Instead of a multiprocessing queue use sockets via asyncore

For a local process based executor usage currently to ensure
that task emitted notifications are proxied we use the multi
processing library and use its queue concept. This sadly creates
a proxy process that gets associated, and this proxy process
handles the queue and messages sent to and from it. Instead of
doing this we can instead just create a temporary local socket
using a random socket and have tasks (which are running in
different processes) use that to communicate back any emitted
notifications instead (and we can use the asyncore module to handle
the emitted notifications since it handles the lower level socket
reading, polling and dispatching).

To ensure that the socket created is somewhat secure we use a
similar process as the multi-processing library uses where we
sign all messages with a hmac that uses a one time key that only
the main process and the child process know about (and reject
any messages that do not validate using this key).

Change-Id: Iff9180054bf14495e5667af00ae2fafbdbc23791

3 files changed, 724 insertions, 412 deletions

diff --git a/taskflow/engines/action_engine/engine.py b/taskflow/engines/action_engine/engine.py
index e31229b..aea4b1f 100644
--- a/taskflow/engines/action_engine/engine.py
+++ b/taskflow/engines/action_engine/engine.py
@@ -31,6 +31,7 @@ import six
 from taskflow.engines.action_engine import builder
 from taskflow.engines.action_engine import compiler
 from taskflow.engines.action_engine import executor
+from taskflow.engines.action_engine import process_executor
 from taskflow.engines.action_engine import runtime
 from taskflow.engines import base
 from taskflow import exceptions as exc
@@ -502,7 +503,7 @@ class ParallelActionEngine(ActionEngine):
 Type provided              Executor used
 =========================  ===============================================
 |cft|.ThreadPoolExecutor   :class:`~.executor.ParallelThreadTaskExecutor`
-|cfp|.ProcessPoolExecutor  :class:`~.executor.ParallelProcessTaskExecutor`
+|cfp|.ProcessPoolExecutor  :class:`~.|pe|.ParallelProcessTaskExecutor`
 |cf|._base.Executor        :class:`~.executor.ParallelThreadTaskExecutor`
 =========================  ===============================================
 
@@ -514,8 +515,8 @@ Type provided              Executor used
 ===========================  ===============================================
 String (case insensitive)    Executor used
 ===========================  ===============================================
-``process``                  :class:`~.executor.ParallelProcessTaskExecutor`
-``processes``                :class:`~.executor.ParallelProcessTaskExecutor`
+``process``                  :class:`~.|pe|.ParallelProcessTaskExecutor`
+``processes``                :class:`~.|pe|.ParallelProcessTaskExecutor`
 ``thread``                   :class:`~.executor.ParallelThreadTaskExecutor`
 ``threaded``                 :class:`~.executor.ParallelThreadTaskExecutor`
 ``threads``                  :class:`~.executor.ParallelThreadTaskExecutor`
@@ -531,7 +532,7 @@ String (case insensitive)    Executor used
       workers that are used to dispatch tasks into (this number is bounded
       by the maximum parallelization your workflow can support).
 
-    * ``dispatch_periodicity``: a float (in seconds) that will affect the
+    * ``wait_timeout``: a float (in seconds) that will affect the
       parallel process task executor (and therefore is **only** applicable when
       the executor provided above is of the process variant). This number
       affects how much time the process task executor waits for messages from
@@ -540,6 +541,7 @@ String (case insensitive)    Executor used
       polling while a higher number will involve less polling but a slower time
       for an engine to notice a task has completed.
 
+    .. |pe|  replace:: process_executor
     .. |cfp| replace:: concurrent.futures.process
     .. |cft| replace:: concurrent.futures.thread
     .. |cf| replace:: concurrent.futures
@@ -555,7 +557,7 @@ String (case insensitive)    Executor used
         _ExecutorTypeMatch((futures.ThreadPoolExecutor,),
                            executor.ParallelThreadTaskExecutor),
         _ExecutorTypeMatch((futures.ProcessPoolExecutor,),
-                           executor.ParallelProcessTaskExecutor),
+                           process_executor.ParallelProcessTaskExecutor),
         _ExecutorTypeMatch((futures.Executor,),
                            executor.ParallelThreadTaskExecutor),
     ]
@@ -565,7 +567,7 @@ String (case insensitive)    Executor used
     # will be lower-cased before checking).
     _executor_str_matchers = [
         _ExecutorTextMatch(frozenset(['processes', 'process']),
-                           executor.ParallelProcessTaskExecutor),
+                           process_executor.ParallelProcessTaskExecutor),
         _ExecutorTextMatch(frozenset(['thread', 'threads', 'threaded']),
                            executor.ParallelThreadTaskExecutor),
         _ExecutorTextMatch(frozenset(['greenthread', 'greenthreads',
diff --git a/taskflow/engines/action_engine/executor.py b/taskflow/engines/action_engine/executor.py
index ff4f400..9e5c6f9 100644
--- a/taskflow/engines/action_engine/executor.py
+++ b/taskflow/engines/action_engine/executor.py
@@ -15,45 +15,19 @@
 #    under the License.
 
 import abc
-import collections
-from multiprocessing import managers
-import os
-import pickle
-import threading
 
 import futurist
-from oslo_utils import excutils
-from oslo_utils import reflection
-from oslo_utils import timeutils
-from oslo_utils import uuidutils
 import six
-from six.moves import queue as compat_queue
 
 from taskflow import logging
-from taskflow import task as task_atom
+from taskflow import task as ta
 from taskflow.types import failure
 from taskflow.types import notifier
-from taskflow.utils import threading_utils
 
