Cluster.extenstions shouldn't have duplications

PUT converts input data to set(), so
ClusterExtensionsHandler will never produce duplications
in a list of enabled extensions.

Change-Id: I3631f9ba8bebc24102c13544d8463afdddd0bef4
Blueprint: extensions-management
Related-Bug: #1614526

nailgun/nailgun/api/v1/handlers/cluster.py

@@ -548,6 +548,6 @@ class ClusterExtensionsHandler(BaseHandler):
                * 404 (cluster not found in db)
         """
         cluster = self._get_cluster_obj(cluster_id)
-        data = self.checked_data()
+        data = set(self.checked_data())
         set_extensions_for_object(cluster, data)
         return cluster.extensions

nailgun/nailgun/extensions/manager.py

@@ -199,7 +199,8 @@ def _collect_data_pipelines_for_cluster(cluster):
 
 
 def _collect_extensions_for_cluster(cluster):
-    return [get_extension(e) for e in set(cluster.extensions)]
+    for e in cluster.extensions:
+        yield get_extension(e)
 
 
 def fire_callback_on_node_serialization_for_deployment(node, node_data):

nailgun/nailgun/test/integration/test_cluster_handler.py

@@ -418,6 +418,25 @@ class TestClusterExtension(BaseIntegrationTest):
         self.assertEqual(resp.status_code, 200)
         self.assertItemsEqual(resp.json_body, enabled_extensions)
 
+    def test_enabling_duplicated_extensions(self):
+        extensions = 'bareon', 'volume_manager'
+        requested_extensions = 2 * extensions
+
+        with mock.patch(
+                'nailgun.api.v1.validators.extension.get_all_extensions',
+                return_value=make_mock_extensions(extensions)):
+            resp = self.app.put(
+                reverse(
+                    'ClusterExtensionsHandler',
+                    kwargs={'cluster_id': self.cluster.id}),
+                jsonutils.dumps(requested_extensions),
+                headers=self.default_headers,
+            )
+        self.assertEqual(resp.status_code, 200)
+
+        self.db.refresh(self.cluster)
+        self.assertItemsEqual(self.cluster.extensions, extensions)
+
     def test_enabling_extensions(self):
         extensions = 'bareon', 'volume_manager'