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[BUG] python setup.py build_ext -j4 fails on macOS with Python 3.8+ #3973

@adamjstewart

Description

@adamjstewart

Describe the bug

It's currently not possible to compile Cython in parallel on macOS with Python 3.8+

To Reproduce

$ python setup.py build_ext -j4
Unable to find pgen, not compiling formal grammar.
running build_ext
Compiling /private/var/folders/21/hwq39zyj4g36x6zjfyl5l8080000gn/T/Adam/spack-stage/spack-stage-py-cython-0.29.21-tiuclkwj7x2mxlolbc7yvkeeo5umquce/spack-src/Cython/Plex/Scanners.py because it changed.
Compiling /private/var/folders/21/hwq39zyj4g36x6zjfyl5l8080000gn/T/Adam/spack-stage/spack-stage-py-cython-0.29.21-tiuclkwj7x2mxlolbc7yvkeeo5umquce/spack-src/Cython/Plex/Actions.py because it changed.
Compiling /private/var/folders/21/hwq39zyj4g36x6zjfyl5l8080000gn/T/Adam/spack-stage/spack-stage-py-cython-0.29.21-tiuclkwj7x2mxlolbc7yvkeeo5umquce/spack-src/Cython/Compiler/Scanning.py because it changed.
Compiling /private/var/folders/21/hwq39zyj4g36x6zjfyl5l8080000gn/T/Adam/spack-stage/spack-stage-py-cython-0.29.21-tiuclkwj7x2mxlolbc7yvkeeo5umquce/spack-src/Cython/Compiler/Visitor.py because it changed.
Compiling /private/var/folders/21/hwq39zyj4g36x6zjfyl5l8080000gn/T/Adam/spack-stage/spack-stage-py-cython-0.29.21-tiuclkwj7x2mxlolbc7yvkeeo5umquce/spack-src/Cython/Compiler/FlowControl.py because it changed.
Compiling /private/var/folders/21/hwq39zyj4g36x6zjfyl5l8080000gn/T/Adam/spack-stage/spack-stage-py-cython-0.29.21-tiuclkwj7x2mxlolbc7yvkeeo5umquce/spack-src/Cython/Runtime/refnanny.pyx because it changed.
Compiling /private/var/folders/21/hwq39zyj4g36x6zjfyl5l8080000gn/T/Adam/spack-stage/spack-stage-py-cython-0.29.21-tiuclkwj7x2mxlolbc7yvkeeo5umquce/spack-src/Cython/Compiler/FusedNode.py because it changed.
Compiling /private/var/folders/21/hwq39zyj4g36x6zjfyl5l8080000gn/T/Adam/spack-stage/spack-stage-py-cython-0.29.21-tiuclkwj7x2mxlolbc7yvkeeo5umquce/spack-src/Cython/Tempita/_tempita.py because it changed.
Unable to find pgen, not compiling formal grammar.
Unable to find pgen, not compiling formal grammar.
Unable to find pgen, not compiling formal grammar.
Unable to find pgen, not compiling formal grammar.
running build_ext
running build_ext
running build_ext
running build_ext
Compiling /private/var/folders/21/hwq39zyj4g36x6zjfyl5l8080000gn/T/Adam/spack-stage/spack-stage-py-cython-0.29.21-tiuclkwj7x2mxlolbc7yvkeeo5umquce/spack-src/Cython/Plex/Scanners.py because it changed.
Compiling /private/var/folders/21/hwq39zyj4g36x6zjfyl5l8080000gn/T/Adam/spack-stage/spack-stage-py-cython-0.29.21-tiuclkwj7x2mxlolbc7yvkeeo5umquce/spack-src/Cython/Plex/Actions.py because it changed.
Compiling /private/var/folders/21/hwq39zyj4g36x6zjfyl5l8080000gn/T/Adam/spack-stage/spack-stage-py-cython-0.29.21-tiuclkwj7x2mxlolbc7yvkeeo5umquce/spack-src/Cython/Compiler/Scanning.py because it changed.
Compiling /private/var/folders/21/hwq39zyj4g36x6zjfyl5l8080000gn/T/Adam/spack-stage/spack-stage-py-cython-0.29.21-tiuclkwj7x2mxlolbc7yvkeeo5umquce/spack-src/Cython/Compiler/Visitor.py because it changed.
Compiling /private/var/folders/21/hwq39zyj4g36x6zjfyl5l8080000gn/T/Adam/spack-stage/spack-stage-py-cython-0.29.21-tiuclkwj7x2mxlolbc7yvkeeo5umquce/spack-src/Cython/Compiler/FlowControl.py because it changed.
