Avoid writing segments to the DB repeatedly

When:
* the segments service plugin is enabled and
* we have multiple rpc worker processes (as in the sum of rpc_workers
  and rpc_state_report_workers, since both kind processes agent
  state_reports) and
* many ovs-agents report physnets,
then rabbitmq dispatches the state_report messages between the workers
in a round robin fashion, therefore eventually the state_reports of the
same agent will hit all rpc workers.

Unfortunately all worker processes have a 'reported_hosts' set to
remember from which host it has seen agent reports already. But right
after a server start when that set is still empty, each worker will
unconditionally write the received physnet-segment information into
the db. This means we multiply the load on the db and rpc workers by
a factor of the rpc worker count.

This patch tries to reduce the load on the db by adding another early
return before the unconditional db write.

Change-Id: I935186b6ee95f0cae8dc05869d9742c8fb3353c3
Closes-Bug: #1952730
(cherry picked from commit 176503e610)
(cherry picked from commit dcb372b041)
(cherry picked from commit 0c909e3b55)

neutron/db/agents_db.py

@@ -397,6 +397,7 @@ class AgentDbMixin(ext_agent.AgentPluginBase, AgentAvailabilityZoneMixin):
                 agent = self._get_agent_by_type_and_host(
                     context, agent_state['agent_type'], agent_state['host'])
                 agent_state_orig = copy.deepcopy(agent_state)
+                agent_state_previous = copy.deepcopy(agent)
                 if not agent.is_active:
                     status = agent_consts.AGENT_REVIVED
                     if 'resource_versions' not in agent_state:
@@ -417,6 +418,7 @@ class AgentDbMixin(ext_agent.AgentPluginBase, AgentAvailabilityZoneMixin):
                 event_type = events.AFTER_UPDATE
             except agent_exc.AgentNotFoundByTypeHost:
                 agent_state_orig = None
+                agent_state_previous = None
                 greenthread.sleep(0)
                 res['created_at'] = current_time
                 res['started_at'] = current_time
@@ -441,7 +443,7 @@ class AgentDbMixin(ext_agent.AgentPluginBase, AgentAvailabilityZoneMixin):
                                  'plugin': self,
                                  'status': status
                              },
-                             states=(agent_state_orig, ),
+                             states=(agent_state_orig, agent_state_previous),
                              desired_state=agent_state,
                              resource_id=agent.id
                          ))

neutron/services/segments/db.py

@@ -298,6 +298,11 @@ def _update_segment_host_mapping_for_agent(resource, event, trigger,
     if host in reported_hosts and not start_flag:
         return
     reported_hosts.add(host)
+    if (len(payload.states) > 1 and
+            payload.states[1] is not None and
+            agent.get('configurations') == payload.states[1].get(
+                'configurations')):
+        return
     segments = get_segments_with_phys_nets(context, phys_nets)
     current_segment_ids = {
         segment['id'] for segment in segments

neutron/tests/unit/extensions/test_segment.py

@@ -947,9 +947,9 @@ class TestMl2HostSegmentMappingAgentServerSynch(HostSegmentMappingTestCase):
         self._register_agent(host, mappings={physical_network: 'br-eth-1'},
                              plugin=self.plugin, start_flag=True)
         self.assertIn(host, db.reported_hosts)
-        self.assertEqual(2, mock_function.call_count)
+        self.assertEqual(1, mock_function.call_count)
         expected_call = mock.call(mock.ANY, host, set())
-        mock_function.assert_has_calls([expected_call, expected_call])
+        mock_function.assert_has_calls([expected_call])
 
     @mock.patch(mock_path)
     def test_no_starting_agent_is_not_processed(self, mock_function):