Don't update DHCP ports in a transaction

Updating ports is incompatible with the transaction
semantics that happen inside of ML2. See partial-bug for
more details.

This transaction served no purpose wrapped around the port
update because there should only be one port per agent per
network and if that port fails to update, the code to delete
the binding won't be hit anyway.

Change-Id: Ica2c5a19e10ae1773e49537b5ff4ae0eacaeb388
Partial-Bug: #1540844

neutron/db/agentschedulers_db.py

@@ -374,29 +374,28 @@ class DhcpAgentSchedulerDbMixin(dhcpagentscheduler
     def remove_network_from_dhcp_agent(self, context, id, network_id,
                                        notify=True):
         agent = self._get_agent(context, id)
-        with context.session.begin(subtransactions=True):
-            try:
-                query = context.session.query(NetworkDhcpAgentBinding)
-                query = query.filter(
-                    NetworkDhcpAgentBinding.network_id == network_id,
-                    NetworkDhcpAgentBinding.dhcp_agent_id == id)
-                # just ensure the binding exists
-                query.one()
-            except exc.NoResultFound:
-                raise dhcpagentscheduler.NetworkNotHostedByDhcpAgent(
-                    network_id=network_id, agent_id=id)
+        try:
+            query = context.session.query(NetworkDhcpAgentBinding)
+            binding = query.filter(
+                NetworkDhcpAgentBinding.network_id == network_id,
+                NetworkDhcpAgentBinding.dhcp_agent_id == id).one()
+        except exc.NoResultFound:
+            raise dhcpagentscheduler.NetworkNotHostedByDhcpAgent(
+                network_id=network_id, agent_id=id)
 
-            # reserve the port, so the ip is reused on a subsequent add
-            device_id = utils.get_dhcp_agent_device_id(network_id,
-                                                       agent['host'])
-            filters = dict(device_id=[device_id])
-            ports = self.get_ports(context, filters=filters)
-            for port in ports:
-                port['device_id'] = constants.DEVICE_ID_RESERVED_DHCP_PORT
-                self.update_port(context, port['id'], dict(port=port))
-            # avoid issues with query.one() object that was
-            # loaded into the session
-            query.delete(synchronize_session=False)
+        # reserve the port, so the ip is reused on a subsequent add
+        device_id = utils.get_dhcp_agent_device_id(network_id,
+                                                   agent['host'])
+        filters = dict(device_id=[device_id])
+        ports = self.get_ports(context, filters=filters)
+        # NOTE(kevinbenton): there should only ever be one port per
+        # DHCP agent per network so we don't have to worry about one
+        # update_port passing and another failing
+        for port in ports:
+            port['device_id'] = constants.DEVICE_ID_RESERVED_DHCP_PORT
+            self.update_port(context, port['id'], dict(port=port))
+        with context.session.begin():
+            context.session.delete(binding)
 
         if not notify:
             return