replace celery task decorators with a kombu-based publisher

this commit implements the bulk of `awx-manage run_dispatcher`, a new
command that binds to RabbitMQ via kombu and balances messages across
a pool of workers that are similar to celeryd workers in spirit.
Specifically, this includes:

- a new decorator, `awx.main.dispatch.task`, which can be used to
  decorate functions or classes so that they can be designated as
  "Tasks"
- support for fanout/broadcast tasks (at this point in time, only
  `conf.Setting` memcached flushes use this functionality)
- support for job reaping
- support for success/failure hooks for job runs (i.e.,
  `handle_work_success` and `handle_work_error`)
- support for auto scaling worker pool that scale processes up and down
  on demand
- minimal support for RPC, such as status checks and pool recycle/reload

awx/main/dispatch/worker/base.py

@@ -0,0 +1,146 @@
+# Copyright (c) 2018 Ansible by Red Hat
+# All Rights Reserved.
+
+import os
+import logging
+import signal
+from uuid import UUID
+from Queue import Empty as QueueEmpty
+
+from kombu import Producer
+from kombu.mixins import ConsumerMixin
+
+from awx.main.dispatch.pool import WorkerPool
+
+logger = logging.getLogger('awx.main.dispatch')
+
+
+def signame(sig):
+    return dict(
+        (k, v) for v, k in signal.__dict__.items()
+        if v.startswith('SIG') and not v.startswith('SIG_')
+    )[sig]
+
+
+class WorkerSignalHandler:
+
+    def __init__(self):
+        self.kill_now = False
+        signal.signal(signal.SIGINT, self.exit_gracefully)
+
+    def exit_gracefully(self, *args, **kwargs):
+        self.kill_now = True
+
+
+class AWXConsumer(ConsumerMixin):
+
+    def __init__(self, name, connection, worker, queues=[], pool=None):
+        self.connection = connection
+        self.total_messages = 0
+        self.queues = queues
+        self.worker = worker
+        self.pool = pool
+        if pool is None:
+            self.pool = WorkerPool()
+        self.pool.init_workers(self.worker.work_loop)
+
+    def get_consumers(self, Consumer, channel):
+        logger.debug(self.listening_on)
+        return [Consumer(queues=self.queues, accept=['json'],
+                         callbacks=[self.process_task])]
+
+    @property
+    def listening_on(self):
+        return 'listening on {}'.format([
+            '{} [{}]'.format(q.name, q.exchange.type) for q in self.queues
+        ])
+
+    def control(self, body, message):
+        logger.warn(body)
+        control = body.get('control')
+        if control in ('status', 'running'):
+            producer = Producer(
+                channel=self.connection,
+                routing_key=message.properties['reply_to']
+            )
+            if control == 'status':
+                msg = '\n'.join([self.listening_on, self.pool.debug()])
+            elif control == 'running':
+                msg = []
+                for worker in self.pool.workers:
+                    worker.calculate_managed_tasks()
+                    msg.extend(worker.managed_tasks.keys())
+            producer.publish(msg)
+        elif control == 'reload':
+            for worker in self.pool.workers:
+                worker.quit()
+        else:
+            logger.error('unrecognized control message: {}'.format(control))
+        message.ack()
+
+    def process_task(self, body, message):
+        if 'control' in body:
+            return self.control(body, message)
+        if len(self.pool):
+            if "uuid" in body and body['uuid']:
+                try:
+                    queue = UUID(body['uuid']).int % len(self.pool)
+                except Exception:
+                    queue = self.total_messages % len(self.pool)
+            else:
+                queue = self.total_messages % len(self.pool)
+        else:
+            queue = 0
+        self.pool.write(queue, body)
+        self.total_messages += 1
+        message.ack()
+
+    def run(self, *args, **kwargs):
+        signal.signal(signal.SIGINT, self.stop)
+        signal.signal(signal.SIGTERM, self.stop)
+        self.worker.on_start()
+        super(AWXConsumer, self).run(*args, **kwargs)
+
+    def stop(self, signum, frame):
+        self.should_stop = True  # this makes the kombu mixin stop consuming
+        logger.debug('received {}, stopping'.format(signame(signum)))
+        self.worker.on_stop()
+        raise SystemExit()
+
+
+class BaseWorker(object):
+
+    def work_loop(self, queue, finished, idx, *args):
+        ppid = os.getppid()
+        signal_handler = WorkerSignalHandler()
+        while not signal_handler.kill_now:
+            # if the parent PID changes, this process has been orphaned
+            # via e.g., segfault or sigkill, we should exit too
+            if os.getppid() != ppid:
+                break
+            try:
+                body = queue.get(block=True, timeout=1)
+                if body == 'QUIT':
+                    break
+            except QueueEmpty:
+                continue
+            except Exception as e:
+                logger.error("Exception on worker, restarting: " + str(e))
+                continue
+            try:
+                self.perform_work(body, *args)
+            finally:
+                if 'uuid' in body:
+                    uuid = body['uuid']
+                    logger.debug('task {} is finished'.format(uuid))
+                    finished.put(uuid)
+        logger.warn('worker exiting gracefully pid:{}'.format(os.getpid()))
+
+    def perform_work(self, body):
+        raise NotImplementedError()
+
+    def on_start(self):
+        pass
+
+    def on_stop(self):
+        pass