Populate binding levels when concurrent ops fail

Concurrent calls to _bind_port_if_needed may lead to a missing RPC
notification which can cause a port stuck in a DOWN state. If the only
caller that succeeds in the concurrency does not specify that an RPC
notification is allowed then no RPC would be sent to the agent. The
other caller which needs to send an RPC notification will fail since the
resulting PortContext instance will not have any binding levels set.

The failure has negative effects on consumers of the L2Population
functionality because the L2Population mechanism driver will not be
triggered to publish that a port is UP on a given compute node. Manual
intervention is required in this case.

This patch proposes to handle this by populating the PortContext with
the current binding levels so that the caller can continue on and have
an RPC notification sent out.

Closes-Bug: #1755810
Story: 2003922
Change-Id: Ie2b813b2bdf181fb3c24743dbd13487ace6ee76a

neutron/plugins/ml2/plugin.py

@@ -683,6 +683,22 @@ class Ml2Plugin(db_base_plugin_v2.NeutronDbPluginV2,
                 # Call the mechanism driver precommit methods, commit
                 # the results, and call the postcommit methods.
                 self.mechanism_manager.update_port_precommit(cur_context)
+            else:
+                # Try to populate the PortContext with the current binding
+                # levels so that the RPC notification won't get suppressed.
+                # This is to avoid leaving ports stuck in a DOWN state.
+                # For more information see bug:
+                # https://bugs.launchpad.net/neutron/+bug/1755810
+                LOG.warning("Concurrent port binding operations failed on "
+                            "port %s", port_id)
+                levels = db.get_binding_level_objs(plugin_context, port_id,
+                                                   cur_binding.host)
+                for level in levels:
+                    cur_context._push_binding_level(level)
+                # refresh context with a snapshot of the current binding state
+                cur_context._binding = driver_context.InstanceSnapshot(
+                    cur_binding)
+
         if commit:
             # Continue, using the port state as of the transaction that
             # just finished, whether that transaction committed new

neutron/tests/unit/plugins/ml2/test_plugin.py

@@ -2218,6 +2218,66 @@ class TestMl2PortBinding(Ml2PluginV2TestCase,
                     # Successful binding should only be attempted once.
                     self.assertEqual(1, at_mock.call_count)
 
+    def test__bind_port_if_needed_concurrent_calls(self):
+        port_vif_type = portbindings.VIF_TYPE_UNBOUND
+        bound_vif_type = portbindings.VIF_TYPE_OVS
+
+        plugin, port_context, bound_context = (
+            self._create_port_and_bound_context(port_vif_type,
+                                                bound_vif_type))
+        bound_context._binding_levels = [mock.Mock(
+            port_id="port_id",
+            level=0,
+            driver='fake_agent',
+            segment_id="11111111-2222-3333-4444-555555555555")]
+
+        # let _commit_port_binding replace the PortContext with a new instance
+        # which does not have any binding levels set to simulate the concurrent
+        # port binding operations fail
+        with mock.patch(
+            'neutron.plugins.ml2.plugin.Ml2Plugin._bind_port',
+            return_value=bound_context),\
+            mock.patch('neutron.plugins.ml2.plugin.Ml2Plugin.'
+                       '_notify_port_updated') as npu_mock,\
+            mock.patch('neutron.plugins.ml2.plugin.Ml2Plugin.'
+                       '_attempt_binding',
+                       side_effect=plugin._attempt_binding) as ab_mock,\
+            mock.patch('neutron.plugins.ml2.plugin.Ml2Plugin.'
+                       '_commit_port_binding', return_value=(
+                            mock.MagicMock(), True, True)) as cpb_mock:
+            ret_context = plugin._bind_port_if_needed(port_context,
+                                                      allow_notify=True)
+            # _attempt_binding will return without doing anything during
+            # the second iteration since _should_bind_port returns False
+            self.assertEqual(2, ab_mock.call_count)
+            self.assertEqual(1, cpb_mock.call_count)
+            # _notify_port_updated will still be called though it does
+            # nothing due to the missing binding levels
+            npu_mock.assert_called_once_with(ret_context)
+
+    def test__commit_port_binding_populating_with_binding_levels(self):
+        port_vif_type = portbindings.VIF_TYPE_OVS
+        bound_vif_type = portbindings.VIF_TYPE_OVS
+
+        plugin, port_context, bound_context = (
+            self._create_port_and_bound_context(port_vif_type,
+                                                bound_vif_type))
+        db_portbinding = port_obj.PortBindingLevel(
+            self.context,
+            port_id=uuidutils.generate_uuid(),
+            level=0,
+            driver='fake_agent',
+            segment_id="11111111-2222-3333-4444-555555555555")
+        bound_context.network.current = {'id': 'net_id'}
+
+        with mock.patch.object(ml2_db, 'get_binding_level_objs',
+                return_value=[db_portbinding]),\
+                mock.patch.object(driver_context.PortContext,
+                '_push_binding_level') as pbl_mock:
+            plugin._commit_port_binding(
+                    port_context, bound_context, True, False)
+            pbl_mock.assert_called_once_with(db_portbinding)
+
     def test_port_binding_profile_not_changed(self):
         profile = {'e': 5}
         profile_arg = {portbindings.PROFILE: profile}