Fix sample config value when set_defaults is used

By calling set_default[1] on a conf object it only applies to opts
registered to that object. This causes an incorrect value to appear in
the generated sample config because it deals with a list of raw opts,
not a conf object.

To fix this, we can call the global set_defaults[2] on the cfg module
which alters the opts directly. This is the method used in the cors
middleware[3] and works as expected there.

This does complicate the unit tests, however. Because we're altering
global state we need to save the original opts and restore them after
the test. Furthermore, the conf.reset() call in the config fixture
doesn't sufficiently reset the conf object to allow it to recognize
the replaced opts. For the purposes of this test we can just create
a standalone conf object though, which gets past that problem.

It's possible that we should fix reset() so it actually removes opts
in groups completely, but I'm unsure what implications that might
have for other users of the function.

1: b5df53543f/oslo_config/cfg.py (L2433)
2: b5df53543f/oslo_config/cfg.py (L391)
3: 8c7fa5bb10/oslo_middleware/cors.py (L88)

Change-Id: I3af9de1b39b6360ecfcb448d8c37b463e1a42ca7
Closes-Bug: 1807184

oslo_policy/opts.py

@@ -120,4 +120,4 @@ def set_defaults(conf, policy_file=None):
     _register(conf)
 
     if policy_file is not None:
-        conf.set_default('policy_file', policy_file, group=_option_group)
+        cfg.set_defaults(_options, policy_file=policy_file)

oslo_policy/tests/test_opts.py

@@ -10,7 +10,9 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-from oslo_config import fixture as config
+import copy
+
+from oslo_config import cfg
 from oslotest import base as test_base
 
 from oslo_policy import opts
@@ -20,7 +22,13 @@ class OptsTestCase(test_base.BaseTestCase):
 
     def setUp(self):
         super(OptsTestCase, self).setUp()
-        self.conf = self.useFixture(config.Config()).conf
+        self.conf = cfg.ConfigOpts()
+        self.original_opts = opts._options
+        opts._options = copy.deepcopy(opts._options)
+
+        def reset():
+            opts._options = self.original_opts
+        self.addCleanup(reset)
 
     def test_set_defaults_policy_file(self):
         opts._register(self.conf)