api: Set min, maxItems for server_group.policies field

As noted inline, the 'policies' field may be a list but it expects one
of two items.

Change-Id: I34c68df1e6330dab1524aa0abec733610211a407
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>
Closes-Bug: #1894966

nova/api/openstack/compute/schemas/server_groups.py

@@ -11,6 +11,7 @@
 #    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 #    License for the specific language governing permissions and limitations
 #    under the License.
+
 import copy
 
 from nova.api.validation import parameter_types
@@ -27,15 +28,18 @@ create = {
                 'name': parameter_types.name,
                 'policies': {
                     # This allows only a single item and it must be one of the
-                    # enumerated values. So this is really just a single string
-                    # value, but for legacy reasons is an array. We could
-                    # probably change the type from array to string with a
-                    # microversion at some point but it's very low priority.
+                    # enumerated values. It's changed to a single string value
+                    # in 2.64.
                     'type': 'array',
-                    'items': [{
-                        'type': 'string',
-                        'enum': ['anti-affinity', 'affinity']}],
+                    'items': [
+                        {
+                            'type': 'string',
+                            'enum': ['anti-affinity', 'affinity'],
+                        },
+                    ],
                     'uniqueItems': True,
+                    'minItems': 1,
+                    'maxItems': 1,
                     'additionalItems': False,
                 }
             },

nova/tests/functional/regressions/test_bug_1894966.py

@@ -37,5 +37,4 @@ class TestCreateServerGroupWithEmptyPolicies(
             client.OpenStackApiException,
             self.api.post_server_groups,
             {'name': 'test group', 'policies': []})
-        # FIXME(stephenfin): This should not be a 500 error
-        self.assertEqual(500, exc.response.status_code)
+        self.assertEqual(400, exc.response.status_code)

releasenotes/notes/bug-1894966-d25c12b1320cb910.yaml

@@ -0,0 +1,10 @@
+---
+fixes:
+  - |
+    Resolved an issue whereby providing an empty list for the ``policies``
+    field in the request body of the ``POST /os-server-groups`` API would
+    result in a server error. This only affects the 2.1 to 2.63 microversions,
+    as the 2.64 microversion replaces the ``policies`` list field with a
+    ``policy`` string field. See `bug #1894966`__ for more information.
+
+    .. __: https://bugs.launchpad.net/nova/+bug/1894966