Investigate Cython support

[maxfischer2781]

It looks like Cython is compatible with asyncio, python-trio/trio#550 and dabeaz/curio#326, as well as bulk of Python level functions (see cython/cython#2273). Since asyncstdlib currently does some heavy lifting using pure Python, optimising this seems prudent.

Rough checkpoints to using Cython:

• Check whether Cython 0.29/0.3 has sufficient support for async
  • Check that current code can be significantly optimised
  • Check whether PyPy can benefit; PyPy support will not be dropped.
• Add unittests using asyncio, trio, curio to ensure compatibility
• Add benchmarks to verify added complexity is worth it

[maxfischer2781]

Quick check shows that Cython can handle all of asyncstdlib, with the exception of two one "clever" comprehensions. These can be refactored to work with Cython.

[maxfischer2781]

Packaging is a lot less straightforward. Basic template would be the Cython guide. Possible example projects are hdbscan and a third-party cython-package-example.

My main concern is that the package still has to work "out of the box". That means at least building wheels for all major platforms; separate GitHub Actions could help here.

[maxfischer2781]

A simple approach to the packaging issue would be to provide a separate package for fast utilities. Similar to the stdlib, asyncstdlib would optionally import it for speedup if available.

[maxfischer2781]

Major showstopper in my tests so far is that Cython's PyTypeObject does not (yet) define the tp_as_async slot.

[maxfischer2781]

Turns out the missing type fields are simple enough to define.

cdef extern from "Python.h":
    ctypedef struct PyAsyncMethods:
        unaryfunc am_await
        unaryfunc am_aiter
        unaryfunc am_anext

    ctypedef struct PyTypeObject:
        ...
        PyAsyncMethods* tp_as_async

    ...

Got it working for a basic aiter out of the box.

[maxfischer2781]

Experimental code is in a separate repo: https://github.com/maxfischer2781/asyncstdlib_cy

[maxfischer2781]

I do have a clear roadmap on how this could be done, but it does not seem worth the effort at this point. There would have to be a separately maintained Cython codebase, yet the optimisations that Cython can perform statically on coroutines (especially the async-neutral ones) are minimal.

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If anyone needs a push of performance but cannot switch to PyPy, it looks like nuitka can compile the entire library for CPython.

python -m nuitka --module asyncstdlib --include-package=asyncstdlib