Handle properly ObjectNotFound while deleting network from DHCP agent

In case when 2 neutron servers are trying to remove network from the
same DHCP agent it may happend that one of them will not be able
to find binding object anymore thus deletion of that binding object
will raise ObjectNotFound exception.
This patch adds proper handling of such case to not raise ugly
stacktrace in neutron logs.

Closes-Bug: #1970759
Change-Id: I67d516c4583aa0c20416114b92a6d69ece5b970c
(cherry picked from commit 8df2f69)

Author: slawqo

neutron/db/agentschedulers_db.py

@@ -405,9 +405,9 @@ def add_network_to_dhcp_agent(self, context, id, network_id):
     def remove_network_from_dhcp_agent(self, context, id, network_id,
                                        notify=True):
         agent = self._get_agent(context, id)
-        binding_obj = network.NetworkDhcpAgentBinding.get_object(
+        deleted_bindings = network.NetworkDhcpAgentBinding.delete_objects(
             context, network_id=network_id, dhcp_agent_id=id)
-        if not binding_obj:
+        if not deleted_bindings:
             raise das_exc.NetworkNotHostedByDhcpAgent(
                 network_id=network_id, agent_id=id)
 
@@ -426,7 +426,6 @@ def remove_network_from_dhcp_agent(self, context, id, network_id,
             except n_exc.PortNotFound:
                 LOG.debug("DHCP port %s has been deleted concurrently",
                           port['id'])
-        binding_obj.delete()
 
         if not notify:
             return

neutron/tests/unit/db/test_agentschedulers_db.py

@@ -1466,6 +1466,11 @@ def test_network_remove_from_dhcp_agent_notification(self):
 
         self._remove_network_from_dhcp_agent(hosta_id,
                                              network_id)
+        # Call it second time, it should be already deleted so should 409 be
+        # returned this time
+        self._remove_network_from_dhcp_agent(
+            hosta_id, network_id,
+            expected_code=exc.HTTPConflict.code)
         self.dhcp_notifier_cast.assert_called_with(
                 mock.ANY, 'network_delete_end',
                 {'network_id': network_id,