Allow passing capsules for draw callbacks

[khaledhosny]

This allow defining the callbacks in C modules and passing the pointers as Python objects.

[behdad]

This is neat.

[justvanrossum]

Should user_data also be allowed to be a capsule?

[khaledhosny]

Should user_data also be allowed to be a capsule?

I wondered about that too, but it seems to work without a change. I guess because user_data is accessed from the callback functions, so that is the part the needs to deal with whether it is a capsule or a another Python object.

[justvanrossum]

I'm just worried that the callback code will have to deal with unpacking the capsule. If it requires a Python C API call that would be unfortunate.

[khaledhosny]

Good point.

[justvanrossum]

Can capsules interoperate with ctypes pointers? It's a bit unclear to me how compatible the two are.

[khaledhosny]

Can capsules interoperate with ctypes pointers? It's a bit unclear to me how compatible the two are.

I think they should work, PyCapsule is essentially wrapping an integer in a Python object. The problem, I think, will be that there is no Python API for dealing with PyCapsule’s, only a C-API.

[anthrotype]

I think ctypes.pythonapi exposes the Python API, including PyCapsule, not sure if that's what you're after.
I do think cython is neater than ctypes in general.

[anthrotype]

found this on https://stackoverflow.com/a/65057267

[justvanrossum]

LGTM, and works well in FontGoggles. Some tests would be nice, but we can do that later.

[khaledhosny]

Testing would be good indeed, but I was hesitant to look into it because it will need some compiled native code and I have no idea how to pull that for tsets.

[justvanrossum]

Could it be faked done with some cdefs in the main .pyx source?

[khaledhosny]

I don’t know, I’m apparently not a Cython expert. @anthrotype probably has some ideas.

[anthrotype]

maybe you could define some dummy move_to, line_to etc. methods inside the main .pyx module (or a new extension module that's only for testing but that complicates the setup), then from inside the pure-python test module you use ctypes to ctypes.cdll.LoadLibrary the uharfbuzz extension module itself in the same way FontGoggles loads its own objc dylib (an extension module is like a shared library, I think) and then use ctypes to make the capsules and pass them on to uharfbuzz DrawFuncs..

[anthrotype]

I tried just for fun to load the uharfbuzz extension via ctypes, looks like it works

In [1]: import ctypes
In [2]: import uharfbuzz._harfbuzz
In [3]: uharfbuzz._harfbuzz.__file__
Out[3]: '/Users/clupo/Github/nanoemoji/.venv/lib/python3.10/site-packages/uharfbuzz/_harfbuzz.cpython-310-darwin.so'
In [4]: lib = ctypes.cdll.LoadLibrary(uharfbuzz._harfbuzz.__file__)
In [5]: hb_version_string = lib.hb_version_string
In [6]: hb_version_string.restype = ctypes.c_char_p
In [7]: hb_version_string.argtypes = ()
In [8]: hb_version_string()
Out[8]: b'5.0.0'

I think the dummy move_to, line_to, etc. for testing should be defined as cdef methods (so they are only visible from C)

[khaledhosny]

Thanks, I’ll give this a try.