[issue46934] A started multiprocessing.Process instance cannot be serialised

[Géry]

Change by Géry :


--
keywords: +patch
pull_requests: +29821
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/31701

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[issue46934] A started multiprocessing.Process instance cannot be serialised

[Géry]

New submission from Géry :

The Python program:

```
import multiprocessing
import time


class Application:

def __init__(self):
self._event = multiprocessing.Event()
self._processes = [
multiprocessing.Process(target=self._worker)
for _ in range(multiprocessing.cpu_count())]

def _worker(self):
while not self._event.is_set():
print(multiprocessing.current_process().name)
time.sleep(1)

def start(self):
for process in self._processes:
print('starting')
process.start()

def stop(self):
self._event.set()
for process in self._processes:
process.join()


if __name__ == '__main__':
application = Application()
application.start()
time.sleep(3)
application.stop()
```

Its output:

```none
starting
starting
Traceback (most recent call last):
  File "/Users/maggyero/Desktop/application.py", line 31, in 
application.start()
  File "/Users/maggyero/Desktop/application.py", line 21, in start
process.start()
  File 
"/usr/local/Cellar/python@3.9/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/process.py",
 line 121, in start
self._popen = self._Popen(self)
  File 
"/usr/local/Cellar/python@3.9/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/context.py",
 line 224, in _Popen
return _default_context.get_context().Process._Popen(process_obj)
  File 
"/usr/local/Cellar/python@3.9/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/context.py",
 line 284, in _Popen
return Popen(process_obj)
  File 
"/usr/local/Cellar/python@3.9/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/popen_spawn_posix.py",
 line 32, in __init__
super().__init__(process_obj)
  File 
"/usr/local/Cellar/python@3.9/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/popen_fork.py",
 line 19, in __init__
self._launch(process_obj)
  File 
"/usr/local/Cellar/python@3.9/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/popen_spawn_posix.py",
 line 47, in _launch
reduction.dump(process_obj, fp)
  File 
"/usr/local/Cellar/python@3.9/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/reduction.py",
 line 60, in dump
ForkingPickler(file, protocol).dump(obj)
TypeError: cannot pickle 'weakref' object
Traceback (most recent call last):
  File "", line 1, in 
  File 
"/usr/local/Cellar/python@3.9/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/spawn.py",
 line 116, in spawn_main
exitcode = _main(fd, parent_sentinel)
  File 
"/usr/local/Cellar/python@3.9/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/spawn.py",
 line 126, in _main
self = reduction.pickle.load(from_parent)
  File 
"/usr/local/Cellar/python@3.9/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/synchronize.py",
 line 110, in __setstate__
self._semlock = _multiprocessing.SemLock._rebuild(*state)
FileNotFoundError: [Errno 2] No such file or directory
```

In the function `Application.__init__`, each call 
`multiprocessing.Process(target=self._worker)` initializes a 
`multiprocessing.Process` instance with the instance method `self._worker` as 
its `target` argument. `self._worker` is bound to `self` which has the instance 
attribute `self._processes`.

In the function `Application.start`, each call `process.start()` serialises the 
`target` argument and therefore `self._processes`. `self._processes` is a list 
of `multiprocessing.Process` instances, initially not started yet. The first 
call `process.start()` starts the first `multiprocessing.Process` instance in 
that list without issue, but the second call `process.start()` fails.

So a started `multiprocessing.Process` instance cannot be serialised.

The root of the problem is that the `start` method of a 
`multiprocessing.Process` instance sets its `_popen` instance attribute to a 
`multiprocessing.popen_*.Popen` instance. The initialization of that instance 
performs these two steps (among others):

1. For a `multiprocessing.popen_spawn_posix.Popen` instance, a 
`multiprocessing.popen_spawn_win32.Popen` instance, or a 
`multiprocessing.popen_forkserver.Popen` instance but not a 
`multiprocessing.popen_fork.Popen` instance (i.e. for the start method 
`'spawn'` or the start method `'forkserver'` but not the start method 
`'fork'`), it 
[serialises](https://github.com/python/cpython/blob/v3.10.2/Lib/multiprocessing/popen_spawn_posix.py#L47)
 the `multiprocessing.Process` instance for writing it to the end of the pipe 
used by the parent process to communicate with the child process so that the 
child process can execute the `run` method of the `multiprocessing.Process` 
instance.

2. It 
[sets](https://github.com/python/cpython/blob/v3.10.2/Lib/multiprocessing/p