New submission from Géry <gery.o...@gmail.com>:
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 <module>
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 "<string>", line 1, in <module>
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/popen_spawn_posix.py#L68)
its `finalizer` instance attribute to a `multiprocessing.util.Finalize`
instance which itself sets its `_weakref` instance attribute to a `weakref.ref`
instance for closing at interpreter exit the ends of the pipes used by the
parent process to communicate with the child process. In other words, it makes
the `multiprocessing.Process` instance hold a weak reference.
Thus if a `multiprocessing.Process` instance holds a reference to a started
`multiprocessing.Process` instance then it holds a weak reference (point 2), so
starting it will fail since it will serialise (point 1) the weak reference and
weak references are not serialisable:
```
import multiprocessing
if __name__ == '__main__':
multiprocessing.set_start_method('spawn') # or 'forkserver' but not 'fork'
process_a = multiprocessing.Process()
process_b = multiprocessing.Process()
process_b.foo = process_a
process_a.start() # creates process_a._popen.finalizer._weakref
process_b.start() # TypeError: cannot pickle 'weakref' object
```
A minimal Python program showing the serialisation issue:
```
import pickle
import weakref
pickle.dumps(weakref.ref(int)) # TypeError: cannot pickle 'weakref' object
```
----------
components: Library (Lib)
messages: 414597
nosy: maggyero
priority: normal
severity: normal
status: open
title: A started multiprocessing.Process instance cannot be serialised
type: enhancement
versions: Python 3.11
_______________________________________
Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue46934>
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