Fix exceptions raised by the diffs worker

Fixes #465

data/queue.py

@@ -185,7 +185,12 @@ class WorkQueue(object):
 
   def complete(self, completed_item):
     with self._transaction_factory(db):
-      completed_item_obj = self._item_by_id_for_update(completed_item.id)
+      try:
+        completed_item_obj = self._item_by_id_for_update(completed_item.id)
+      except QueueItem.DoesNotExist:
+        self._currently_processing = False
+        return
+
       completed_item_obj.delete_instance(recursive=True)
       self._currently_processing = False
 

workers/diffsworker.py

@@ -23,6 +23,12 @@ class DiffsWorker(QueueWorker):
       msg = ('Image does not exist in database \'%s\' for repo \'%s/\'%s\'' %
              (image_id, namespace, repository))
       logger.warning(msg)
+    except IOError:
+      # This exception is unrecoverable, and the item should continue and be
+      # marked as complete.
+      msg = ("Data could not be retrieved for image %s under repo %s/%s" %
+             (image_id, namespace, repository))
+      logger.exception(msg)
 
     return True