Refactors periodic tasks to use a decorator.

Additional work:

    1. Added support for tasks being scheduled at differing rates via the
       `ticks_between_runs` argument.

    2. Fixed `reclaim_queued_deletes` so that it doesn't run if
       `FLAGS.reclaim_instance_interval` is 0.

Change-Id: I18c01baf07bd06301a6fe26a7b29dc2452a4fa96

nova/manager.py

@@ -66,16 +66,98 @@ FLAGS = flags.FLAGS
 LOG = logging.getLogger('nova.manager')
 
 
+def periodic_task(*args, **kwargs):
+    """Decorator to indicate that a method is a periodic task.
+
+    This decorator can be used in two ways:
+
+        1. Without arguments '@periodic_task', this will be run on every tick
+           of the periodic scheduler.
+
+        2. With arguments, @periodic_task(ticks_between_runs=N), this will be
+           run on every N ticks of the periodic scheduler.
+    """
+    def decorator(f):
+        f._periodic_task = True
+        f._ticks_between_runs = kwargs.pop('ticks_between_runs', 0)
+        return f
+
+    # NOTE(sirp): The `if` is necessary to allow the decorator to be used with
+    # and without parens.
+    #
+    # In the 'with-parens' case (with kwargs present), this function needs to
+    # return a decorator function since the interpreter will invoke it like:
+    #
+    #   periodic_task(*args, **kwargs)(f)
+    #
+    # In the 'without-parens' case, the original function will be passed
+    # in as the first argument, like:
+    #
+    #   periodic_task(f)
+    if kwargs:
+        return decorator
+    else:
+        return decorator(args[0])
+
+
+class ManagerMeta(type):
+    def __init__(cls, names, bases, dict_):
+        """Metaclass that allows us to collect decorated periodic tasks."""
+        super(ManagerMeta, cls).__init__(names, bases, dict_)
+
+        # NOTE(sirp): if the attribute is not present then we must be the base
+        # class, so, go ahead an initialize it. If the attribute is present,
+        # then we're a subclass so make a copy of it so we don't step on our
+        # parent's toes.
+        try:
+            cls._periodic_tasks = cls._periodic_tasks[:]
+        except AttributeError:
+            cls._periodic_tasks = []
+
+        try:
+            cls._ticks_to_skip = cls._ticks_to_skip.copy()
+        except AttributeError:
+            cls._ticks_to_skip = {}
+
+        for value in cls.__dict__.values():
+            if getattr(value, '_periodic_task', False):
+                task = value
+                name = task.__name__
+                cls._periodic_tasks.append((name, task))
+                cls._ticks_to_skip[name] = task._ticks_between_runs
+
+
 class Manager(base.Base):
+    __metaclass__ = ManagerMeta
+
     def __init__(self, host=None, db_driver=None):
         if not host:
             host = FLAGS.host
         self.host = host
         super(Manager, self).__init__(db_driver)
 
-    def periodic_tasks(self, context=None):
+    def periodic_tasks(self, context, raise_on_error=False):
         """Tasks to be run at a periodic interval."""
-        pass
+        for task_name, task in self._periodic_tasks:
+            full_task_name = '.'.join([self.__class__.__name__, task_name])
+
+            ticks_to_skip = self._ticks_to_skip[task_name]
+            if ticks_to_skip > 0:
+                LOG.debug(_("Skipping %(full_task_name)s, %(ticks_to_skip)s"
+                            " ticks left until next run"), locals())
+                self._ticks_to_skip[task_name] -= 1
+                continue
+
+            self._ticks_to_skip[task_name] = task._ticks_between_runs
+            LOG.debug(_("Running periodic task %(full_task_name)s"), locals())
+
+            try:
+                task(self, context)
+            except Exception as e:
+                if raise_on_error:
+                    raise
+                LOG.exception(_("Error during %(full_task_name)s: %(e)s"),
+                              locals())
 
     def init_host(self):
         """Handle initialization if this is a standalone service.
@@ -105,11 +187,10 @@ class SchedulerDependentManager(Manager):
         """Remember these capabilities to send on next periodic update."""
         self.last_capabilities = capabilities
 
-    def periodic_tasks(self, context=None):
+    @periodic_task
+    def _publish_service_capabilities(self, context):
         """Pass data back to the scheduler at a periodic interval."""
         if self.last_capabilities:
             LOG.debug(_('Notifying Schedulers of capabilities ...'))
             api.update_service_capabilities(context, self.service_name,
                                 self.host, self.last_capabilities)
-
-        super(SchedulerDependentManager, self).periodic_tasks(context)