Return None from generic plugin if failure

The create_plugin method can be called multiple times with different
version values and the generic plugin should return an object if it is
suitable. By raising an error here when you have invalid parameters it
prevents the generic handler attempting additional version options.

Change-Id: I3391c6607fc53a154e10bee7e741bf73afeae5fa

keystoneauth1/identity/generic/password.py

@@ -12,7 +12,6 @@
 
 from keystoneauth1 import _utils as utils
 from keystoneauth1 import discover
-from keystoneauth1 import exceptions
 from keystoneauth1.identity.generic import base
 from keystoneauth1.identity import v2
 from keystoneauth1.identity import v3
@@ -45,8 +44,7 @@ class Password(base.BaseGenericPlugin):
     def create_plugin(self, session, version, url, raw_status=None):
         if discover.version_match((2,), version):
             if self._user_domain_id or self._user_domain_name:
-                raise exceptions.DiscoveryFailure(
-                    'Cannot use v2 authentication with domain scope')
+                return None
 
             return v2.Password(auth_url=url,
                                user_id=self._user_id,

keystoneauth1/tests/unit/loading/test_generic.py

@@ -10,7 +10,12 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
+import uuid
+
+from keystoneauth1 import fixture
+from keystoneauth1 import identity
 from keystoneauth1.loading._plugins.identity import generic
+from keystoneauth1 import session
 from keystoneauth1.tests.unit.loading import utils
 
 
@@ -37,6 +42,24 @@ class PasswordTests(utils.TestCase):
         self.assertEqual(set(allowed_opts), set(opts))
         self.assertEqual(len(allowed_opts), len(opts))
 
+    def test_loads_v3_with_user_domain(self):
+        auth_url = 'http://keystone.test:5000'
+        disc = fixture.DiscoveryList(href=auth_url)
+        sess = session.Session()
+        self.requests_mock.get(auth_url, json=disc)
+
+        plugin = generic.Password().load_from_options(
+            auth_url=auth_url,
+            user_id=uuid.uuid4().hex,
+            password=uuid.uuid4().hex,
+            project_id=uuid.uuid4().hex,
+            user_domain_id=uuid.uuid4().hex)
+
+        inner_plugin = plugin._do_create_plugin(sess)
+
+        self.assertIsInstance(inner_plugin, identity.V3Password)
+        self.assertEqual(inner_plugin.auth_url, auth_url + '/v3')
+
 
 class TokenTests(utils.TestCase):