Support passing in LowLevelCallable as argument to jitted function

With the recent support for passing in numba jitted functions as arguments (https://github.com/numba/numba/pull/2801), it would also be nice to also support passing in scipy.LowLevelCallable objects. These have become common in the scipy ecosystem, and would be useful for downstream projects considering numba adoption.

[pv]

The PyCapsule is the canonical format of the function pointer, and any ctypes/cffi/Cython inputs are converted to that form by LowLevelCallable.__init__. The function attribute exists mainly for function pointer object life cycle management, and should not be used as the way to call the function in Numba, as you then throw Cython etc. compatibility away — you can just ignore it’s there.

The intended usage is that you know beforehand the C signature(s) you support — then you can cast the LowLevelCallable to a suitable function pointer (or fail doing so, if your signature is not equivalent match). In particular, giving a generic way to call a given arbitrary LowLevelCallable is not really part of the design. While that may sound strange in the context of Numba, it should make sense when you consider that this is intended to be usable from hand-written C without JIT and with minimal hassle and speed impact.

Insofar as what Numba might do here, one possibility is to offer a convenience function that takes a Numba-format signature(s) and a LowLevelCallable object, and produces a function pointer (or NULL/error). One particular issue that you probably encounter here is allowing C integer types of the same size on the platform be considered equivalent.

In addition, something numba-specific could be added here: https://github.com/scipy/scipy/blob/master/scipy/_lib/_ccallback.py#L134 so that you could write scipy.LowLevelCallable(some_numba_cfunc) instead of scipy.LowLevelCallable(some_numba_cfunc.ctypes).

https://mail.python.org/pipermail/scipy-dev/2018-July/022919.html https://mail.python.org/pipermail/scipy-dev/2018-July/022920.html

[sklam]

This is definitely something we can do fairly quickly; esp if all you need to do on a LowLevelCallable is just to call it.