 # Execution and reversion outcomes.
 EXECUTED = 'executed'
 REVERTED = 'reverted'
 
-# See http://bugs.python.org/issue1457119 for why this is so complex...
-_PICKLE_ERRORS = [pickle.PickleError, TypeError]
-try:
-    import cPickle as _cPickle
-    _PICKLE_ERRORS.append(_cPickle.PickleError)
-except ImportError:
-    pass
-_PICKLE_ERRORS = tuple(_PICKLE_ERRORS)
-_SEND_ERRORS = (IOError, EOFError)
-_UPDATE_PROGRESS = task_atom.EVENT_UPDATE_PROGRESS
-
-# Message types/kind sent from worker/child processes...
-_KIND_COMPLETE_ME = 'complete_me'
-_KIND_EVENT = 'event'
-
 LOG = logging.getLogger(__name__)
 
 
@@ -75,7 +49,7 @@ def _revert_retry(retry, arguments):
 
 def _execute_task(task, arguments, progress_callback=None):
     with notifier.register_deregister(task.notifier,
-                                      _UPDATE_PROGRESS,
+                                      ta.EVENT_UPDATE_PROGRESS,
                                       callback=progress_callback):
         try:
             task.pre_execute()
@@ -91,10 +65,10 @@ def _execute_task(task, arguments, progress_callback=None):
 
 def _revert_task(task, arguments, result, failures, progress_callback=None):
     arguments = arguments.copy()
-    arguments[task_atom.REVERT_RESULT] = result
-    arguments[task_atom.REVERT_FLOW_FAILURES] = failures
+    arguments[ta.REVERT_RESULT] = result
+    arguments[ta.REVERT_FLOW_FAILURES] = failures
     with notifier.register_deregister(task.notifier,
-                                      _UPDATE_PROGRESS,
+                                      ta.EVENT_UPDATE_PROGRESS,
                                       callback=progress_callback):
         try:
             task.pre_revert()
@@ -108,235 +82,6 @@ def _revert_task(task, arguments, result, failures, progress_callback=None):
     return (REVERTED, result)
 
 
-class _ViewableSyncManager(managers.SyncManager):
-    """Manager that exposes its state as methods."""
-
-    def is_shutdown(self):
-        return self._state.value == managers.State.SHUTDOWN
-
-    def is_running(self):
-        return self._state.value == managers.State.STARTED
-
-
-class _Channel(object):
-    """Helper wrapper around a multiprocessing queue used by a worker."""
-
-    def __init__(self, queue, identity):
-        self._queue = queue
-        self._identity = identity
-        self._sent_messages = collections.defaultdict(int)
-        self._pid = None
-
-    @property
-    def sent_messages(self):
-        return self._sent_messages
-
-    def put(self, message):
-        # NOTE(harlowja): this is done in late in execution to ensure that this
-        # happens in the child process and not the parent process (where the
-        # constructor is called).
-        if self._pid is None:
-            self._pid = os.getpid()
-        message.update({
-            'sent_on': timeutils.utcnow(),
-            'sender': {
-                'pid': self._pid,
-                'id': self._identity,
-            },
-        })
-        if 'body' not in message:
-            message['body'] = {}
-        try:
-            self._queue.put(message)
-        except _PICKLE_ERRORS:
-            LOG.warn("Failed serializing message %s", message, exc_info=True)
-            return False
-        except _SEND_ERRORS:
-            LOG.warn("Failed sending message %s", message, exc_info=True)
-            return False
-        else:
-            self._sent_messages[message['kind']] += 1
-            return True
-
-
-class _WaitWorkItem(object):
-    """The piece of work that will executed by a process executor.
-
-    This will call the target function, then wait until the tasks emitted
-    events/items have been depleted before offically being finished.
-
-    NOTE(harlowja): this is done so that the task function will *not* return
-    until all of its notifications have been proxied back to its originating
-    task. If we didn't do this then the executor would see this task as done
-    and then potentially start tasks that are successors of the task that just
-    finished even though notifications are still left to be sent from the
-    previously finished task...
-    """
-
-    def __init__(self, channel, barrier,
-                 func, task, *args, **kwargs):
-        self._channel = channel
-        self._barrier = barrier
-        self._func = func
-        self._task = task
-        self._args = args
-        self._kwargs = kwargs
-
-    def _on_finish(self):
-        sent_events = self._channel.sent_messages.get(_KIND_EVENT, 0)
-        if sent_events:
-            message = {
-                'created_on': timeutils.utcnow(),
-                'kind': _KIND_COMPLETE_ME,
-            }
-            if self._channel.put(message):
-                watch = timeutils.StopWatch()
-                watch.start()
-                self._barrier.wait()
-                LOG.trace("Waited %s seconds until task '%s' %s emitted"
-                          " notifications were depleted", watch.elapsed(),
-                          self._task, sent_events)
-
-    def __call__(self):
-        args = self._args
-        kwargs = self._kwargs
