Provide compatibliity for db.compute_node_statistics

As some computes are providing the host field and others are not, let's modify
how we calculate the statistics by merging both versions.

Note: This change won't impact how the API is getting stastistics as the result
is not a ComputeNode or a list of ComputeNodes but instead a mix of stats.

Partially-Implements blueprint detach-service-from-computenode

Change-Id: I889efc41babb2c82018122b5f255eb0fca9b452b

nova/db/sqlalchemy/api.py

@@ -634,6 +634,12 @@ def compute_node_delete(context, compute_id):
 
 def compute_node_statistics(context):
     """Compute statistics over all compute nodes."""
+
+    # TODO(sbauza): Remove the service_id filter in a later release
+    # once we are sure that all compute nodes report the host field
+    _filter = or_(models.Service.host == models.ComputeNode.host,
+                  models.Service.id == models.ComputeNode.service_id)
+
     result = model_query(context,
                          models.ComputeNode, (
                              func.count(models.ComputeNode.id),
@@ -650,9 +656,7 @@ def compute_node_statistics(context):
                              func.sum(models.ComputeNode.disk_available_least),
                          ), read_deleted="no").\
                          filter(models.Service.disabled == false()).\
-                         filter(
+                         filter(_filter).\
-                            models.Service.id ==
-                            models.ComputeNode.service_id).\
                          first()
 
     # Build a dict of the info--making no assumptions about result

nova/tests/unit/db/test_db_api.py

@@ -6430,6 +6430,25 @@ class ComputeNodeTestCase(test.TestCase, ModelsObjectComparatorMixin):
         stats = db.compute_node_statistics(self.ctxt)
         self.assertEqual(stats.pop('count'), 0)
 
+    def test_compute_node_statistics_with_old_service_id(self):
+        # NOTE(sbauza): This test is only for checking backwards compatibility
+        # with old versions of compute_nodes not providing host column.
+        # This test could be removed once we are sure that all compute nodes
+        # are populating the host field thanks to the ResourceTracker
+
+        service2 = self.service_dict.copy()
+        service2['host'] = 'host2'
+        db_service2 = db.service_create(self.ctxt, service2)
+        compute_node_old_host = self.compute_node_dict.copy()
+        compute_node_old_host['stats'] = jsonutils.dumps(self.stats)
+        compute_node_old_host['hypervisor_hostname'] = 'node_2'
+        compute_node_old_host['service_id'] = db_service2['id']
+        compute_node_old_host.pop('host')
+
+        db.compute_node_create(self.ctxt, compute_node_old_host)
+        stats = db.compute_node_statistics(self.ctxt)
+        self.assertEqual(2, stats.pop('count'))
+
     def test_compute_node_not_found(self):
         self.assertRaises(exception.ComputeHostNotFound, db.compute_node_get,
                           self.ctxt, 100500)