Context managers and multiprocessing pools

Question

Suppose you are using a multiprocessing.Pool object, and you are using the initializer setting of the constructor to pass an initializer function that then creates a resource in the global namespace. Assume resource has a context manager. How would you handle the life-cycle of the context managed resource provided it has to live through the life of the process, but be properly cleaned up at the end?

So far, I have something somewhat like this:

resource_cm = None
resource = None


def _worker_init(args):
    global resource
    resource_cm = open_resource(args)
    resource = resource_cm.__enter__()

From here on, the pool processes can use the resource. So far so good. But handling clean up is a bit trickier, since the multiprocessing.Pool class does not provide a destructor or deinitializer argument.

One of my ideas is to use the atexit module, and register the clean up in the initializer. Something like this:

def _worker_init(args):
    global resource
    resource_cm = open_resource(args)
    resource = resource_cm.__enter__()

    def _clean_up():
        resource_cm.__exit__()

    import atexit
    atexit.register(_clean_up)

Is this a good approach? Is there an easier way of doing this?

EDIT: atexit does not seem to work. At least not in the way I am using it above, so as of right now I still do not have a solution for this problem.

Answer 1

You can subclass Process and override its run() method so that it performs cleanup before exit. Then you should subclass Pool so that it uses your subclassed process:

from multiprocessing import Process
from multiprocessing.pool import Pool

class SafeProcess(Process):
    """ Process that will cleanup before exit """
    def run(self, *args, **kw):
        result = super().run(*args, **kw)
        # cleanup however you want here
        return result


class SafePool(Pool):
    Process = SafeProcess


pool = SafePool(4)  # use it as standard Pool

Answer 2

First, this is a really great question! After digging around a bit in the multiprocessing code, I think I've found a way to do this:

When you start a multiprocessing.Pool, internally the Pool object creates a multiprocessing.Process object for each member of the pool. When those sub-processes are starting up, they call a _bootstrap function, which looks like this:

def _bootstrap(self):
    from . import util
    global _current_process
    try:
        # ... (stuff we don't care about)
        util._finalizer_registry.clear()
        util._run_after_forkers()
        util.info('child process calling self.run()')
        try:
            self.run()
            exitcode = 0 
        finally:
            util._exit_function()
        # ... (more stuff we don't care about)

The run method is what actually runs the target you gave the Process object. For a Pool process that's a method with a long-running while loop that waits for work items to come in over an internal queue. What's really interesting for us is what happened after self.run: util._exit_function() is called.

As it turns out, that function does some clean up that sounds a lot like what you're looking for:

def _exit_function(info=info, debug=debug, _run_finalizers=_run_finalizers,
                   active_children=active_children,
                   current_process=current_process):
    # NB: we hold on to references to functions in the arglist due to the
    # situation described below, where this function is called after this
    # module's globals are destroyed.

    global _exiting

    info('process shutting down')
    debug('running all "atexit" finalizers with priority >= 0')  # Very interesting!
    _run_finalizers(0)

Here's the docstring of _run_finalizers:

def _run_finalizers(minpriority=None):
    '''
    Run all finalizers whose exit priority is not None and at least minpriority

    Finalizers with highest priority are called first; finalizers with
    the same priority will be called in reverse order of creation.
    '''

The method actually runs through a list of finalizer callbacks and executes them:

items = [x for x in _finalizer_registry.items() if f(x)]
items.sort(reverse=True)

for key, finalizer in items:
    sub_debug('calling %s', finalizer)
    try:
        finalizer()
    except Exception:
        import traceback
        traceback.print_exc()

Perfect. So how do we get into the _finalizer_registry? There's an undocumented object called Finalize in multiprocessing.util that is responsible for adding a callback to the registry:

class Finalize(object):
    '''
    Class which supports object finalization using weakrefs
    '''
    def __init__(self, obj, callback, args=(), kwargs=None, exitpriority=None):
        assert exitpriority is None or type(exitpriority) is int

        if obj is not None:
            self._weakref = weakref.ref(obj, self)
        else:
            assert exitpriority is not None

        self._callback = callback
        self._args = args
        self._kwargs = kwargs or {}
        self._key = (exitpriority, _finalizer_counter.next())
        self._pid = os.getpid()

        _finalizer_registry[self._key] = self  # That's what we're looking for!

Ok, so putting it all together into an example:

import multiprocessing
from multiprocessing.util import Finalize

resource_cm = None
resource = None

class Resource(object):
    def __init__(self, args):
        self.args = args

    def __enter__(self):
        print("in __enter__ of %s" % multiprocessing.current_process())
        return self

    def __exit__(self, *args, **kwargs):
        print("in __exit__ of %s" % multiprocessing.current_process())

def open_resource(args):
    return Resource(args)

def _worker_init(args):
    global resource
    print("calling init")
    resource_cm = open_resource(args)
    resource = resource_cm.__enter__()
    # Register a finalizer
    Finalize(resource, resource.__exit__, exitpriority=16)

def hi(*args):
    print("we're in the worker")

if __name__ == "__main__":
    pool = multiprocessing.Pool(initializer=_worker_init, initargs=("abc",))
    pool.map(hi, range(pool._processes))
    pool.close()
    pool.join()

Output:

calling init
in __enter__ of <Process(PoolWorker-1, started daemon)>
calling init
calling init
in __enter__ of <Process(PoolWorker-2, started daemon)>
in __enter__ of <Process(PoolWorker-3, started daemon)>
calling init
in __enter__ of <Process(PoolWorker-4, started daemon)>
we're in the worker
we're in the worker
we're in the worker
we're in the worker
in __exit__ of <Process(PoolWorker-1, started daemon)>
in __exit__ of <Process(PoolWorker-2, started daemon)>
in __exit__ of <Process(PoolWorker-3, started daemon)>
in __exit__ of <Process(PoolWorker-4, started daemon)>

As you can see __exit__ gets called in all our workers when we join() the pool.