-        try:
-            return self._func(self._task, *args, **kwargs)
-        finally:
-            self._on_finish()
-
-
-class _EventSender(object):
-    """Sends event information from a child worker process to its creator."""
-
-    def __init__(self, channel):
-        self._channel = channel
-
-    def __call__(self, event_type, details):
-        message = {
-            'created_on': timeutils.utcnow(),
-            'kind': _KIND_EVENT,
-            'body': {
-                'event_type': event_type,
-                'details': details,
-            },
-        }
-        self._channel.put(message)
-
-
-class _Target(object):
-    """An immutable helper object that represents a target of a message."""
-
-    def __init__(self, task, barrier, identity):
-        self.task = task
-        self.barrier = barrier
-        self.identity = identity
-        # Counters used to track how many message 'kinds' were proxied...
-        self.dispatched = collections.defaultdict(int)
-
-    def __repr__(self):
-        return "<%s at 0x%x targeting '%s' with identity '%s'>" % (
-            reflection.get_class_name(self), id(self),
-            self.task, self.identity)
-
-
-class _Dispatcher(object):
-    """Dispatches messages received from child worker processes."""
-
-    # When the run() method is busy (typically in a thread) we want to set
-    # these so that the thread can know how long to sleep when there is no
-    # active work to dispatch.
-    _SPIN_PERIODICITY = 0.01
-
-    def __init__(self, dispatch_periodicity=None):
-        if dispatch_periodicity is None:
-            dispatch_periodicity = self._SPIN_PERIODICITY
-        if dispatch_periodicity <= 0:
-            raise ValueError("Provided dispatch periodicity must be greater"
-                             " than zero and not '%s'" % dispatch_periodicity)
-        self._targets = {}
-        self._dead = threading.Event()
-        self._dispatch_periodicity = dispatch_periodicity
-        self._stop_when_empty = False
-
-    def register(self, identity, target):
-        self._targets[identity] = target
-
-    def deregister(self, identity):
-        try:
-            target = self._targets.pop(identity)
-        except KeyError:
-            pass
-        else:
-            # Just incase set the barrier to unblock any worker...
-            target.barrier.set()
-            if LOG.isEnabledFor(logging.TRACE):
-                LOG.trace("Dispatched %s messages %s to target '%s' during"
-                          " the lifetime of its existence in the dispatcher",
-                          sum(six.itervalues(target.dispatched)),
-                          dict(target.dispatched), target)
-
-    def reset(self):
-        self._stop_when_empty = False
-        self._dead.clear()
-        if self._targets:
-            leftover = set(six.iterkeys(self._targets))
-            while leftover:
-                self.deregister(leftover.pop())
-
-    def interrupt(self):
-        self._stop_when_empty = True
-        self._dead.set()
-
-    def _dispatch(self, message):
-        if LOG.isEnabledFor(logging.TRACE):
-            LOG.trace("Dispatching message %s (it took %s seconds"
-                      " for it to arrive for processing after being"
-                      " sent)", message,
-                      timeutils.delta_seconds(message['sent_on'],
-                                              timeutils.utcnow()))
-        try:
-            kind = message['kind']
-            sender = message['sender']
-            body = message['body']
-        except (KeyError, ValueError, TypeError):
-            LOG.warn("Badly formatted message %s received", message,
-                     exc_info=True)
-            return
-        target = self._targets.get(sender['id'])
-        if target is None:
-            # Must of been removed...
-            return
-        if kind == _KIND_COMPLETE_ME:
-            target.dispatched[kind] += 1
-            target.barrier.set()
-        elif kind == _KIND_EVENT:
-            task = target.task
-            target.dispatched[kind] += 1
-            task.notifier.notify(body['event_type'], body['details'])
-        else:
-            LOG.warn("Unknown message '%s' found in message from sender"
-                     " %s to target '%s'", kind, sender, target)
-
-    def run(self, queue):
-        watch = timeutils.StopWatch(duration=self._dispatch_periodicity)
-        while (not self._dead.is_set() or
-               (self._stop_when_empty and self._targets)):
-            watch.restart()
-            leftover = watch.leftover()
-            while leftover:
-                try:
-                    message = queue.get(timeout=leftover)
-                except compat_queue.Empty:
-                    break
-                else:
-                    self._dispatch(message)
-                    leftover = watch.leftover()
-            leftover = watch.leftover()
-            if leftover:
-                self._dead.wait(leftover)
-
-
 class SerialRetryExecutor(object):
     """Executes and reverts retries."""
 
