Add additional assertions and further checks in storage code

data/model/image.py

@@ -489,7 +489,8 @@ def find_or_create_derived_storage(source_image, transformation_name, preferred_
                                             transformation=trans, uniqueness_hash=uniqueness_hash)
   except IntegrityError:
     # Storage was created while this method executed. Just return the existing.
-    new_storage.delete_instance(recursive=True)
+    ImageStoragePlacement.delete().where(ImageStoragePlacement.storage == new_storage).execute()
+    new_storage.delete_instance()
     return find_derived_storage_for_image(source_image, transformation_name, varying_metadata)
 
   return derived