During DST transition, handle ambiguous dates as pre-transition. Fixes #196

CHANGELOG.md

@@ -7,6 +7,9 @@ All notable changes to this project will be documented in this file.
 - Load settings from environment variables
 - Add "List-Unsubscribe" header to alert and report emails
 
+### Bug Fixes
+- During DST transition, handle ambiguous dates as pre-transition
+
 
 ## 1.2.0 - 2018-10-20
 

hc/api/models.py

@@ -116,7 +116,7 @@ class Check(models.Model):
             last_naive = timezone.make_naive(self.last_ping)
             it = croniter(self.schedule, last_naive)
             next_naive = it.get_next(datetime)
-            return timezone.make_aware(next_naive, is_dst=False)
+            return timezone.make_aware(next_naive, is_dst=True)
 
     def get_status(self, now=None):
         """ Return "up" if the check is up or in grace, otherwise "down". """

hc/front/tests/test_cron_preview.py

@@ -1,4 +1,8 @@
+from datetime import datetime
+
 from hc.test import BaseTestCase
+from mock import patch
+import pytz
 
 
 class CronPreviewTestCase(BaseTestCase):
@@ -30,3 +34,21 @@ class CronPreviewTestCase(BaseTestCase):
     def test_it_rejects_get(self):
         r = self.client.get("/checks/cron_preview/", {})
         self.assertEqual(r.status_code, 405)
+
+    @patch("hc.front.views.timezone.now")
+    def test_it_handles_dst_transition(self, mock_now):
+        # Consider year 2018, Riga, Latvia:
+        # The daylight-saving-time ends at 4AM on October 28.
+        # At that time, the clock is turned back one hour.
+        # So, on that date,  3AM happens *twice* and saying
+        # "3AM on October 28" is ambiguous.
+        mock_now.return_value = datetime(2018, 10, 26, tzinfo=pytz.UTC)
+
+        # This schedule will hit the ambiguous date. Cron preview must
+        # be able to handle this:
+        payload = {
+            "schedule": "0 3 * * *",
+            "tz": "Europe/Riga"
+        }
+        r = self.client.post("/checks/cron_preview/", payload)
+        self.assertNotContains(r, "Invalid cron expression", status_code=200)

hc/front/views.py

@@ -281,7 +281,7 @@ def cron_preview(request):
             it = croniter(schedule, now_naive)
             for i in range(0, 6):
                 naive = it.get_next(datetime)
-                aware = timezone.make_aware(naive)
+                aware = timezone.make_aware(naive, is_dst=True)
                 ctx["dates"].append((naive, aware))
     except UnknownTimeZoneError:
         ctx["bad_tz"] = True