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I am process A and I own object Obj. Now I decide to fork(): process B is born.

How do I do to make process B call the methods of A's object Obj (and not the methods of its own copy of Obj...) ?

This is what this module does: you create the "CallPipe" for Obj prior to fork()ing, and then process B can call any method of A's object Obj through it.

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#  @brief An IPC channel allowing to call methods of an object from a
#  remote process. It is based on python's multiprocessing pipes:
#  instead of sending data, we send method call requests to an object
#  and transmit back the results (or exceptions). Works between
#  processes created by fork() (eg. multiprocessing.Process::start()).
#
# A CallPipe is bound to a 'target' object, and is made of 2 endpoints
# and one multiprocessing.Pipe inbetween:
# - a CallPipe_Callee, which is basically a thread listening to method
#   call requests coming on the pipe, calling the method on the target
#   object and returning the result onto the pipe to the caller
# - a CallPipe_Caller, which is a proxy object transforming the calls
#   to its __getattr__ into a MethodProxy object which in turns
#   transforms the calls to its __call__ method into request/response on
#   the pipe.
#
# A callpipe is associated with 1 'target' object, and should be used
# by 2 different processes: the one that contains the object, and the
# one that remotely calls the methods of the object. If there are more
# processes that have to call the methods on the target object, more
# callpipes have to be created (associated with the same target
# object).
#
# On the 'target' object side (CallPipe_Callee), each callpipe
# corresponds to a thread that may call its methods. As a consequence,
# the methods that might be called from the callpipe threads have to be
# thread-safe.
#
# On the remote side, the callpipe is multithread-safe. Currently, the
# implementation is very crude: only one remote thread is allowed to
# call a method from the target object at any given time (mutual
# exclusion).
#
# Note: Once the remote processes have a reference to the
# CallPipe_Caller object, they can discard any reference to the real
# object that might still exist in their image (due to the fork()
# nature of multiprocessing.Process::start()). In any case, if they
# call the methods of the (copy they have of the) target object,
# instead of the methods of the proxy, then they will act on their
# private copy of the object, not on the remote target object.

import threading, thread, os, select

try:
    # Tested with python 2.6 b3
    import multiprocessing
except ImportError:
    # Tested with standalone processing 0.52
    import processing as multiprocessing


class CallPipe_MethodProxy:
    """The remote proxy whose __call__ method sends the request and
    waits for the answer on the pipe"""
    def __init__(self, callpipe_caller, attr_name, cb_unregister = None):
        self.__caller     = callpipe_caller
        self.__attr_name  = attr_name
        self.__unregister = cb_unregister

    def __call__(self, *args, **kw):
        # We need to acquire the lock because if 2 threads concurrently
        # try to send/recv concurrently on the same pipe, the callee
        # will not be able to tell who is calling what and who it needs to
        # answer to.
        self.__caller._lock.acquire()
        try:
            self.__caller._endpoint.send((self.__attr_name, args, kw))
            recvdata = self.__caller._endpoint.recv()
        finally:
            self.__caller._lock.release()

        if len(recvdata) != 2:
            self.__caller._unregister(self.__attr_name)
            raise AttributeError(self.__attr_name)

        status, r = recvdata
        if status != "OK":
            raise r
        return r


class CallPipe_CallerProxy:
    """The remote proxy whose __getattr__ method simply calls
    CallPipe_Caller::get_method_proxy()"""
    def __init__(self, gateway):
        self.__gateway = gateway

    def __getattr__(self, attr_name):
        return self.__gateway._get_method_proxy(attr_name)


class CallPipe_Caller:
    """The callpipe endpoint which binds a CallPipe_CallerProxy to a
    request/response session over a pipe"""
    def __init__(self, endpoint):
        """Endpoint is an endpoint of a bidirectional multiprocessing.Pipe"""
        self._endpoint        = endpoint
        self._lock            = threading.Lock()
        self.__method_proxies = dict() # Cache of MethodProxy objects
        self.__proxy          = CallPipe_CallerProxy(self)

    def get_proxy(self):
        """Returns the associated CallPipe_CallerProxy object"""
        return self.__proxy

