Return no active network if the agent has not been learnt yet

Sometimes the agent message to get the active networks is
processed before the agent is learnt by Neutron. Neutron throws an
error, but it should simply return no network

Change-Id: I6da0ca578cfe1fe0885138a5e2da76278f791491
Closes-bug: #1302838

neutron/db/agentschedulers_db.py

@@ -23,6 +23,7 @@ from neutron.common import constants
 from neutron.common import utils
 from neutron.db import agents_db
 from neutron.db import model_base
+from neutron.extensions import agent as ext_agent
 from neutron.extensions import dhcpagentscheduler
 from neutron.openstack.common import log as logging
 
@@ -184,8 +185,13 @@ class DhcpAgentSchedulerDbMixin(dhcpagentscheduler
             return {'networks': []}
 
     def list_active_networks_on_active_dhcp_agent(self, context, host):
-        agent = self._get_agent_by_type_and_host(
-            context, constants.AGENT_TYPE_DHCP, host)
+        try:
+            agent = self._get_agent_by_type_and_host(
+                context, constants.AGENT_TYPE_DHCP, host)
+        except ext_agent.AgentNotFoundByTypeHost:
+            LOG.debug("DHCP Agent not found on host %s", host)
+            return []
+
         if not agent.admin_state_up:
             return []
         query = context.session.query(NetworkDhcpAgentBinding.network_id)

neutron/tests/unit/openvswitch/test_agent_scheduler.py

@@ -576,6 +576,12 @@ class OvsAgentSchedulerTestCase(OvsAgentSchedulerTestCaseBase):
         self.assertEqual(1, num_before_remove)
         self.assertEqual(0, num_after_remove)
 
+    def test_list_active_networks_on_not_registered_yet_dhcp_agent(self):
+        plugin = manager.NeutronManager.get_plugin()
+        nets = plugin.list_active_networks_on_active_dhcp_agent(
+            self.adminContext, host=DHCP_HOSTA)
+        self.assertEqual([], nets)
+
     def test_reserved_port_after_network_remove_from_dhcp_agent(self):
         dhcp_hosta = {
             'binary': 'neutron-dhcp-agent',