Compiling /private/var/folders/21/hwq39zyj4g36x6zjfyl5l8080000gn/T/Adam/spack-stage/spack-stage-py-cython-0.29.21-tiuclkwj7x2mxlolbc7yvkeeo5umquce/spack-src/Cython/Runtime/refnanny.pyx because it changed.
Compiling /private/var/folders/21/hwq39zyj4g36x6zjfyl5l8080000gn/T/Adam/spack-stage/spack-stage-py-cython-0.29.21-tiuclkwj7x2mxlolbc7yvkeeo5umquce/spack-src/Cython/Compiler/FusedNode.py because it changed.
Compiling /private/var/folders/21/hwq39zyj4g36x6zjfyl5l8080000gn/T/Adam/spack-stage/spack-stage-py-cython-0.29.21-tiuclkwj7x2mxlolbc7yvkeeo5umquce/spack-src/Cython/Tempita/_tempita.py because it changed.
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/python-3.9.1-gzkonfmwz2asjl54j245h7hyzowrtzhr/lib/python3.9/multiprocessing/spawn.py", line 116, in spawn_main
    exitcode = _main(fd, parent_sentinel)
  File "/Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/python-3.9.1-gzkonfmwz2asjl54j245h7hyzowrtzhr/lib/python3.9/multiprocessing/spawn.py", line 125, in _main
    prepare(preparation_data)
  File "/Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/python-3.9.1-gzkonfmwz2asjl54j245h7hyzowrtzhr/lib/python3.9/multiprocessing/spawn.py", line 236, in prepare
    _fixup_main_from_path(data['init_main_from_path'])
  File "/Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/python-3.9.1-gzkonfmwz2asjl54j245h7hyzowrtzhr/lib/python3.9/multiprocessing/spawn.py", line 287, in _fixup_main_from_path
    main_content = runpy.run_path(main_path,
  File "/Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/python-3.9.1-gzkonfmwz2asjl54j245h7hyzowrtzhr/lib/python3.9/runpy.py", line 268, in run_path
    return _run_module_code(code, init_globals, run_name,
  File "/Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/python-3.9.1-gzkonfmwz2asjl54j245h7hyzowrtzhr/lib/python3.9/runpy.py", line 97, in _run_module_code
    _run_code(code, mod_globals, init_globals,
  File "/Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/python-3.9.1-gzkonfmwz2asjl54j245h7hyzowrtzhr/lib/python3.9/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/private/var/folders/21/hwq39zyj4g36x6zjfyl5l8080000gn/T/Adam/spack-stage/spack-stage-py-cython-0.29.21-tiuclkwj7x2mxlolbc7yvkeeo5umquce/spack-src/setup.py", line 228, in <module>
    setup(
  File "/Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/py-setuptools-50.3.2-5wqt7cliuz2gugkukxe3gitnfwaeviyk/lib/python3.9/site-packages/setuptools/__init__.py", line 153, in setup
    return distutils.core.setup(**attrs)
  File "/Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/python-3.9.1-gzkonfmwz2asjl54j245h7hyzowrtzhr/lib/python3.9/distutils/core.py", line 148, in setup
    dist.run_commands()
  File "/Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/python-3.9.1-gzkonfmwz2asjl54j245h7hyzowrtzhr/lib/python3.9/distutils/dist.py", line 966, in run_commands
    self.run_command(cmd)
  File "/Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/python-3.9.1-gzkonfmwz2asjl54j245h7hyzowrtzhr/lib/python3.9/distutils/dist.py", line 984, in run_command
    cmd_obj.ensure_finalized()
  File "/Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/python-3.9.1-gzkonfmwz2asjl54j245h7hyzowrtzhr/lib/python3.9/distutils/cmd.py", line 107, in ensure_finalized
    self.finalize_options()
  File "/private/var/folders/21/hwq39zyj4g36x6zjfyl5l8080000gn/T/Adam/spack-stage/spack-stage-py-cython-0.29.21-tiuclkwj7x2mxlolbc7yvkeeo5umquce/spack-src/Cython/Distutils/build_ext.py", line 20, in finalize_options
