Fix unrecognized subparser args

Someone thought a transient attr would be a clever way to smuggle data
from a _SubParsersAction up to the main ArgumentsParser. Unfortunately
they use a really weird way to set the attr:

    vars(namespace).setdefault(_UNRECOGNIZED_ARGS_ATTR, [])

This bypasses __setattr__ so when they try to delattr later,
__delattr__ can't find it and bombs. Even though the delattr is
protected by a hasattr, because hasattr uses getattr which checks the
private object dict first before calling __getattr__.

Handle this by falling back to super in __getattr__ and __delattr__.

Change-Id: Id89a0c00fd32b6580d891ccf69d58fa17f11ec49
Closes-Bug: #1540959

oslo_config/cfg.py

@@ -1776,15 +1776,13 @@ class _Namespace(argparse.Namespace):
         try:
             return self._cli[name]
         except KeyError:
-            raise AttributeError(
-                "'_Namespace' object has no attribute '%s'" % name)
+            return super(_Namespace, self).__getattr__(name)
 
     def __delattr__(self, name):
         try:
             del self._cli[name]
         except KeyError:
-            raise AttributeError(
-                "'_Namespace' object has no attribute '%s'" % name)
+            return super(_Namespace, self).__delattr__(name)
 
     def _parse_cli_opts_from_config_file(self, sections, normalized):
         """Parse CLI options from a config file.

oslo_config/tests/test_cfg.py

@@ -3457,6 +3457,18 @@ class NamespaceTestCase(BaseTestCase):
         self.assertRaises(AttributeError, self.ns.__getattr__, 'boo')
         self.assertRaises(AttributeError, self.ns.__delattr__, 'boo')
 
+    def test_attrs_subparser(self):
+        CONF = cfg.ConfigOpts()
+        CONF.register_cli_opt(cfg.SubCommandOpt(
+            'foo', handler=lambda sub: sub.add_parser('foo')))
+        CONF(['foo'])
+
+    def test_attrs_subparser_failure(self):
+        CONF = cfg.ConfigOpts()
+        CONF.register_cli_opt(cfg.SubCommandOpt(
+            'foo', handler=lambda sub: sub.add_parser('foo')))
+        self.assertRaises(SystemExit, CONF, ['foo', 'bar'])
+
 
 class TildeExpansionTestCase(BaseTestCase):