Python 3.8/3.9 exception for multiprocessing

[Ashex]

I've been doing all my testing under 3.7, when running on 3.8.8 and 3.9.2 will fails to start with the following exception.

Python versions:

• 3.9.2
• 3.8.8

Platform: OSX 11.2.3

CRITICAL:root:Error starting io adapter will.backends.io_adapters.slack: 
Traceback (most recent call last):
  File "/Users/evelyn/git/will/will/backends/io_adapters/base.py", line 119, in _start
    self.__event_listener_thread.start()
  File "/usr/local/homebrew/Cellar/python@3.8/3.8.8_1/Frameworks/Python.framework/Versions/3.8/lib/python3.8/multiprocessing/process.py", line 121, in start
    self._popen = self._Popen(self)
  File "/usr/local/homebrew/Cellar/python@3.8/3.8.8_1/Frameworks/Python.framework/Versions/3.8/lib/python3.8/multiprocessing/context.py", line 224, in _Popen
    return _default_context.get_context().Process._Popen(process_obj)
  File "/usr/local/homebrew/Cellar/python@3.8/3.8.8_1/Frameworks/Python.framework/Versions/3.8/lib/python3.8/multiprocessing/context.py", line 284, in _Popen
    return Popen(process_obj)
  File "/usr/local/homebrew/Cellar/python@3.8/3.8.8_1/Frameworks/Python.framework/Versions/3.8/lib/python3.8/multiprocessing/popen_spawn_posix.py", line 32, in __init__
    super().__init__(process_obj)
  File "/usr/local/homebrew/Cellar/python@3.8/3.8.8_1/Frameworks/Python.framework/Versions/3.8/lib/python3.8/multiprocessing/popen_fork.py", line 19, in __init__
    self._launch(process_obj)
  File "/usr/local/homebrew/Cellar/python@3.8/3.8.8_1/Frameworks/Python.framework/Versions/3.8/lib/python3.8/multiprocessing/popen_spawn_posix.py", line 47, in _launch
    reduction.dump(process_obj, fp)
  File "/usr/local/homebrew/Cellar/python@3.8/3.8.8_1/Frameworks/Python.framework/Versions/3.8/lib/python3.8/multiprocessing/reduction.py", line 60, in dump
    ForkingPickler(file, protocol).dump(obj)
TypeError: cannot pickle 'hiredis.Reader' object
CRITICAL:root:Error starting io adapter will.backends.io_adapters.slack: 
Traceback (most recent call last):
  File "/Users/evelyn/git/will/will/backends/io_adapters/base.py", line 119, in _start
    self.__event_listener_thread.start()
  File "/usr/local/homebrew/Cellar/python@3.8/3.8.8_1/Frameworks/Python.framework/Versions/3.8/lib/python3.8/multiprocessing/process.py", line 121, in start
    self._popen = self._Popen(self)
  File "/usr/local/homebrew/Cellar/python@3.8/3.8.8_1/Frameworks/Python.framework/Versions/3.8/lib/python3.8/multiprocessing/context.py", line 224, in _Popen
    return _default_context.get_context().Process._Popen(process_obj)
  File "/usr/local/homebrew/Cellar/python@3.8/3.8.8_1/Frameworks/Python.framework/Versions/3.8/lib/python3.8/multiprocessing/context.py", line 284, in _Popen
    return Popen(process_obj)
  File "/usr/local/homebrew/Cellar/python@3.8/3.8.8_1/Frameworks/Python.framework/Versions/3.8/lib/python3.8/multiprocessing/popen_spawn_posix.py", line 32, in __init__
    super().__init__(process_obj)
  File "/usr/local/homebrew/Cellar/python@3.8/3.8.8_1/Frameworks/Python.framework/Versions/3.8/lib/python3.8/multiprocessing/popen_fork.py", line 19, in __init__
    self._launch(process_obj)
  File "/usr/local/homebrew/Cellar/python@3.8/3.8.8_1/Frameworks/Python.framework/Versions/3.8/lib/python3.8/multiprocessing/popen_spawn_posix.py", line 47, in _launch
    reduction.dump(process_obj, fp)
  File "/usr/local/homebrew/Cellar/python@3.8/3.8.8_1/Frameworks/Python.framework/Versions/3.8/lib/python3.8/multiprocessing/reduction.py", line 60, in dump
    ForkingPickler(file, protocol).dump(obj)
TypeError: cannot pickle 'hiredis.Reader' object
CRITICAL:root:Error starting io adapter will.backends.io_adapters.slack: 
Traceback (most recent call last):
  File "/Users/evelyn/git/will/will/backends/io_adapters/base.py", line 119, in _start
    self.__event_listener_thread.start()
  File "/usr/local/homebrew/Cellar/python@3.8/3.8.8_1/Frameworks/Python.framework/Versions/3.8/lib/python3.8/multiprocessing/process.py", line 121, in start
    self._popen = self._Popen(self)
  File "/usr/local/homebrew/Cellar/python@3.8/3.8.8_1/Frameworks/Python.framework/Versions/3.8/lib/python3.8/multiprocessing/context.py", line 224, in _Popen
    return _default_context.get_context().Process._Popen(process_obj)
  File "/usr/local/homebrew/Cellar/python@3.8/3.8.8_1/Frameworks/Python.framework/Versions/3.8/lib/python3.8/multiprocessing/context.py", line 284, in _Popen
    return Popen(process_obj)
  File "/usr/local/homebrew/Cellar/python@3.8/3.8.8_1/Frameworks/Python.framework/Versions/3.8/lib/python3.8/multiprocessing/popen_spawn_posix.py", line 32, in __init__
    super().__init__(process_obj)
  File "/usr/local/homebrew/Cellar/python@3.8/3.8.8_1/Frameworks/Python.framework/Versions/3.8/lib/python3.8/multiprocessing/popen_fork.py", line 19, in __init__
    self._launch(process_obj)
  File "/usr/local/homebrew/Cellar/python@3.8/3.8.8_1/Frameworks/Python.framework/Versions/3.8/lib/python3.8/multiprocessing/popen_spawn_posix.py", line 47, in _launch
    reduction.dump(process_obj, fp)
  File "/usr/local/homebrew/Cellar/python@3.8/3.8.8_1/Frameworks/Python.framework/Versions/3.8/lib/python3.8/multiprocessing/reduction.py", line 60, in dump
    ForkingPickler(file, protocol).dump(obj)
TypeError: cannot pickle 'hiredis.Reader' object

[Ashex]

This is an OSX specific issue as multiprocessing behavior changed in 3.8:

https://chrissardegna.com/blog/multiprocessing-changes-python-3-8/