@@ -492,149 +237,3 @@ class ParallelGreenThreadTaskExecutor(ParallelThreadTaskExecutor):
         if max_workers is None:
             max_workers = self.DEFAULT_WORKERS
         return futurist.GreenThreadPoolExecutor(max_workers=max_workers)
-
-
-class ParallelProcessTaskExecutor(ParallelTaskExecutor):
-    """Executes tasks in parallel using a process pool executor.
-
-    NOTE(harlowja): this executor executes tasks in external processes, so that
-    implies that tasks that are sent to that external process are pickleable
-    since this is how the multiprocessing works (sending pickled objects back
-    and forth) and that the bound handlers (for progress updating in
-    particular) are proxied correctly from that external process to the one
-    that is alive in the parent process to ensure that callbacks registered in
-    the parent are executed on events in the child.
-    """
-
-    constructor_options = [
-        ('max_workers', lambda v: v if v is None else int(v)),
-        ('dispatch_periodicity', lambda v: v if v is None else float(v)),
-    ]
-    """
-    Optional constructor keyword arguments this executor supports. These will
-    typically be passed via engine options (by a engine user) and converted
-    into the correct type before being sent into this
-    classes ``__init__`` method.
-    """
-
-    def __init__(self, executor=None, max_workers=None,
-                 dispatch_periodicity=None):
-        super(ParallelProcessTaskExecutor, self).__init__(
-            executor=executor, max_workers=max_workers)
-        self._manager = _ViewableSyncManager()
-        self._dispatcher = _Dispatcher(
-            dispatch_periodicity=dispatch_periodicity)
-        # Only created after starting...
-        self._worker = None
-        self._queue = None
-
-    def _create_executor(self, max_workers=None):
-        return futurist.ProcessPoolExecutor(max_workers=max_workers)
-
-    def start(self):
-        if threading_utils.is_alive(self._worker):
-            raise RuntimeError("Worker thread must be stopped via stop()"
-                               " before starting/restarting")
-        super(ParallelProcessTaskExecutor, self).start()
-        # These don't seem restartable; make a new one...
-        if self._manager.is_shutdown():
-            self._manager = _ViewableSyncManager()
-        if not self._manager.is_running():
-            self._manager.start()
-        self._dispatcher.reset()
-        self._queue = self._manager.Queue()
-        self._worker = threading_utils.daemon_thread(self._dispatcher.run,
-                                                     self._queue)
-        self._worker.start()
-
-    def stop(self):
-        self._dispatcher.interrupt()
-        super(ParallelProcessTaskExecutor, self).stop()
-        if threading_utils.is_alive(self._worker):
-            self._worker.join()
-            self._worker = None
-            self._queue = None
-        self._dispatcher.reset()
-        self._manager.shutdown()
-        self._manager.join()
-
-    def _rebind_task(self, task, clone, channel, progress_callback=None):
-        # Creates and binds proxies for all events the task could receive
-        # so that when the clone runs in another process that this task
-        # can recieve the same notifications (thus making it look like the
-        # the notifications are transparently happening in this process).
-        needed = set()
-        for (event_type, listeners) in task.notifier.listeners_iter():
-            if listeners:
-                needed.add(event_type)
-        if progress_callback is not None:
-            needed.add(_UPDATE_PROGRESS)
-        if needed:
-            sender = _EventSender(channel)
-            for event_type in needed:
-                clone.notifier.register(event_type, sender)
-
-    def _submit_task(self, func, task, *args, **kwargs):
-        """Submit a function to run the given task (with given args/kwargs).
-
-        NOTE(harlowja): Adjust all events to be proxies instead since we want
-        those callbacks to be activated in this process, not in the child,
-        also since typically callbacks are functors (or callables) we can
-        not pickle those in the first place...
-
-        To make sure people understand how this works, the following is a
-        lengthy description of what is going on here, read at will:
-
-        So to ensure that we are proxying task triggered events that occur
-        in the executed subprocess (which will be created and used by the
-        thing using the multiprocessing based executor) we need to establish
-        a link between that process and this process that ensures that when a
-        event is triggered in that task in that process that a corresponding
-        event is triggered on the original task that was requested to be ran
-        in this process.
-
-        To accomplish this we have to create a copy of the task (without
-        any listeners) and then reattach a new set of listeners that will
-        now instead of calling the desired listeners just place messages
-        for this process (a dispatcher thread that is created in this class)
-        to dispatch to the original task (using a common queue + per task
-        sender identity/target that is used and associated to know which task
-        to proxy back too, since it is possible that there many be *many*
-        subprocess running at the same time, each running a different task
-        and using the same common queue to submit messages back to).
-
-        Once the subprocess task has finished execution, the executor will
-        then trigger a callback that will remove the task + target from the
-        dispatcher (which will stop any further proxying back to the original
-        task).
-        """
-        progress_callback = kwargs.pop('progress_callback', None)
-        clone = task.copy(retain_listeners=False)
-        identity = uuidutils.generate_uuid()
-        target = _Target(task, self._manager.Event(), identity)
-        channel = _Channel(self._queue, identity)
-        self._rebind_task(task, clone, channel,
-                          progress_callback=progress_callback)
-
-        def register():
-            if progress_callback is not None:
-                task.notifier.register(_UPDATE_PROGRESS, progress_callback)
-            self._dispatcher.register(identity, target)
-
-        def deregister():
-            if progress_callback is not None:
-                task.notifier.deregister(_UPDATE_PROGRESS, progress_callback)
-            self._dispatcher.deregister(identity)
-
-        register()
-        work = _WaitWorkItem(channel, target.barrier,
-                             func, clone, *args, **kwargs)
-        try:
-            fut = self._executor.submit(work)
