work around a limitation in postgres notify to properly support copying

postgres has a limitation on its notify message size (8k), and the
messages we generate for deep copying functionality easily go over this
limit; instead of passing a giant nested data structure across the
message bus, this change makes it so that we temporarily store the JSON
structure in memcached, and look it up from *within* the task

see: https://github.com/ansible/tower/issues/4162

awx/api/generics.py

@@ -5,10 +5,12 @@
 import inspect
 import logging
 import time
+import uuid
 import urllib.parse
 
 # Django
 from django.conf import settings
+from django.core.cache import cache
 from django.db import connection
 from django.db.models.fields import FieldDoesNotExist
 from django.db.models.fields.related import OneToOneRel
@@ -973,6 +975,11 @@ class CopyAPIView(GenericAPIView):
         if hasattr(new_obj, 'admin_role') and request.user not in new_obj.admin_role.members.all():
             new_obj.admin_role.members.add(request.user)
         if sub_objs:
+            # store the copied object dict into memcached, because it's
+            # often too large for postgres' notification bus
+            # (which has a default maximum message size of 8k)
+            key = 'deep-copy-{}'.format(str(uuid.uuid4()))
+            cache.set(key, sub_objs, timeout=3600)
             permission_check_func = None
             if hasattr(type(self), 'deep_copy_permission_check_func'):
                 permission_check_func = (
@@ -980,7 +987,7 @@ class CopyAPIView(GenericAPIView):
                 )
             trigger_delayed_deep_copy(
                 self.model.__module__, self.model.__name__,
-                obj.pk, new_obj.pk, request.user.pk, sub_objs,
+                obj.pk, new_obj.pk, request.user.pk, key,
                 permission_check_func=permission_check_func
             )
         serializer = self._get_copy_return_serializer(new_obj)

awx/main/tasks.py

@@ -2856,8 +2856,13 @@ def _reconstruct_relationships(copy_mapping):
 @task(queue=get_local_queuename)
 def deep_copy_model_obj(
     model_module, model_name, obj_pk, new_obj_pk,
-    user_pk, sub_obj_list, permission_check_func=None
+    user_pk, uuid, permission_check_func=None
 ):
+    sub_obj_list = cache.get(uuid)
+    if sub_obj_list is None:
+        logger.error('Deep copy {} from {} to {} failed unexpectedly.'.format(model_name, obj_pk, new_obj_pk))
+        return
+
     logger.debug('Deep copy {} from {} to {}.'.format(model_name, obj_pk, new_obj_pk))
     from awx.api.generics import CopyAPIView
     from awx.main.signals import disable_activity_stream