Add a chunk cleanup queue for async GC of empty chunks

Instead of having the Swift storage engine try to delete the empty chunk(s) synchronously, we simply queue them and have a worker come along after 30s to delete the empty chunks. This has a few key benefits: it is async (doesn't slow down the push code), helps deal with Swift's eventual consistency (less retries necessary) and is generic for other storage engines if/when they need this as well

storage/__init__.py

@@ -15,27 +15,36 @@ STORAGE_DRIVER_CLASSES = {
   'SwiftStorage': SwiftStorage,
 }
 
-def get_storage_driver(metric_queue, storage_params):
+def get_storage_driver(location, metric_queue, chunk_cleanup_queue, storage_params):
   """ Returns a storage driver class for the given storage configuration
       (a pair of string name and a dict of parameters). """
   driver = storage_params[0]
   parameters = storage_params[1]
   driver_class = STORAGE_DRIVER_CLASSES.get(driver, FakeStorage)
-  return driver_class(metric_queue, **parameters)
+  context = StorageContext(location, metric_queue, chunk_cleanup_queue)
+  return driver_class(context, **parameters)
+
+
+class StorageContext(object):
+  def __init__(self, location, metric_queue, chunk_cleanup_queue):
+    self.location = location
+    self.metric_queue = metric_queue
+    self.chunk_cleanup_queue = chunk_cleanup_queue
 
 
 class Storage(object):
-  def __init__(self, app=None, metric_queue=None, instance_keys=None):
+  def __init__(self, app=None, metric_queue=None, chunk_cleanup_queue=None, instance_keys=None):
     self.app = app
     if app is not None:
-      self.state = self.init_app(app, metric_queue, instance_keys)
+      self.state = self.init_app(app, metric_queue, chunk_cleanup_queue, instance_keys)
     else:
       self.state = None
 
-  def init_app(self, app, metric_queue, instance_keys):
+  def init_app(self, app, metric_queue, chunk_cleanup_queue, instance_keys):
     storages = {}
     for location, storage_params in app.config.get('DISTRIBUTED_STORAGE_CONFIG').items():
-      storages[location] = get_storage_driver(metric_queue, storage_params)
+      storages[location] = get_storage_driver(location, metric_queue, chunk_cleanup_queue,
+                                              storage_params)
 
     preference = app.config.get('DISTRIBUTED_STORAGE_PREFERENCE', None)
     if not preference: