Invalidate AZ cache when the instance AZ information is different

When a new instance is created, the host value is set to None. As a corrolar,
its AZ field is set to the default_availability_zone value.

That's only when the host field is provided (ie. once the instance is started) that
the AZ information is also updated. Unfornately, as the AZ module is using a cache
system, it leaves the old AZ unchanged which can make the instance AZ data wrong.

In order to fix that corner case, we need to compare both cache and instance values
and if different, invalidate the cache key and leave it updated correctly by calling
the AZ related information of the aggregate the host belongs to.

Change-Id: I1b47ae07cf8a4245df00b9dc25e6000428bdc4f2
Closes-Bug: #1390033

nova/availability_zones.py

@@ -178,6 +178,20 @@ def get_instance_availability_zone(context, instance):
     cache_key = _make_cache_key(host)
     cache = _get_cache()
     az = cache.get(cache_key)
+    az_inst = instance.get('availability_zone')
+    if az_inst is not None and az != az_inst:
+        # NOTE(sbauza): Cache is wrong, we need to invalidate it by fetching
+        # again the right AZ related to the aggregate the host belongs to.
+        # As the API is also calling this method for setting the instance
+        # AZ field, we don't need to update the instance.az field.
+        # This case can happen because the cache is populated before the
+        # instance has been assigned to the host so that it would keep the
+        # former reference which was incorrect. Instead of just taking the
+        # instance AZ information for refilling the cache, we prefer to
+        # invalidate the cache and fetch it again because there could be some
+        # corner cases where this method could be called before the instance
+        # has been assigned to the host also.
+        az = None
     if not az:
         elevated = context.elevated()
         az = get_host_availability_zone(elevated, host)

nova/tests/unit/test_availability_zones.py

@@ -17,6 +17,7 @@
 Tests for availability zones
 """
 
+import mock
 from oslo_config import cfg
 import six
 
@@ -239,7 +240,8 @@ class AvailabilityZoneTestCases(test.TestCase):
 
     def test_get_instance_availability_zone_default_value(self):
         """Test get right availability zone by given an instance."""
-        fake_inst = objects.Instance(host=self.host)
+        fake_inst = objects.Instance(host=self.host,
+                                     availability_zone=None)
 
         self.assertEqual(self.default_az,
                 az.get_instance_availability_zone(self.context, fake_inst))
@@ -250,11 +252,25 @@ class AvailabilityZoneTestCases(test.TestCase):
         service = self._create_service_with_topic('compute', host)
         self._add_to_aggregate(service, self.agg)
 
-        fake_inst = objects.Instance(host=host)
+        fake_inst = objects.Instance(host=host,
+                                     availability_zone=self.availability_zone)
 
         self.assertEqual(self.availability_zone,
                 az.get_instance_availability_zone(self.context, fake_inst))
 
+    @mock.patch.object(az._get_cache(), 'get')
+    def test_get_instance_availability_zone_cache_differs(self, cache_get):
+        host = 'host170'
+        service = self._create_service_with_topic('compute', host)
+        self._add_to_aggregate(service, self.agg)
+        cache_get.return_value = self.default_az
+
+        fake_inst = objects.Instance(host=host,
+                                     availability_zone=self.availability_zone)
+        self.assertEqual(
+            self.availability_zone,
+            az.get_instance_availability_zone(self.context, fake_inst))
+
     def test_get_instance_availability_zone_no_host(self):
         """Test get availability zone from instance if host not set."""
         fake_inst = objects.Instance(host=None, availability_zone='inst-az')

nova/tests/unit/test_metadata.py

@@ -83,7 +83,8 @@ def fake_inst_obj(context):
         metadata={},
         default_ephemeral_device=None,
         default_swap_device=None,
-        system_metadata={})
+        system_metadata={},
+        availability_zone=None)
     nwinfo = network_model.NetworkInfo([])
     inst.info_cache = objects.InstanceInfoCache(context=context,
                                                 instance_uuid=inst.uuid,