post live migration: don't call Neutron needlessly

In bug 1879787, the call to network_api.get_instance_nw_info() in
_post_live_migration() on the source compute manager eventually calls
out to the Neutron REST API. If this fails, the exception is
unhandled, and the migrating instance - which is fully running on the
destination at this point - will never be updated in the database.
This update normally happens later in
post_live_migration_at_destination().

The network_info variable obtained from get_instance_nw_info() is used
for two things: notifications - which aren't critical - and unplugging
the instance's vifs on the source - which is very important!

It turns out that at the time of the get_instance_nw_info() call, the
network info in the instance info cache is still valid for unplugging
the source vifs. The port bindings on the destination are only
activated by the network_api.migrate_instance_start() [1] call that
happens shortly *after* the problematic get_instance_nw_info() call.
In other words, get_instance_nw_info() will always return the source
ports. Because of that, we can replace it with a call to
instance.get_network_info().

[1] d9e04c4ff0/nova/network/neutronv2/api.py (L2493-L2522)

Change-Id: If0fbae33ce2af198188c91638afef939256c2556
Closes-bug: 1879787
(cherry picked from commit 6488a5dfb2)

nova/compute/manager.py

@@ -8409,7 +8409,13 @@ class ComputeManager(manager.Manager):
         # Releasing vlan.
         # (not necessary in current implementation?)
 
-        network_info = self.network_api.get_instance_nw_info(ctxt, instance)
+        # NOTE(artom) At this point in time we have not bound the ports to the
+        # destination host yet (this happens in migrate_instance_start()
+        # below). Therefore, the "old" source network info that's still in the
+        # instance info cache is safe to use here, since it'll be used below
+        # during driver.post_live_migration_at_source() to unplug the VIFs on
+        # the source.
+        network_info = instance.get_network_info()
 
         self._notify_about_instance_usage(ctxt, instance,
                                           "live_migration._post.start",

nova/tests/functional/compute/test_live_migration.py

@@ -182,7 +182,8 @@ class TestVolAttachmentsDuringLiveMigration(
         self.assertEqual(1, len(attachments))
 
 
-class LiveMigrationNeutronFailure(integrated_helpers._IntegratedTestBase):
+class LiveMigrationNeutronInteractionsTest(
+        integrated_helpers._IntegratedTestBase):
     # NOTE(artom) We need the admin API to force the host when booting the test
     # server.
     ADMIN_API = True
@@ -192,38 +193,29 @@ class LiveMigrationNeutronFailure(integrated_helpers._IntegratedTestBase):
         self._start_compute('src')
         self._start_compute('dest')
 
-    def test_live_migrate_get_nw_info_fails(self):
-        """Test that if the driver.post_live_migration() call fails (for
-        example by not being able to connect to Neutron), the exception goes
-        unhandled and results in the live-migration erroring out. This is bug
-        1879787.
+    def test_live_migrate_vifs_from_info_cache(self):
+        """Test that bug 1879787 can no longer manifest itself because we get
+        the network_info from the instance info cache, and not Neutron.
         """
+        def stub_notify(context, instance, event_suffix,
+                        network_info=None, extra_usage_info=None, fault=None):
+            vif = network_info[0]
+            # Make sure we have the correct VIF (the NeutronFixture
+            # deterministically uses port_2 for networks=auto) and that the
+            # profile does not contain `migrating_to`, indicating that we did
+            # not obtain it from the Neutron API.
+            self.assertEqual(self.neutron.port_2['id'], vif['id'])
+            self.assertNotIn('migrating_to', vif['profile'])
+
         server = self._create_server(networks='auto',
                                      host=self.computes['src'].host)
 
-        orig_plm = self.computes['src'].manager._post_live_migration
-
-        def stub_plm(*args, **kwargs):
-            """We simulate a failure in driver.post_live_migration() on the
-            source by stubbing the source compute's _post_live_migration() with
-            a method that, within a context that mocks
-            driver.post_live_migration() to raise an exception, calls the
-            original compute.manager._post_live_migration(). This is needed to
-            make sure driver.post_live_migration() raises only once, and only
-            on the source.
-            """
-            with mock.patch.object(self.computes['src'].manager.network_api,
-                                   'get_instance_nw_info',
-                                   side_effect=ConnectionError):
-                return orig_plm(*args, **kwargs)
-
         with mock.patch.object(self.computes['src'].manager,
-                               '_post_live_migration',
-                               side_effect=stub_plm):
-            # FIXME(artom) Until bug 1879787 is fixed, the raised
-            # ConnectionError will go unhandled, the migration will fail, and
-            # the instance will still be reported as being on the source, even
-            # though it's actually running on the destination.
-            self._live_migrate(server, 'error', server_expected_state='ERROR')
+                              '_notify_about_instance_usage',
+                              side_effect=stub_notify) as mock_notify:
+            self._live_migrate(server, 'completed')
             server = self.api.get_server(server['id'])
-            self.assertEqual('src', server['OS-EXT-SRV-ATTR:host'])
+            self.assertEqual('dest', server['OS-EXT-SRV-ATTR:host'])
+            # We don't care about call arguments here, we just want to be sure
+            # our stub actually got called.
+            mock_notify.assert_called()

nova/tests/unit/compute/test_compute.py

@@ -6562,12 +6562,15 @@ class ComputeTestCase(BaseTestCase,
                               'clear_events_for_instance'),
             mock.patch.object(self.compute, 'update_available_resource'),
             mock.patch.object(migration_obj, 'save'),
+            mock.patch.object(instance, 'get_network_info'),
         ) as (
             post_live_migration,
             migrate_instance_start, post_live_migration_at_destination,
             post_live_migration_at_source, setup_networks_on_host,
-            clear_events, update_available_resource, mig_save
+            clear_events, update_available_resource, mig_save, get_nw_info,
         ):
+            nw_info = network_model.NetworkInfo.hydrate([])
+            get_nw_info.return_value = nw_info
             self.compute._post_live_migration(c, instance, dest,
                                               migrate_data=migrate_data,
                                               source_bdms=bdms)
@@ -6589,7 +6592,7 @@ class ComputeTestCase(BaseTestCase,
             post_live_migration_at_destination.assert_has_calls([
                 mock.call(c, instance, False, dest)])
             post_live_migration_at_source.assert_has_calls(
-                [mock.call(c, instance, [])])
+                [mock.call(c, instance, nw_info)])
             clear_events.assert_called_once_with(instance)
             update_available_resource.assert_has_calls([mock.call(c)])
             self.assertEqual('completed', migration_obj.status)