-        except RuntimeError:
-            with excutils.save_and_reraise_exception():
-                deregister()
-
-        fut.atom = task
-        fut.add_done_callback(lambda fut: deregister())
-        return fut
diff --git a/taskflow/engines/action_engine/process_executor.py b/taskflow/engines/action_engine/process_executor.py
new file mode 100644
index 0000000..c45a92d
--- /dev/null
+++ b/taskflow/engines/action_engine/process_executor.py
@@ -0,0 +1,711 @@
+# -*- coding: utf-8 -*-
+
+#    Copyright (C) 2015 Yahoo! Inc. All Rights Reserved.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+import asyncore
+import binascii
+import collections
+import errno
+import functools
+import hmac
+import math
+import os
+import pickle
+import socket
+import struct
+import time
+
+import futurist
+from oslo_utils import excutils
+import six
+
+from taskflow.engines.action_engine import executor as base
+from taskflow import logging
+from taskflow import task as ta
+from taskflow.utils import iter_utils
+from taskflow.utils import misc
+from taskflow.utils import schema_utils as su
+from taskflow.utils import threading_utils
+
+LOG = logging.getLogger(__name__)
+
+# Internal parent <-> child process protocol schema, message constants...
+MAGIC_HEADER = 0xDECAF
+CHALLENGE = 'identify_yourself'
+CHALLENGE_RESPONSE = 'worker_reporting_in'
+ACK = 'ack'
+EVENT = 'event'
+SCHEMAS = {
+    # Basic jsonschemas for verifying that the data we get back and
+    # forth from parent <-> child observes at least a basic expected
+    # format.
+    CHALLENGE: {
+        "type": "string",
+        "minLength": 1,
+    },
+    ACK: {
+        "type": "string",
+        "minLength": 1,
+    },
+    CHALLENGE_RESPONSE: {
+        "type": "string",
+        "minLength": 1,
+    },
+    EVENT: {
+        "type": "object",
+        "properties": {
+            'event_type': {
+                "type": "string",
+            },
+            'sent_on': {
+                "type": "number",
+            },
+        },
+        "required": ['event_type', 'sent_on'],
+        "additionalProperties": True,
+    },
+}
+
+# See http://bugs.python.org/issue1457119 for why this is so complex...
+_DECODE_ENCODE_ERRORS = [pickle.PickleError, TypeError]
+try:
+    import cPickle
+    _DECODE_ENCODE_ERRORS.append(cPickle.PickleError)
+    del cPickle
+except (ImportError, AttributeError):
+    pass
+_DECODE_ENCODE_ERRORS = tuple(_DECODE_ENCODE_ERRORS)
+
+# Use the best pickle from here on out...
+from six.moves import cPickle as pickle
+
+
+class UnknownSender(Exception):
+    """Exception raised when message from unknown sender is recvd."""
+
+
+class ChallengeIgnored(Exception):
+    """Exception raised when challenge has not been responded to."""
+
+
+class Reader(object):
+    """Reader machine that streams & parses messages that it then dispatches.
+
+    TODO(harlowja): Use python-suitcase in the future when the following
+    are addressed/resolved and released:
+
+    - https://github.com/digidotcom/python-suitcase/issues/28
+    - https://github.com/digidotcom/python-suitcase/issues/29
+
+    Binary format format is the following (no newlines in actual format)::
+
+        <magic-header> (4 bytes)
+        <mac-header-length> (4 bytes)
+        <mac> (1 or more variable bytes)
+        <identity-header-length> (4 bytes)
+        <identity> (1 or more variable bytes)
+        <msg-header-length> (4 bytes)
+        <msg> (1 or more variable bytes)
+    """
+
+    #: Per state memory initializers.
+    _INITIALIZERS = {
+        'magic_header_left': 4,
+        'mac_header_left': 4,
+        'identity_header_left': 4,
+        'msg_header_left': 4,
+    }
+
+    #: Linear steps/transitions (order matters here).
+    _TRANSITIONS = tuple([
+        'magic_header_left',
+        'mac_header_left',
+        'mac_left',
+        'identity_header_left',
+        'identity_left',
+        'msg_header_left',
+        'msg_left',
+    ])
+
+    def __init__(self, auth_key, dispatch_func, msg_limit=-1):
+        if not six.callable(dispatch_func):
+            raise ValueError("Expected provided dispatch function"
+                             " to be callable")
+        self.auth_key = auth_key
+        self.dispatch_func = dispatch_func
+        msg_limiter = iter_utils.iter_forever(msg_limit)
+        self.msg_count = six.next(msg_limiter)
+        self._msg_limiter = msg_limiter
+        self._buffer = misc.BytesIO()
+        self._state = None
+        # Local machine variables and such are stored in here.
+        self._memory = {}
+        self._transitions = collections.deque(self._TRANSITIONS)
+        # This is the per state callback handler set. The first entry reads
+        # the data and the second entry is called after reading is completed,
+        # typically to save that data into object memory, or to validate
+        # it.
+        self._handlers = {
+            'magic_header_left': (self._read_field_data,
+                                  self._save_and_validate_magic),
+            'mac_header_left': (self._read_field_data,
+                                functools.partial(self._save_pos_integer,
+                                                  'mac_left')),
+            'mac_left': (functools.partial(self._read_data, 'mac'),
+                         functools.partial(self._save_data, 'mac')),
+            'identity_header_left': (self._read_field_data,
+                                     functools.partial(self._save_pos_integer,
+                                                       'identity_left')),
+            'identity_left': (functools.partial(self._read_data, 'identity'),
+                              functools.partial(self._save_data, 'identity')),
+            'msg_header_left': (self._read_field_data,
+                                functools.partial(self._save_pos_integer,
+                                                  'msg_left')),
+            'msg_left': (functools.partial(self._read_data, 'msg'),
+                         self._dispatch_and_reset),
+        }
+        # Force transition into first state...
+        self._transition()
+
+    def _save_pos_integer(self, key_name, data):
+        key_val = struct.unpack("!i", data)[0]
+        if key_val <= 0:
+            raise IOError("Invalid %s length received for key '%s', expected"
+                          " greater than zero length" % (key_val, key_name))
+        self._memory[key_name] = key_val
+        return True
+
+    def _save_data(self, key_name, data):
+        self._memory[key_name] = data
+        return True
+
+    def _dispatch_and_reset(self, data):
+        self.dispatch_func(
+            self._memory['identity'],
+            # Lazy evaluate so the message can be thrown out as needed
+            # (instead of the receiver discarding it after the fact)...
