# -*- coding: utf-8 -*-

"""
Problem:

Accessing some Manager types (e.g. Lock) through a list, dict or Namespace proxy
is not possible if both the mutable container (e.g. list) and contained type
instance (e.g. Lock) have been created in the same process.

Symptoms:

In accessing process:
multiprocessing.managers.RemoteError on access, e.g.:
Unserializable message: ('#RETURN', <thread.lock object at 0x7fcf40d31370>)

.. and in underlying manager:
_pickle.PicklingError: Can't pickle <class '_thread.lock'>: attribute lookup
lock on _thread failed

See also:
https://mail.python.org/pipermail/python-list/2009-September/552988.html
"""

from __future__ import unicode_literals, print_function

from traceback import print_exc
from multiprocessing import Manager, Process, Event
from multiprocessing.managers import RemoteError


def use_manager(obj, type_name, mgr=None, started=None, end=None):
    if not mgr:
        mgr = Manager()
    obj.append(getattr(mgr, type_name)())
    print('(use_manager) obj[0]: %s' % obj[0])
    if started:
        started.set()
    if end:
        end.wait()


def add_type_in_child_process(mgr, type_name):
    started = Event()
    end = Event()
    obj = mgr.list()
    proc = Process(target=use_manager, args=(obj, type_name),
                   kwargs={'started': started, 'end': end})
    proc.start()
    # wait for child process to populate proxied object with a value
    started.wait()
    print('(from child) obj[0]: %s' % obj[0])
    end.set()
    proc.join()


def add_type_in_own_process(mgr, type_name):
    # also affects dict, Queue & namespace
    obj = mgr.list()

    use_manager(obj, type_name, mgr=mgr)
    print('(own) obj[0]: %s' % obj[0])


def main():
    mgr = Manager()

    for type_name in ('list', 'dict', 'Namespace', 'Lock', 'RLock', 'Semaphore',
                      'BoundedSemaphore', 'Condition', 'Event', 'Queue'):
        # works
        add_type_in_child_process(mgr, type_name)
        # fails with pickling error
        try:
            add_type_in_own_process(mgr, type_name)
        except RemoteError:
            print('Manager failure (for %s)' % type_name)
            # print_exc()


if __name__ == '__main__':
    main()