    self.distribution.ext_modules[:] = cythonize(
  File "/private/var/folders/21/hwq39zyj4g36x6zjfyl5l8080000gn/T/Adam/spack-stage/spack-stage-py-cython-0.29.21-tiuclkwj7x2mxlolbc7yvkeeo5umquce/spack-src/Cython/Build/Dependencies.py", line 1078, in cythonize
    pool = multiprocessing.Pool(
  File "/Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/python-3.9.1-gzkonfmwz2asjl54j245h7hyzowrtzhr/lib/python3.9/multiprocessing/context.py", line 119, in Pool
    return Pool(processes, initializer, initargs, maxtasksperchild,
  File "/Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/python-3.9.1-gzkonfmwz2asjl54j245h7hyzowrtzhr/lib/python3.9/multiprocessing/pool.py", line 212, in __init__
    self._repopulate_pool()
  File "/Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/python-3.9.1-gzkonfmwz2asjl54j245h7hyzowrtzhr/lib/python3.9/multiprocessing/pool.py", line 303, in _repopulate_pool
    return self._repopulate_pool_static(self._ctx, self.Process,
  File "/Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/python-3.9.1-gzkonfmwz2asjl54j245h7hyzowrtzhr/lib/python3.9/multiprocessing/pool.py", line 326, in _repopulate_pool_static
    w.start()
  File "/Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/python-3.9.1-gzkonfmwz2asjl54j245h7hyzowrtzhr/lib/python3.9/multiprocessing/process.py", line 121, in start
    self._popen = self._Popen(self)
  File "/Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/python-3.9.1-gzkonfmwz2asjl54j245h7hyzowrtzhr/lib/python3.9/multiprocessing/context.py", line 284, in _Popen
    return Popen(process_obj)
  File "/Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/python-3.9.1-gzkonfmwz2asjl54j245h7hyzowrtzhr/lib/python3.9/multiprocessing/popen_spawn_posix.py", line 32, in __init__
    super().__init__(process_obj)
  File "/Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/python-3.9.1-gzkonfmwz2asjl54j245h7hyzowrtzhr/lib/python3.9/multiprocessing/popen_fork.py", line 19, in __init__
    self._launch(process_obj)
  File "/Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/python-3.9.1-gzkonfmwz2asjl54j245h7hyzowrtzhr/lib/python3.9/multiprocessing/popen_spawn_posix.py", line 42, in _launch
    prep_data = spawn.get_preparation_data(process_obj._name)
  File "/Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/python-3.9.1-gzkonfmwz2asjl54j245h7hyzowrtzhr/lib/python3.9/multiprocessing/spawn.py", line 154, in get_preparation_data
    _check_not_importing_main()
  File "/Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/python-3.9.1-gzkonfmwz2asjl54j245h7hyzowrtzhr/lib/python3.9/multiprocessing/spawn.py", line 134, in _check_not_importing_main
    raise RuntimeError('''
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.
...

Expected behavior

I would expect to be able to build Cython in parallel.

Environment:

  • OS: macOS
  • Python version: 3.9.1
  • Cython version: 0.29.21

Additional context

As you're probably aware, the multiprocessing package changed in Python 3.8 and now defaults to the spawn start method on macOS: https://docs.python.org/3/library/multiprocessing.html#contexts-and-start-methods

I'm guessing this is the issue, although I could be wrong.

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