+            functools.partial(_decode_message, self.auth_key, data,
+                              self._memory['mac']))
+        self.msg_count = six.next(self._msg_limiter)
+        self._memory.clear()
+
+    def _transition(self):
+        try:
+            self._state = self._transitions.popleft()
+        except IndexError:
+            self._transitions.extend(self._TRANSITIONS)
+            self._state = self._transitions.popleft()
+        try:
+            self._memory[self._state] = self._INITIALIZERS[self._state]
+        except KeyError:
+            pass
+        self._handle_func, self._post_handle_func = self._handlers[self._state]
+
+    def _save_and_validate_magic(self, data):
+        magic_header = struct.unpack("!i", data)[0]
+        if magic_header != MAGIC_HEADER:
+            raise IOError("Invalid magic header received, expected 0x%x but"
+                          " got 0x%x for message %s" % (MAGIC_HEADER,
+                                                        magic_header,
+                                                        self.msg_count + 1))
+        self._memory['magic'] = magic_header
+        return True
+
+    def _read_data(self, save_key_name, data):
+        data_len_left = self._memory[self._state]
+        self._buffer.write(data[0:data_len_left])
+        if len(data) < data_len_left:
+            data_len_left -= len(data)
+            self._memory[self._state] = data_len_left
+            return ''
+        else:
+            self._memory[self._state] = 0
+            buf_data = self._buffer.getvalue()
+            self._buffer.reset()
+            self._post_handle_func(buf_data)
+            self._transition()
+            return data[data_len_left:]
+
+    def _read_field_data(self, data):
+        return self._read_data(self._state, data)
+
+    @property
+    def bytes_needed(self):
+        return self._memory.get(self._state, 0)
+
+    def feed(self, data):
+        while len(data):
+            data = self._handle_func(data)
+
+
+class BadHmacValueError(ValueError):
+    """Value error raised when an invalid hmac is discovered."""
+
+
+def _create_random_string(desired_length):
+    if desired_length <= 0:
+        return b''
+    data_length = int(math.ceil(desired_length / 2.0))
+    data = os.urandom(data_length)
+    hex_data = binascii.hexlify(data)
+    return hex_data[0:desired_length]
+
+
+def _calculate_hmac(auth_key, body):
+    mac = hmac.new(auth_key, body).hexdigest()
+    if isinstance(mac, six.text_type):
+        mac = mac.encode("ascii")
+    return mac
+
+
+def _encode_message(auth_key, message, identity, reverse=False):
+    message = pickle.dumps(message, 2)
+    message_mac = _calculate_hmac(auth_key, message)
+    pieces = [
+        struct.pack("!i", MAGIC_HEADER),
+        struct.pack("!i", len(message_mac)),
+        message_mac,
+        struct.pack("!i", len(identity)),
+        identity,
+        struct.pack("!i", len(message)),
+        message,
+    ]
+    if reverse:
+        pieces.reverse()
+    return tuple(pieces)
+
+
+def _decode_message(auth_key, message, message_mac):
+    tmp_message_mac = _calculate_hmac(auth_key, message)
+    if tmp_message_mac != message_mac:
+        raise BadHmacValueError('Invalid message hmac')
+    return pickle.loads(message)
+
+
+class Channel(object):
+    """Object that workers use to communicate back to their creator."""
+
+    def __init__(self, port, identity, auth_key):
+        self.identity = identity
+        self.port = port
+        self.auth_key = auth_key
+        self.dead = False
+        self._sent = self._received = 0
+        self._socket = None
+        self._read_pipe = None
+        self._write_pipe = None
+
+    def close(self):
+        if self._socket is not None:
+            self._socket.close()
+            self._socket = None
+            self._read_pipe = None
+            self._write_pipe = None
+
+    def _ensure_connected(self):
+        if self._socket is None:
+            s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+            s.setblocking(1)
+            try:
+                s.connect(("", self.port))
+            except socket.error as e:
+                with excutils.save_and_reraise_exception():
+                    s.close()
+                    if e.errno in (errno.ECONNREFUSED, errno.ENOTCONN,
+                                   errno.ECONNRESET):
+                        # Don't bother with further connections...
+                        self.dead = True
+            read_pipe = s.makefile("rb", 0)
+            write_pipe = s.makefile("wb", 0)
+            try:
+                msg = self._do_recv(read_pipe=read_pipe)
+                su.schema_validate(msg, SCHEMAS[CHALLENGE])
+                if msg != CHALLENGE:
+                    raise IOError("Challenge expected not received")
+                else:
+                    pieces = _encode_message(self.auth_key,
+                                             CHALLENGE_RESPONSE,
+                                             self.identity)
+                    self._do_send_and_ack(pieces, write_pipe=write_pipe,
+                                          read_pipe=read_pipe)
+            except Exception:
+                with excutils.save_and_reraise_exception():
+                    s.close()
+            else:
+                self._socket = s
+                self._read_pipe = read_pipe
+                self._write_pipe = write_pipe
+
+    def recv(self):
+        self._ensure_connected()
+        return self._do_recv()
+
+    def _do_recv(self, read_pipe=None):
+        if read_pipe is None:
+            read_pipe = self._read_pipe
+        msg_capture = collections.deque(maxlen=1)
+        msg_capture_func = (lambda _from_who, msg_decoder_func:
+                            msg_capture.append(msg_decoder_func()))
+        reader = Reader(self.auth_key, msg_capture_func, msg_limit=1)
+        try:
+            maybe_msg_num = self._received + 1
+            bytes_needed = reader.bytes_needed
+            while True:
+                blob = read_pipe.read(bytes_needed)
+                if len(blob) != bytes_needed:
+                    raise EOFError("Read pipe closed while reading %s"
+                                   " bytes for potential message %s"
+                                   % (bytes_needed, maybe_msg_num))
+                reader.feed(blob)
+                bytes_needed = reader.bytes_needed
+        except StopIteration:
+            pass
+        msg = msg_capture[0]
+        self._received += 1
+        return msg
+
+    def _do_send(self, pieces, write_pipe=None):
+        if write_pipe is None:
+            write_pipe = self._write_pipe
+        for piece in pieces:
+            write_pipe.write(piece)
+        write_pipe.flush()
+
