docker: access system_metadata as a dict

This bit of code causes a problem when moving to objects.  The Instance
object presents system_metadata as you would normally expect (a dict).
The data is in a different format when working with a db instance.  Use
the handy helper utility to normalize it into a dict.

Change-Id: Ieca9ab449a07374ed2c9cdaffdfd54783df7a866

nova/tests/utils.py

@@ -28,6 +28,7 @@ from nova import exception
 from nova.image import glance
 from nova.network import minidns
 from nova.network import model as network_model
+from nova.objects import instance as instance_obj
 
 CONF = cfg.CONF
 CONF.import_opt('use_ipv6', 'nova.netconf')
@@ -71,7 +72,7 @@ def get_test_instance_type(context=None, options=None):
     return instance_type_ref
 
 
-def get_test_instance(context=None, instance_type=None):
+def get_test_instance(context=None, instance_type=None, obj=False):
     if not context:
         context = get_test_admin_context()
 
@@ -93,8 +94,12 @@ def get_test_instance(context=None, instance_type=None):
                      'system_metadata': metadata,
                      'extra_specs': {}}
 
-    instance_ref = nova.db.instance_create(context, test_instance)
-    return instance_ref
+    if obj:
+        instance = instance_obj.Instance(context, **test_instance)
+        instance.create()
+    else:
+        instance = nova.db.instance_create(context, test_instance)
+    return instance
 
 
 def get_test_network_info(count=1):

nova/tests/virt/docker/test_driver.py

@@ -165,3 +165,13 @@ class DockerDriverTestCase(_VirtDriverTestCase, test.TestCase):
         instance_href = utils.get_test_instance()
         self.connection.destroy(self.context, instance_href,
                 'fake_networkinfo')
+
+    def test_get_memory_limit_from_sys_meta_in_object(self):
+        instance = utils.get_test_instance(obj=True)
+        limit = self.connection._get_memory_limit_bytes(instance)
+        self.assertEqual(2048 * unit.Mi, limit)
+
+    def test_get_memory_limit_from_sys_meta_in_db_instance(self):
+        instance = utils.get_test_instance(obj=False)
+        limit = self.connection._get_memory_limit_bytes(instance)
+        self.assertEqual(2048 * unit.Mi, limit)

nova/virt/docker/driver.py

@@ -259,12 +259,8 @@ class DockerDriver(driver.ComputeDriver):
             undo_mgr.rollback_and_reraise(msg=msg, instance=instance)
 
     def _get_memory_limit_bytes(self, instance):
-        for metadata in instance.get('system_metadata', []):
-            if metadata['deleted']:
-                continue
-            if metadata['key'] == 'instance_type_memory_mb':
-                return int(metadata['value']) * unit.Mi
-        return 0
+        system_meta = utils.instance_sys_meta(instance)
+        return int(system_meta.get('instance_type_memory_mb', 0)) * unit.Mi
 
     def _get_image_name(self, context, instance, image):
         fmt = image['container_format']