    def _get_method_proxy(self, method_name):
        """Called by the proxy object: creates a CallPipe_MethodProxy
        object for the given method name and store it into the local
        cache (__method_proxies). Returns the MethodProxy object."""
        self._lock.acquire()
        try:
            method_proxy = self.__method_proxies[method_name]
        except KeyError:
            method_proxy = CallPipe_MethodProxy(self, method_name)
            self.__method_proxies[method_name] = method_proxy
        finally:
            self._lock.release()
        return method_proxy

    def start(self):
        pass

    def stop(self):
        """Send a termination request to the CallPipe_Callee"""
        self.__endpoint.send(("TERMINATE",))
        self.__endpoint.close()

    def _unregister(self, attr_name):
        """Called by the CallPipe_MethodProxy object when the remote
        object signals an AttributeError, to remove it from the
        __method_proxies cache"""
        del self.__method_proxies[attr_name]


class CallPipe_CalleeThread(threading.Thread):
    """The thread that waits for the remote requests coming from the
    CallPipe_Caller at the other end of the pipe and performes the
    requested local method calls on the target object"""
    def __init__(self, termfd, endpoint, obj):
        """
        \param termfd is a file descriptor on which to select() to
        wait for termination requests from the main CallPipe_Callee
        thread
        \param Endpoint is an endpoint of a bidirectional multiprocessing.Pipe
        \param obj is the object on which to perform the method calls
        """
        threading.Thread.__init__(self)
        self.__endpoint = endpoint
        self.__obj      = obj
        self.__waitset  = select.poll()
        eventmask = select.POLLIN | select.POLLERR \
                    | select.POLLHUP | select.POLLPRI
        self.__waitset.register(self.__endpoint.fileno(), eventmask)
        self.__waitset.register(termfd, eventmask)

    def run(self):
        while True:
            # Check whether we received something from either the
            # callpipe or the terminating pipe
            fds = set([fd for fd, evt in self.__waitset.poll()])
            if len(fds) > 1:
                break
            if self.__endpoint.fileno() not in fds:
                break

            request = self.__endpoint.recv()
            if not isinstance(request, tuple) or len(request) != 3:
                # We have a problem. stopping ourselves
                try:
                    if request[0] == "TERMINATE":
                        raise SystemExit("Received a termination request.")
                except IndexError:
                    pass
                raise SystemExit("Invalid requests received by Callee.")

            method_name, args, kw = request
            try:
                method = None
                try:
                    method = getattr(self.__obj, method_name)
                except AttributeError:
                    self.__endpoint.send(("ATTRIBUTE_ERROR",))
                if method:
                    result = method(*args, **kw)
                    self.__endpoint.send(("OK", result))
            except Exception, ex:
                self.__endpoint.send(("EXCEPTION", ex))
            except:
                self.__endpoint.send(("EXCEPTION",
                                      RuntimeError("Uncaught exception")))

        # Closing the endpoint (never reached)
        self.__endpoint.close()


class CallPipe_Callee:
    """The object that binds the local target object to a thread
    listening for the method requests coming from the pipe"""
    def __init__(self, endpoint, obj):
        """
        \param Endpoint is an endpoint of a bidirectional multiprocessing.Pipe
        \param obj is the object on which to perform the method calls
        """
        term_r, term_w  = os.pipe()
        self.__obj      = obj
        self.__termpipe = term_w
        self.__thread   = CallPipe_CalleeThread(term_r, endpoint, obj)

    def get_object(self):
        return self.__obj

    def get_request_handler(self):
        return self.__thread

    def start(self):
        self.__thread.start()

    def stop(self):
        """Stop the thread performing the requests. Returns when the
        thread has been stopped"""
        # Send a terminate request to the terminating pipe.
        # Has to be called from outside the thread (otherwise: deadlock)
        os.write(self.__termpipe, "TERMINATE")
        self.__thread.join()


def CallPipe(obj):
    """
    Returns the endpoints of the callpipe:

    result[0]: a CallPipe_Callee object, the local endpoint referencing
               the target object (listening for requests)

    result[1]: a CallPipe_Caller object, the endpoint to remotely call
               the methods on the target object (sending requests)
    """
    src, dst = multiprocessing.Pipe()
    return (CallPipe_Callee(dst, obj), CallPipe_Caller(src))


if __name__ == "__main__":
    import time

    class MyObject:
        def __init__(self, name):
            self.__name  = name
            self.__trace = []
            self.__lock  = threading.Lock()

