doit auto crashes with python 3.8 on MacOS

Describe the bug

Considering the following dodo.py file :

/private/tmp$ cat dodo.py 
def task_hello():
    """hello"""

    def python_hello(targets):
        with open(targets[0], "a") as output:
            output.write("Python says Hello World!!!\n")

    return {
        'actions': [python_hello],
        'targets': ["hello.txt"],
        }

doit auto works with python 3.7 :

/private/tmp$ doit --version
0.33.1
lib @ /private/tmp/.env/lib/python3.7/site-packages/doit
/private/tmp$ doit auto
.  hello
^CProcess Process-1:
Traceback (most recent call last):
  File "/usr/local/opt/python@3.7/Frameworks/Python.framework/Versions/3.7/lib/python3.7/multiprocessing/process.py", line 297, in _bootstrap
    self.run()
  File "/usr/local/opt/python@3.7/Frameworks/Python.framework/Versions/3.7/lib/python3.7/multiprocessing/process.py", line 99, in run
    self._target(*self._args, **self._kwargs)
  File "/private/tmp/.env/lib/python3.7/site-packages/doit/cmd_auto.py", line 131, in run_watch
    file_watcher.loop()
  File "/private/tmp/.env/lib/python3.7/site-packages/doit/filewatch.py", line 100, in loop
    self._loop_darwin()
  File "/private/tmp/.env/lib/python3.7/site-packages/doit/filewatch.py", line 72, in _loop_darwin
    observer.run()
  File "/private/tmp/.env/lib/python3.7/site-packages/fsevents.py", line 116, in run
    self.event.wait()
  File "/usr/local/opt/python@3.7/Frameworks/Python.framework/Versions/3.7/lib/python3.7/threading.py", line 552, in wait
    signaled = self._cond.wait(timeout)
  File "/usr/local/opt/python@3.7/Frameworks/Python.framework/Versions/3.7/lib/python3.7/threading.py", line 296, in wait
    waiter.acquire()
KeyboardInterrupt

but crashes with python 3.8 :

/private/tmp$ doit --version
0.33.1
lib @ /private/tmp/.env-3.8/lib/python3.8/site-packages/doit
/private/tmp$ doit auto
Traceback (most recent call last):
  File "/private/tmp/.env-3.8/lib/python3.8/site-packages/doit/doit_cmd.py", line 190, in run
    return command.parse_execute(args)
  File "/private/tmp/.env-3.8/lib/python3.8/site-packages/doit/cmd_base.py", line 150, in parse_execute
    return self.execute(params, args)
  File "/private/tmp/.env-3.8/lib/python3.8/site-packages/doit/cmd_auto.py", line 139, in execute
    proc.start()
  File "/Users/boileau/opt/anaconda3/lib/python3.8/multiprocessing/process.py", line 121, in start
    self._popen = self._Popen(self)
  File "/Users/boileau/opt/anaconda3/lib/python3.8/multiprocessing/context.py", line 224, in _Popen
    return _default_context.get_context().Process._Popen(process_obj)
  File "/Users/boileau/opt/anaconda3/lib/python3.8/multiprocessing/context.py", line 283, in _Popen
    return Popen(process_obj)
  File "/Users/boileau/opt/anaconda3/lib/python3.8/multiprocessing/popen_spawn_posix.py", line 32, in __init__
    super().__init__(process_obj)
  File "/Users/boileau/opt/anaconda3/lib/python3.8/multiprocessing/popen_fork.py", line 19, in __init__
    self._launch(process_obj)
  File "/Users/boileau/opt/anaconda3/lib/python3.8/multiprocessing/popen_spawn_posix.py", line 47, in _launch
    reduction.dump(process_obj, fp)
  File "/Users/boileau/opt/anaconda3/lib/python3.8/multiprocessing/reduction.py", line 60, in dump
    ForkingPickler(file, protocol).dump(obj)
TypeError: cannot pickle '_io.TextIOWrapper' object

Environment

1. OS: MacOS 10.15.6
2. python version: Python 3.8.3
3. doit version: 0.33.1

[Kwpolska]

Yes, but it can be fixed by manually setting it back to fork with multiprocess.set_start_method('spawn') before spawning a Process. I tested with the above snipper and auto, and it worked in spawn mode with multiprocess.

[schettino72]

@bollwyvl moving discussion to #404