Add StableObjectJsonFixture and use it in our base test class

This will ensure that our object JSON is stable during unit tests.
Specifically, the changes field, which is listified from a set, which
can vary in order.

Closes-Bug: #1545177
Change-Id: Ib40ca16c41d5422c18f4fc7d658273a8059100ab

nova/test.py

@@ -226,6 +226,8 @@ class TestCase(testtools.TestCase):
             objects_base.NovaObjectRegistry._registry._obj_classes)
         self.addCleanup(self._restore_obj_registry)
 
+        self.useFixture(nova_fixtures.StableObjectJsonFixture())
+
         # NOTE(mnaser): All calls to utils.is_neutron() are cached in
         # nova.utils._IS_NEUTRON.  We set it to None to avoid any
         # caching of that value.

nova/tests/fixtures.py

@@ -446,3 +446,31 @@ class BannedDBSchemaOperations(fixtures.Fixture):
             self.useFixture(fixtures.MonkeyPatch(
                 'sqlalchemy.%s.alter' % thing,
                 lambda *a, **k: self._explode(thing, 'alter')))
+
+
+class StableObjectJsonFixture(fixtures.Fixture):
+    """Fixture that makes sure we get stable JSON object representations.
+
+    Since objects contain things like set(), which can't be converted to
+    JSON, we have some situations where the representation isn't fully
+    deterministic. This doesn't matter at all at runtime, but does to
+    unit tests that try to assert things at a low level.
+
+    This fixture mocks the obj_to_primitive() call and makes sure to
+    sort the list of changed fields (which came from a set) before
+    returning it to the caller.
+    """
+    def __init__(self):
+        self._original_otp = obj_base.NovaObject.obj_to_primitive
+
+    def setUp(self):
+        super(StableObjectJsonFixture, self).setUp()
+
+        def _doit(obj, *args, **kwargs):
+            result = self._original_otp(obj, *args, **kwargs)
+            if 'nova_object.changes' in result:
+                result['nova_object.changes'].sort()
+            return result
+
+        self.useFixture(fixtures.MonkeyPatch(
+            'nova.objects.base.NovaObject.obj_to_primitive', _doit))