get rid of celery/celerybeat

alternative to https://github.com/ansible/awx/pull/2530 which makes use
of https://pypi.org/project/schedule/

this doesn't have support for any persistence (like how celery beat uses
a shelve file), because all of our periodic jobs run at most every few
minutes

awx/main/management/commands/run_dispatcher.py

@@ -16,6 +16,7 @@ from awx.main.dispatch.control import Control
 from awx.main.dispatch.kombu import Connection
 from awx.main.dispatch.pool import AutoscalePool
 from awx.main.dispatch.worker import AWXConsumer, TaskWorker
+from awx.main.dispatch import periodic
 
 logger = logging.getLogger('awx.main.dispatch')
 
@@ -36,71 +37,6 @@ class Command(BaseCommand):
                             help=('cause the dispatcher to recycle all of its worker processes;'
                                   'running jobs will run to completion first'))
 
-    def beat(self):
-        from celery import Celery
-        from celery.beat import PersistentScheduler
-        from celery.apps import beat
-
-        class AWXScheduler(PersistentScheduler):
-
-            def __init__(self, *args, **kwargs):
-                self.ppid = os.getppid()
-                super(AWXScheduler, self).__init__(*args, **kwargs)
-
-            def setup_schedule(self):
-                super(AWXScheduler, self).setup_schedule()
-                self.update_from_dict(settings.CELERYBEAT_SCHEDULE)
-
-            def tick(self, *args, **kwargs):
-                if os.getppid() != self.ppid:
-                    # if the parent PID changes, this process has been orphaned
-                    # via e.g., segfault or sigkill, we should exit too
-                    raise SystemExit()
-                return super(AWXScheduler, self).tick(*args, **kwargs)
-
-            def apply_async(self, entry, producer=None, advance=True, **kwargs):
-                for conn in connections.all():
-                    # If the database connection has a hiccup, re-establish a new
-                    # connection
-                    conn.close_if_unusable_or_obsolete()
-                task = TaskWorker.resolve_callable(entry.task)
-                result, queue = task.apply_async()
-
-                class TaskResult(object):
-                    id = result['uuid']
-
-                return TaskResult()
-
-        sched_file = '/var/lib/awx/beat.db'
-        app = Celery()
-        app.conf.BROKER_URL = settings.BROKER_URL
-        app.conf.CELERY_TASK_RESULT_EXPIRES = False
-
-        # celery in py3 seems to have a bug where the celerybeat schedule
-        # shelve can become corrupted; we've _only_ seen this in Ubuntu and py36
-        # it can be avoided by detecting and removing the corrupted file
-        # at some point, we'll just stop using celerybeat, because it's clearly
-        # buggy, too -_-
-        #
-        # https://github.com/celery/celery/issues/4777
-        sched = AWXScheduler(schedule_filename=sched_file, app=app)
-        try:
-            sched.setup_schedule()
-        except Exception:
-            logger.exception('{} is corrupted, removing.'.format(sched_file))
-            sched._remove_db()
-        finally:
-            try:
-                sched.close()
-            except Exception:
-                logger.exception('{} failed to sync/close'.format(sched_file))
-
-        beat.Beat(
-            30,
-            app,
-            schedule=sched_file, scheduler_cls=AWXScheduler
-        ).run()
-
     def handle(self, *arg, **options):
         if options.get('status'):
             print(Control('dispatcher').status())
@@ -116,9 +52,10 @@ class Command(BaseCommand):
         # for the DB and memcached connections (that way lies race conditions)
         django_connection.close()
         django_cache.close()
-        beat = Process(target=self.beat)
-        beat.daemon = True
-        beat.start()
+
+        # spawn a daemon thread to periodically enqueues scheduled tasks
+        # (like the node heartbeat)
+        cease_continuous_run = periodic.run_continuously()
 
         reaper.reap()
         consumer = None
@@ -152,6 +89,7 @@ class Command(BaseCommand):
                 )
                 consumer.run()
             except KeyboardInterrupt:
+                cease_continuous_run.set()
                 logger.debug('Terminating Task Dispatcher')
                 if consumer:
                     consumer.stop()