+    def _do_send_and_ack(self, pieces, write_pipe=None, read_pipe=None):
+        self._do_send(pieces, write_pipe=write_pipe)
+        self._sent += 1
+        msg = self._do_recv(read_pipe=read_pipe)
+        su.schema_validate(msg, SCHEMAS[ACK])
+        if msg != ACK:
+            raise IOError("Failed receiving ack for sent"
+                          " message %s" % self._metrics['sent'])
+
+    def send(self, message):
+        self._ensure_connected()
+        self._do_send_and_ack(_encode_message(self.auth_key, message,
+                                              self.identity))
+
+
+class EventSender(object):
+    """Sends event information from a child worker process to its creator."""
+
+    def __init__(self, channel):
+        self._channel = channel
+        self._pid = None
+
+    def __call__(self, event_type, details):
+        if not self._channel.dead:
+            if self._pid is None:
+                self._pid = os.getpid()
+            message = {
+                'event_type': event_type,
+                'details': details,
+                'sent_on': time.time(),
+            }
+            LOG.trace("Sending %s (from child %s)", message, self._pid)
+            self._channel.send(message)
+
+
+class DispatcherHandler(asyncore.dispatcher):
+    """Dispatches from a single connection into a target."""
+
+    #: Read/write chunk size.
+    CHUNK_SIZE = 8192
+
+    def __init__(self, sock, addr, dispatcher):
+        if six.PY2:
+            asyncore.dispatcher.__init__(self, map=dispatcher.map, sock=sock)
+        else:
+            super(DispatcherHandler, self).__init__(map=dispatcher.map,
+                                                    sock=sock)
+        self.blobs_to_write = list(dispatcher.challenge_pieces)
+        self.reader = Reader(dispatcher.auth_key, self._dispatch)
+        self.targets = dispatcher.targets
+        self.tied_to = None
+        self.challenge_responded = False
+        self.ack_pieces = _encode_message(dispatcher.auth_key, ACK,
+                                          dispatcher.identity,
+                                          reverse=True)
+        self.addr = addr
+
+    def handle_close(self):
+        self.close()
+
+    def writable(self):
+        return bool(self.blobs_to_write)
+
+    def handle_write(self):
+        try:
+            blob = self.blobs_to_write.pop()
+        except IndexError:
+            pass
+        else:
+            sent = self.send(blob[0:self.CHUNK_SIZE])
+            if sent < len(blob):
+                self.blobs_to_write.append(blob[sent:])
+
+    def _send_ack(self):
+        self.blobs_to_write.extend(self.ack_pieces)
+
+    def _dispatch(self, from_who, msg_decoder_func):
+        if not self.challenge_responded:
+            msg = msg_decoder_func()
+            su.schema_validate(msg, SCHEMAS[CHALLENGE_RESPONSE])
+            if msg != CHALLENGE_RESPONSE:
+                raise ChallengeIgnored("Discarding connection from %s"
+                                       " challenge was not responded to"
+                                       % self.addr)
+            else:
+                LOG.trace("Peer %s (%s) has passed challenge sequence",
+                          self.addr, from_who)
+                self.challenge_responded = True
+                self.tied_to = from_who
+                self._send_ack()
+        else:
+            if self.tied_to != from_who:
+                raise UnknownSender("Sender %s previously identified as %s"
+                                    " changed there identity to %s after"
+                                    " challenge sequence" % (self.addr,
+                                                             self.tied_to,
+                                                             from_who))
+            try:
+                task = self.targets[from_who]
+            except KeyError:
+                raise UnknownSender("Unknown message from %s (%s) not matched"
+                                    " to any known target" % (self.addr,
+                                                              from_who))
+            msg = msg_decoder_func()
+            su.schema_validate(msg, SCHEMAS[EVENT])
+            if LOG.isEnabledFor(logging.TRACE):
+                msg_delay = max(0, time.time() - msg['sent_on'])
+                LOG.trace("Dispatching message from %s (%s) (it took %0.3f"
+                          " seconds for it to arrive for processing after"
+                          " being sent)", self.addr, from_who, msg_delay)
+            task.notifier.notify(msg['event_type'], msg.get('details'))
+            self._send_ack()
+
+    def handle_read(self):
+        data = self.recv(self.CHUNK_SIZE)
+        if len(data) == 0:
+            self.handle_close()
+        else:
+            try:
+                self.reader.feed(data)
+            except (IOError, UnknownSender):
+                LOG.warn("Invalid received message", exc_info=True)
+                self.handle_close()
+            except _DECODE_ENCODE_ERRORS:
+                LOG.warn("Badly formatted message", exc_info=True)
+                self.handle_close()
+            except (ValueError, su.ValidationError):
+                LOG.warn("Failed validating message", exc_info=True)
+                self.handle_close()
+            except ChallengeIgnored:
+                LOG.warn("Failed challenge sequence", exc_info=True)
+                self.handle_close()
+
+
+class Dispatcher(asyncore.dispatcher):
+    """Accepts messages received from child worker processes."""
+
+    #: See https://docs.python.org/2/library/socket.html#socket.socket.listen
+    MAX_BACKLOG = 5
+
+    def __init__(self, map, auth_key, identity):
+        if six.PY2:
+            asyncore.dispatcher.__init__(self, map=map)
+        else:
+            super(Dispatcher, self).__init__(map=map)
+        self.identity = identity
+        self.challenge_pieces = _encode_message(auth_key, CHALLENGE,
+                                                identity, reverse=True)
+        self.auth_key = auth_key
+        self.targets = {}
+
+    @property
+    def port(self):
+        if self.socket is not None:
+            return self.socket.getsockname()[1]
+        else:
+            return None
+
+    def setup(self):
+        self.targets.clear()
+        self.create_socket(socket.AF_INET, socket.SOCK_STREAM)
+        self.bind(("", 0))
+        LOG.trace("Accepting dispatch requests on port %s", self.port)
+        self.listen(self.MAX_BACKLOG)
+
+    def writable(self):
+        return False
+
+    @property
+    def map(self):
+        return self._map
+
+    def handle_close(self):
+        if self.socket is not None:
+            self.close()
+
+    def handle_accept(self):
+        pair = self.accept()
+        if pair is not None:
