Cython builds on python3.8+ on MacOS appears to be broken

[jakeschurch]

Describe the bug

Building cython from python3.8+ with OSs (like darwin) that default multiprocessing start method set to spawn creates infinite build loop.

Known upstream bug of cython

Blurb from upstream issue:

When

Cython.Build.cythonize(..., nthreads=1) is called
from a typical setup.py script
that does not guard its execution with the if name == 'main' idiom
and multiprocessing's start method is set to "spawn",
then

starting a new process will run the initial code again up until the next cythonize/multiprocessing.Pool call,
which will then repeatedly fail with a
RuntimeError:
An attempt has been made to start a new process before the
current process has finished its bootstrapping phase.

    This probably means that you are not using fork to start your
    child processes and you have forgotten to use the proper idiom
    in the main module:

        if __name__ == '__main__':
            freeze_support()
            ...

    The "freeze_support()" line can be omitted if the program
    is not going to be frozen to produce an executable.

Steps To Reproduce

Steps to reproduce the behavior:

1. attempt to build package with cython as dependency from python3.10 set on m1 mac

Expected behavior

Cython builds, exits.

Additional context

Please note there is a bugfix that has been sitting for 4 years - cython/cython#3263. I have applied to cython patches for 0.29.36 version and seems to fix fine. I'm not sure if we want to consider this as an option, but have added my voice to the upstream conversation to see if that fix can be merged; and if not what I can do to support.

Notify maintainers

@FRidh

Metadata

Please run nix-shell -p nix-info --run "nix-info -m" and paste the result.

• system: "aarch64-darwin"
• host os: Darwin 22.1.0, macOS 13.0.1
• multi-user?: yes
• sandbox: no
• version: nix-env (Nix) 2.11.1
• channels(jake): "darwin, home-manager-22.11.tar.gz"
• channels(root): "nixpkgs"
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixpkgs