushes_fd method signature

[hoodmane]

Python getters are supposed to take a void* closure argument.

I am coming from Pyodide. Web assembly has no native support for function pointer casting -- a function declared with a certain signature must be called with the same signature, or it will fail with RuntimeError: function signature mismatch. Emscripten has support for emulating function pointer casts which we are currently using, but it imposes a significant performance and stack space cost and we are trying to remove the pointer cast emulation pyodide/pyodide#1677. To do this, we need to patch Python packages so that they have function signatures that match the calling conventions they declare.

[hoodmane]

I also removed all of the (getter) casts, but I'm not sure if that will work correctly since strictly speaking most of the function pointers are defined with type e.g.,

PyObject *
get_some_attr(SpecificTypeOfObject *decoder, void *closure)

whereas the first argument is expected to be a PyObject *. So depending on what error / warning flags are set, this may generate a bunch of warnings or a compile error.

Probably it's better to put the casts back, but it's unfortunate that we can't get better type safety here -- it's so easy to mess up these signatures.

[radarhere]

It is indeed generating a bunch of warnings - https://github.com/python-pillow/Pillow/runs/4202395176#step:9:26

src/_imaging.c:3571:14: warning: initialization of ‘PyObject * (*)(PyObject *, void )’ {aka ‘struct _object * ()(struct _object *, void )’} from incompatible pointer type ‘PyObject * ()(ImagingObject *, void )’ {aka ‘struct _object * ()(struct *, void *)’} [-Wincompatible-pointer-types]
3571 | {"mode", _getattr_mode},
| ^~~~~~~~~~~~~
src/_imaging.c:3571:14: note: (near initialization for ‘getsetters[0].get’)

[hoodmane]

Okay I put the casts back.

[hugovk]

Please could you rebase or redo those two commits, to clean the history up a little bit?

One way:

git reset --soft HEAD~2
git push --force

[hugovk]

Thanks!