+            sock, addr = pair
+            addr = "%s:%s" % (addr[0], addr[1])
+            LOG.trace("Potentially accepted new connection from %s", addr)
+            DispatcherHandler(sock, addr, self)
+
+
+class ParallelProcessTaskExecutor(base.ParallelTaskExecutor):
+    """Executes tasks in parallel using a process pool executor.
+
+    NOTE(harlowja): this executor executes tasks in external processes, so that
+    implies that tasks that are sent to that external process are pickleable
+    since this is how the multiprocessing works (sending pickled objects back
+    and forth) and that the bound handlers (for progress updating in
+    particular) are proxied correctly from that external process to the one
+    that is alive in the parent process to ensure that callbacks registered in
+    the parent are executed on events in the child.
+    """
+
+    #: Default timeout used by asyncore io loop (and eventually select/poll).
+    WAIT_TIMEOUT = 0.01
+
+    constructor_options = [
+        ('max_workers', lambda v: v if v is None else int(v)),
+        ('wait_timeout', lambda v: v if v is None else float(v)),
+    ]
+    """
+    Optional constructor keyword arguments this executor supports. These will
+    typically be passed via engine options (by a engine user) and converted
+    into the correct type before being sent into this
+    classes ``__init__`` method.
+    """
+
+    def __init__(self, executor=None,
+                 max_workers=None, wait_timeout=None):
+        super(ParallelProcessTaskExecutor, self).__init__(
+            executor=executor, max_workers=max_workers)
+        self._auth_key = _create_random_string(32)
+        self._dispatcher = Dispatcher({}, self._auth_key,
+                                      _create_random_string(32))
+        if wait_timeout is None:
+            self._wait_timeout = self.WAIT_TIMEOUT
+        else:
+            if wait_timeout <= 0:
+                raise ValueError("Provided wait timeout must be greater"
+                                 " than zero and not '%s'" % wait_timeout)
+            self._wait_timeout = wait_timeout
+        # Only created after starting...
+        self._worker = None
+
+    def _create_executor(self, max_workers=None):
+        return futurist.ProcessPoolExecutor(max_workers=max_workers)
+
+    def start(self):
+        if threading_utils.is_alive(self._worker):
+            raise RuntimeError("Worker thread must be stopped via stop()"
+                               " before starting/restarting")
+        super(ParallelProcessTaskExecutor, self).start()
+        self._dispatcher.setup()
+        self._worker = threading_utils.daemon_thread(
+            asyncore.loop, map=self._dispatcher.map,
+            timeout=self._wait_timeout)
+        self._worker.start()
+
+    def stop(self):
+        super(ParallelProcessTaskExecutor, self).stop()
+        self._dispatcher.close()
+        if threading_utils.is_alive(self._worker):
+            self._worker.join()
+            self._worker = None
+
+    def _submit_task(self, func, task, *args, **kwargs):
+        """Submit a function to run the given task (with given args/kwargs).
+
+        NOTE(harlowja): Adjust all events to be proxies instead since we want
+        those callbacks to be activated in this process, not in the child,
+        also since typically callbacks are functors (or callables) we can
+        not pickle those in the first place...
+
+        To make sure people understand how this works, the following is a
+        lengthy description of what is going on here, read at will:
+
+        So to ensure that we are proxying task triggered events that occur
+        in the executed subprocess (which will be created and used by the
+        thing using the multiprocessing based executor) we need to establish
+        a link between that process and this process that ensures that when a
+        event is triggered in that task in that process that a corresponding
+        event is triggered on the original task that was requested to be ran
+        in this process.
+
+        To accomplish this we have to create a copy of the task (without
+        any listeners) and then reattach a new set of listeners that will
+        now instead of calling the desired listeners just place messages
+        for this process (a dispatcher thread that is created in this class)
+        to dispatch to the original task (using a common accepting socket and
+        per task sender socket that is used and associated to know
+        which task to proxy back too, since it is possible that there many
+        be *many* subprocess running at the same time).
+
+        Once the subprocess task has finished execution, the executor will
+        then trigger a callback that will remove the task + target from the
+        dispatcher (which will stop any further proxying back to the original
+        task).
+        """
+        progress_callback = kwargs.pop('progress_callback', None)
+        clone = task.copy(retain_listeners=False)
+        identity = _create_random_string(32)
+        channel = Channel(self._dispatcher.port, identity, self._auth_key)
+
+        def rebind_task():
+            # Creates and binds proxies for all events the task could receive
+            # so that when the clone runs in another process that this task
+            # can recieve the same notifications (thus making it look like the
+            # the notifications are transparently happening in this process).
+            needed = set()
+            for (event_type, listeners) in task.notifier.listeners_iter():
+                if listeners:
+                    needed.add(event_type)
+            if progress_callback is not None:
+                needed.add(ta.EVENT_UPDATE_PROGRESS)
+            if needed:
+                sender = EventSender(channel)
+                for event_type in needed:
+                    clone.notifier.register(event_type, sender)
+
+        def register():
+            if progress_callback is not None:
+                task.notifier.register(ta.EVENT_UPDATE_PROGRESS,
+                                       progress_callback)
+            self._dispatcher.targets[identity] = task
+
+        def deregister(fut=None):
+            if progress_callback is not None:
+                task.notifier.deregister(ta.EVENT_UPDATE_PROGRESS,
+                                         progress_callback)
+            self._dispatcher.targets.pop(identity, None)
+
+        rebind_task()
+        register()
+        try:
+            fut = self._executor.submit(func, clone, *args, **kwargs)
+        except RuntimeError:
+            with excutils.save_and_reraise_exception():
+                deregister()
+
+        fut.atom = task
+        fut.add_done_callback(deregister)
+        return fut