BUG: Test failed in macOS Monterey due to a bug in default multiprocessing module

[vxst]

Describe the issue:

In a full test,(numpy.test('full')), using pthread model(the default) with Darwin kernel:

lib/tests/test_io.py::TestSaveTxt::test_large_zip test is FAILED due to an AttributeError.

FAILED  - AttributeError: Can't pickle local object 'TestSaveTxt.test_large_zip.<locals>.check_large_zip'

The failed function is:

    def dump(obj, file, protocol=None):
        '''Replacement for pickle.dump() using ForkingPickler.'''
>       ForkingPickler(file, protocol).dump(obj)
E       AttributeError: Can't pickle local object 'TestSaveTxt.test_large_zip.<locals>.check_large_zip'

file       = <_io.BytesIO object at 0x16fc39ae0>
obj        = <Process name='Process-1' parent=1569 initial>
protocol   = None

/opt/homebrew/Cellar/python@3.9/3.9.13_4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/reduction.py:60: AttributeError

The stack trace at multiprocessing is

/opt/homebrew/Cellar/python@3.9/3.9.13_4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/process.py:121: in start
    self._popen = self._Popen(self)
        self       = <Process name='Process-1' parent=1569 initial>
/opt/homebrew/Cellar/python@3.9/3.9.13_4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/context.py:224: in _Popen
    return _default_context.get_context().Process._Popen(process_obj)
        process_obj = <Process name='Process-1' parent=1569 initial>
/opt/homebrew/Cellar/python@3.9/3.9.13_4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/context.py:284: in _Popen
    return Popen(process_obj)
        Popen      = <class 'multiprocessing.popen_spawn_posix.Popen'>
        process_obj = <Process name='Process-1' parent=1569 initial>
/opt/homebrew/Cellar/python@3.9/3.9.13_4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/popen_spawn_posix.py:32: in __init__
    super().__init__(process_obj)
        __class__  = <class 'multiprocessing.popen_spawn_posix.Popen'>
        process_obj = <Process name='Process-1' parent=1569 initial>
        self       = <multiprocessing.popen_spawn_posix.Popen object at 0x16fbbb070>
/opt/homebrew/Cellar/python@3.9/3.9.13_4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/popen_fork.py:19: in __init__
    self._launch(process_obj)
        process_obj = <Process name='Process-1' parent=1569 initial>
        self       = <multiprocessing.popen_spawn_posix.Popen object at 0x16fbbb070>
/opt/homebrew/Cellar/python@3.9/3.9.13_4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/popen_spawn_posix.py:47: in _launch
    reduction.dump(process_obj, fp)
        fp         = <_io.BytesIO object at 0x16fc39ae0>
        prep_data  = {'authkey': b'\xe7\xf6K\x8ceo\xe6se\x82\x11#q\xa9\xde\x19SX\x917\xf0M\x07\xa9-\xfd\x15\xa8~\xc5\x98\x14', 'dir': '/Use...s', 'init_main_from_path': '/Us
ers/vxst/vxst_env_verified/py39_sci_test/tests/0_numpy.py', 'log_to_stderr': False, ...}
        process_obj = <Process name='Process-1' parent=1569 initial>
        resource_tracker = <module 'multiprocessing.resource_tracker' from '/opt/homebrew/Cellar/python@3.9/3.9.13_4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/m
ultiprocessing/resource_tracker.py'>
        self       = <multiprocessing.popen_spawn_posix.Popen object at 0x16fbbb070>
        tracker_fd = 19

Test hardwares include M1 Max with 32 core GPU and 64GB memory and M1 with 16GB memory.

Test python version include cpython 3.8 and cpython 3.9.

Four configurations all fail in the same manner.

Reproduce the code example:

py.test numpy/lib/tests/test_io.py

Error message:

../../../Library/Python/3.9/lib/python/site-packages/numpy/lib/tests/test_io.py:598:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
/opt/homebrew/Cellar/python@3.9/3.9.13_4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/process.py:121: in start
    self._popen = self._Popen(self)
/opt/homebrew/Cellar/python@3.9/3.9.13_4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/context.py:224: in _Popen
    return _default_context.get_context().Process._Popen(process_obj)
/opt/homebrew/Cellar/python@3.9/3.9.13_4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/context.py:284: in _Popen
    return Popen(process_obj)
/opt/homebrew/Cellar/python@3.9/3.9.13_4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/popen_spawn_posix.py:32: in __init__
    super().__init__(process_obj)
/opt/homebrew/Cellar/python@3.9/3.9.13_4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/popen_fork.py:19: in __init__
    self._launch(process_obj)
/opt/homebrew/Cellar/python@3.9/3.9.13_4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/popen_spawn_posix.py:47: in _launch
    reduction.dump(process_obj, fp)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

obj = <Process name='Process-1' parent=38364 initial>, file = <_io.BytesIO object at 0x1055a08b0>, protocol = None

    def dump(obj, file, protocol=None):
        '''Replacement for pickle.dump() using ForkingPickler.'''
>       ForkingPickler(file, protocol).dump(obj)
E       AttributeError: Can't pickle local object 'TestSaveTxt.test_large_zip.<locals>.check_large_zip'

/opt/homebrew/Cellar/python@3.9/3.9.13_4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/reduction.py:60: AttributeError

NumPy/Python version information:

1.21.6 at both Python 3.8 and Python 3.9 environments, built by homebrew.

1.21.6 3.9.13 (main, Aug 28 2022, 18:56:22)
[Clang 13.1.6 (clang-1316.0.21.2.5)]

and

1.21.6 3.8.13 (default, Aug 28 2022, 18:53:35)
[Clang 13.1.6 (clang-1316.0.21.2.5)]

The system uname is:

Darwin Mac 21.6.0 Darwin Kernel Version 21.6.0: Wed Aug 10 14:28:23 PDT 2022; root:xnu-8020.141.5~2/RELEASE_ARM64_T6000 arm64 arm Darwin

Context for the issue:

It cause numpy.test('full') to fail on macOS.

[vxst]

The reason behind the failure is since Python 3.8, the default start method for multiprocessing has been changed from 'fork' to 'spawn' on macOS. The default start method is still 'fork' on Linux. It will cause a memory-sharing problem for the test "test_large_zip" on macOS. The fix is to change the start method for this context back to 'fork' so that all platforms will have the same memory sharing model and thus fix the failed test.