Prevent StopIteration exception during parse response

- Parse response body only if there is one top-level element
in JSON object because if there is nullable JSON in response body
six will throw us an StopIteration exception from six.next

Closes-Bug: #1529144

Change-Id: Iea2d0950b25cb8f576ca7e15383a205e04d7c9e3

tempest_lib/common/rest_client.py

@@ -461,7 +461,9 @@ class RestClient(object):
         #  }
         try:
             # Ensure there are not more than one top-level keys
-            if len(body.keys()) > 1:
+            # NOTE(freerunner): Ensure, that JSON is not nullable to
+            # to prevent StopIteration Exception
+            if len(body.keys()) != 1:
                 return body
             # Just return the "wrapped" element
             first_key, first_item = six.next(six.iteritems(body))

tempest_lib/tests/test_rest_client.py

@@ -244,6 +244,7 @@ class TestRestClientParseRespJSON(BaseRestClientTestClass):
         keys[0]: values[0],
         keys[1]: values[1],
     }}
+    null_dict = {}
 
     def setUp(self):
         self.fake_http = fake_http.fake_httplib2()
@@ -273,6 +274,10 @@ class TestRestClientParseRespJSON(BaseRestClientTestClass):
         body = self.rest_client._parse_resp(json.dumps(data))
         self.assertEqual(data, body)
 
+    def test_parse_nullable_dict(self):
+        body = self.rest_client._parse_resp(json.dumps(self.null_dict))
+        self.assertEqual(self.null_dict, body)
+
 
 class TestRestClientErrorCheckerJSON(base.TestCase):
     c_type = "application/json"