fixes inventory update deadlock

* all inventory updates continue to occur in parallel up to the point
that they update the database with their results.
* the "funnel" is achieved by using a global per-inventory postgres
named lock

awx/main/management/commands/inventory_import.py

@@ -41,6 +41,7 @@ from awx.main.utils import (
 from awx.main.utils.common import _get_ansible_version
 from awx.main.signals import disable_activity_stream
 from awx.main.constants import STANDARD_INVENTORY_UPDATE_ENV
+from awx.main.utils.pglock import advisory_lock
 
 logger = logging.getLogger('awx.main.commands.inventory_import')
 
@@ -870,6 +871,7 @@ class Command(BaseCommand):
         Load inventory from in-memory groups to the database, overwriting or
         merging as appropriate.
         '''
+        with advisory_lock('inventory_{}_update'.format(self.inventory.id)):
             # FIXME: Attribute changes to superuser?
             # Perform __in queries in batches (mainly for unit tests using SQLite).
             self._batch_size = 500