PyInstaller-built Windows EXE fails with multiprocessing

Enabling --onedir multiprocessing support on Windows (does NOT work with --onefile on Windows, but otherwise safe on all platforms/configurations):

if __name__ == '__main__':
    # On Windows calling this function is necessary.
    multiprocessing.freeze_support()

Enabling --onefile multiprocessing support on Windows (safe on all platforms/configurations, compatible with --onedir):

import multiprocessing.forking
import os
import sys

class _Popen(multiprocessing.forking.Popen):
    def __init__(self, *args, **kw):
        if hasattr(sys, 'frozen'):
            # We have to set original _MEIPASS2 value from sys._MEIPASS
            # to get --onefile mode working.
            os.putenv('_MEIPASS2', sys._MEIPASS)
        try:
            super(_Popen, self).__init__(*args, **kw)
        finally:
            if hasattr(sys, 'frozen'):
                # On some platforms (e.g. AIX) 'os.unsetenv()' is not
                # available. In those cases we cannot delete the variable
                # but only set it to the empty string. The bootloader
                # can handle this case.
                if hasattr(os, 'unsetenv'):
                    os.unsetenv('_MEIPASS2')
                else:
                    os.putenv('_MEIPASS2', '')

class Process(multiprocessing.Process):
    _Popen = _Popen

# ...

if __name__ == '__main__':
    # On Windows calling this function is necessary.
    multiprocessing.freeze_support()

    # Use your new Process class instead of multiprocessing.Process

References
https://stackoverflow.com/questions/24944558/pyinstaller-built-windows-exe-fails-with-multiprocessing

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