        def add_trace(self, item):
            self.__lock.acquire()
            try:
                self.__trace.append(item)
            finally:
                self.__lock.release()

        def get_traces(self):
            self.__lock.acquire()
            try:
                return self.__trace
            finally:
                self.__lock.release()

        def raise_exception(self):
            return 1/0

    class Process1(multiprocessing.Process):
        def __init__(self, obj_proxy):
            multiprocessing.Process.__init__(self)
            self.__obj = obj_proxy

        def run(self):
            for i in xrange(10):
                print "[%d] Calling add_trace %d..." % (os.getpid(), i)
                self.__obj.add_trace("I am pid %d in loop %d" % (os.getpid(),
                                                                 i))
                time.sleep(1)

            print "[%d] Now trying to get an exception..." % os.getpid()
            try:
                self.__obj.raise_exception()
            except ZeroDivisionError, ex:
                self.__obj.add_trace('I am pid %d and got the expected exception "%s".' % (os.getpid(), ex))
            finally:
                print "[%d] Exception test executed." % os.getpid()
            print "[%d] End of process." % os.getpid()

    class Process2(multiprocessing.Process):
        def __init__(self, obj_proxy):
            multiprocessing.Process.__init__(self)
            self.__obj = obj_proxy

        def run(self):
            for i in xrange(10):
                print "[%d] Calling add_trace %d..." % (os.getpid(), i)
                self.__obj.add_trace("I am pid %d in loop %d" % (os.getpid(),
                                                                 i))
                print "[%d] Calling get_traces %d..." % (os.getpid(), i)
                self.__obj.add_trace("I am pid %d and see %d traces before."\
                                     % (os.getpid(),
                                        len(self.__obj.get_traces())))
                time.sleep(1)

            print "[%d] Pausing 3 seconds..." % os.getpid()
            time.sleep(3)
            print "[%d] Calling final get_traces:" % os.getpid()
            for t in self.__obj.get_traces():
                print "[%d]   Trace: '%s'" % (os.getpid(), t)
            print "[%d] End of process." % os.getpid()

    # Creating object called by everybody
    obj = MyObject("The object")
    obj.add_trace("[%d] I am the parent of everything." % os.getpid())

    # Creating the callpipes
    print "[%d] Creating callpipes and processes..." % os.getpid()
    callpipe1 = CallPipe(obj)
    callpipe2 = CallPipe(obj)
    p1 = Process1(callpipe1[1].get_proxy())
    p2 = Process2(callpipe2[1].get_proxy())

    #
    # The next 2 steps can be done in any order
    #

    # Starting the children processes
    print "Starting children processes..."
    p1.start()
    p2.start()

    # Starting the callpipe threads
    print "Starting callpipes..."
    callpipe1[0].start()
    callpipe2[0].start()

    #
    # The previous 2 steps could be done in any order
    #

    # Waiting for children processes to finish
    print "Waiting for processes to terminate..."
    try:
        for p in p1,p2:
            p.join()
    except KeyboardInterrupt:
        for p in p1,p2:
            p.terminate()

    print "Processes terminated. Cleaning up..."
    obj.add_trace("[%d] I am the parent, and my children are done." \
                  % os.getpid())

    # Stopping the call pipe threads
    callpipe1[0].stop()
    callpipe2[0].stop()

    # Dump the contents of the object
    print "[%d] Children done. Dumping object:" % os.getpid()
    for t in obj.get_traces():
        print "  Trace: '%s'" % t
    print "Bye."

This has been tested on Linux with processing 0.52/python 2.5 and with multiprocessing/python 2.6b3.

The example should start 2 children processes that send some text messages to the same "trace recorder" in the parent process and/or call other of its methods. The trace recorder object methods have to be thread-safe (see explanation at the beginning of the code). And exceptions occurring in the parent process while the methods of the object are remotely called, are correctly reported to the children processes.

Note that the CallPipe objects have to be created before Process::start() is called.

Also note that in the example the object to be accessed is living in the parent's image. But nothing prevents us from using the copy of the object in a child process as the main "target", and access it remotely from the parent.

1 comment

david decotigny (author) 13 years, 2 months ago  # | flag

I am thinking of extending this package so that:

  • object attributes can be remotely accessed (not just method calls)
  • it is possible to have threads in the remote process calling concurrently methods over the same callpipe (to the same target object)