Add CORS filter to versions pipeline

This patch exposes the root (version) resource to CORS-enabled
browser clients that wish to perform API version discovery.

This feature is required for js-openstack-lib, and its devstack
tests. https://review.openstack.org/#/c/370389/

Change-Id: I4052186bffa3bddb4d28e378e6a135694a71e05b
Needed-By: Ia19afa530ae8c2df60db740cec9267fe240978c0

etc/nova/api-paste.ini

@@ -61,7 +61,7 @@ paste.filter_factory = nova.api.openstack:LegacyV2CompatibleWrapper.factory
 paste.app_factory = nova.api.openstack.compute:APIRouterV21.factory
 
 [pipeline:oscomputeversions]
-pipeline = faultwrap http_proxy_to_wsgi oscomputeversionapp
+pipeline = cors faultwrap http_proxy_to_wsgi oscomputeversionapp
 
 [app:oscomputeversionapp]
 paste.app_factory = nova.api.openstack.compute.versions:Versions.factory

nova/tests/functional/test_middleware.py

@@ -96,3 +96,27 @@ class TestCORSMiddleware(api_sample_base.ApiSampleTestBaseV21):
 
         self.assertEqual(response.status_code, 200)
         self.assertNotIn('Access-Control-Allow-Origin', response.headers)
+
+    def test_valid_cors_get_versions_request(self):
+        response = self._do_get('',
+                                strip_version=True,
+                                headers={
+                                    'Origin': 'http://valid.example.com',
+                                    'Access-Control-Request-Method': 'GET'
+                                })
+
+        self.assertEqual(response.status_code, 200)
+        self.assertIn('Access-Control-Allow-Origin', response.headers)
+        self.assertEqual('http://valid.example.com',
+                         response.headers['Access-Control-Allow-Origin'])
+
+    def test_invalid_cors_get_versions_request(self):
+        response = self._do_get('',
+                                strip_version=True,
+                                headers={
+                                    'Origin': 'http://invalid.example.com',
+                                    'Access-Control-Request-Method': 'GET'
+                                })
+
+        self.assertEqual(response.status_code, 200)
+        self.assertNotIn('Access-Control-Allow-Origin', response.headers)

releasenotes/notes/add-cors-to-versions-pipeline-56277ca66e796569.yaml

@@ -0,0 +1,6 @@
+---
+upgrade:
+  - CORS headers have been added to the versions REST API, in order
+    to support versions discovery. If using 'oscomputeversions'
+    pipeline in '/etc/nova/api-paste.ini', it is required to update the
+    pipeline to include 'cors' filter.