Check for None in _get_agent_fdb for agent

get_agent_by_host can return None in the l2pop
driver so we need to check for that case before
we blindly try to decode configuration values on
the result.

There are a couple of cases that can lead to this.
* The deployment can be misconfigured and is missing
  either a tunneling_ip option for the agent on a
  host or is missing an L2 agent with that host_id
  entirely.
* Multiple mech drivers are in use and a port is being
  deleted from an agentless host.

Related-Bug: #1533013
Closes-Bug: #1672564
Change-Id: I1e79f600172edad1e31e8231a0a6a2c55f46804c
(cherry picked from commit c7fb24b3cb)

neutron/plugins/ml2/drivers/l2pop/mech_driver.py

@@ -312,6 +312,10 @@ class L2populationMechanismDriver(api.MechanismDriver):
 
         agent = l2pop_db.get_agent_by_host(session,
                                            agent_host)
+        if not agent:
+            LOG.warning(_LW("Unable to retrieve active L2 agent on host %s"),
+                        agent_host)
+            return
         if not self._validate_segment(segment, port['id'], agent):
             return
 

neutron/tests/unit/plugins/ml2/drivers/l2pop/test_mech_driver.py

@@ -1098,15 +1098,24 @@ class TestL2PopulationRpcTestCase(test_plugin.Ml2PluginV2TestCase):
             self.assertTrue(upd_port_down.called)
 
     def test_delete_unbound_port(self):
+        self._test_delete_port_handles_agentless_host_id(None)
+
+    def test_delete_port_bound_to_agentless_host(self):
+        self._test_delete_port_handles_agentless_host_id('test')
+
+    def _test_delete_port_handles_agentless_host_id(self, host):
         l2pop_mech = l2pop_mech_driver.L2populationMechanismDriver()
         l2pop_mech.initialize()
 
         with self.port() as port:
+            port['port'][portbindings.HOST_ID] = host
+            bindings = [mock.Mock()]
             port_context = driver_context.PortContext(
                 self.driver, self.context, port['port'],
                 self.driver.get_network(
                     self.context, port['port']['network_id']),
-                None, None)
+                None, bindings)
+            mock.patch.object(port_context, '_expand_segment').start()
             # The point is to provide coverage and to assert that no exceptions
             # are raised.
             l2pop_mech.delete_port_postcommit(port_context)