[Fix] Fix issues in verification.tempest.diff

Changes in rally.verification.tempest.diff._diff_values:
 * fix ZeroDivisionError
 * fix time threshold calculation

This patch also adds unit test

Change-Id: Ib52d89397ab491d2bc6c40197cb8222fc35cadd7

rally/verification/tempest/diff.py

@@ -69,15 +69,19 @@ class Diff(object):
         return diffs
 
     def _diff_values(self, name, result1, result2):
-        th = self.threshold
         fields = ["status", "time", "output"]
         diffs = []
         for field in fields:
             val1 = result1[field]
             val2 = result2[field]
-            if val1 != val2 and not (field == "time"
+            if val1 != val2:
-                                     and abs(((val2 - val1) / val1) * 100)
+                if field == "time":
-                                     < th):
+                    max_ = max(val1, val2)
+                    min_ = min(val1, val2)
+                    time_threshold = ((max_ - min_) / (min_ or 1)) * 100
+                    if time_threshold < self.threshold:
+                        continue
+
                 diffs.append({
                     "field": field,
                     "type": "value_changed",

tests/unit/verification/test_diff.py

@@ -9,6 +9,7 @@
 #    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 #    License for the specific language governing permissions and limitations
 #    under the License.
+
 from rally.verification.tempest import diff
 from tests.unit import test
 
@@ -87,3 +88,20 @@ class DiffTestCase(test.TestCase):
         assert diff_.to_csv() != ""
         assert diff_.to_html() != ""
         assert diff_.to_json() != ""
+
+    def test_zero_values(self):
+        results1 = {"test.one": {"name": "test.one",
+                                 "output": "test.one",
+                                 "status": "OK",
+                                 "time": 1}}
+
+        results2 = {"test.one": {"name": "test.one",
+                                 "output": "test.one",
+                                 "status": "FAIL",
+                                 "time": 0}}
+
+        # This must NOT raise ZeroDivisionError
+        diff_ = diff.Diff(results1, results2, 0)
+        self.assertEqual(2, len(diff_.diffs))
+        diff_ = diff.Diff(results2, results1, 0)
+        self.assertEqual(2, len(diff_.diffs))