Trivial: avoid KeyError while processing ports

The Neutron OVS agent logs can get flooded with KeyErrors as the
'_get_port_info' method skips the added/removed dict items if no
ports have been added/removed, which are expected to be present,
even if those are just empty sets.

This change ensures that those port info dict fields are always set.

Closes-Bug: #1783556

Change-Id: I9e5325aa2d8525231353ba451e8ea895be51b1ca

neutron/plugins/ml2/drivers/openvswitch/agent/ovs_neutron_agent.py

@@ -1263,7 +1263,9 @@ class OVSNeutronAgent(l2population_rpc.L2populationRpcCallBackTunnelMixin,
 
     def _get_port_info(self, registered_ports, cur_ports,
                        readd_registered_ports):
-        port_info = {'current': cur_ports}
+        port_info = {'current': cur_ports,
+                     'added': set(),
+                     'removed': set()}
         # FIXME(salv-orlando): It's not really necessary to return early
         # if nothing has changed.
         if not readd_registered_ports and cur_ports == registered_ports:

neutron/tests/unit/plugins/ml2/drivers/openvswitch/agent/test_ovs_neutron_agent.py

@@ -387,7 +387,9 @@ class TestOvsNeutronAgent(object):
     def test_scan_ports_returns_current_only_for_unchanged_ports(self):
         vif_port_set = set([1, 3])
         registered_ports = set([1, 3])
-        expected = {'current': vif_port_set}
+        expected = {'current': vif_port_set,
+                    'added': set(),
+                    'removed': set()}
         actual = self.mock_scan_ports(vif_port_set, registered_ports)
         self.assertEqual(expected, actual)
 
@@ -438,7 +440,8 @@ class TestOvsNeutronAgent(object):
         vif_port_set = set([1, 2, 3])
         registered_ports = set([1, 2, 3])
         updated_ports = set([2])
-        expected = dict(current=vif_port_set, updated=set([2]))
+        expected = dict(current=vif_port_set, updated=set([2]),
+                        added=set(), removed=set())
         actual = self.mock_scan_ports(vif_port_set, registered_ports,
                                       updated_ports)
         self.assertEqual(expected, actual)
@@ -2452,7 +2455,9 @@ class AncillaryBridgesTest(object):
     def test_scan_ancillary_ports_returns_cur_only_for_unchanged_ports(self):
         vif_port_set = set([1, 2])
         registered_ports = set([1, 2])
-        expected = dict(current=vif_port_set)
+        expected = dict(current=vif_port_set,
+                        added=set(),
+                        removed=set())
         actual = self.mock_scan_ancillary_ports(vif_port_set, registered_ports)
         self.assertEqual(expected, actual)