Fix get min interval error

When we use get_min_interval method, the exit condition is
the execution time is bigger than the rate time. But default
minutes rate is 2, this may leads errors when the interval
is bigger than 2 minutes, it always return None. So we can
not use min interval to judge if the interval is smaller than
that we configured in the configuration file. This should be
fixed.
Story: 2006848
Task: 37436

Change-Id: I268051f05f9b89e1b6013cfc8cecf90e9dc78581

karbor/services/operationengine/engine/triggers/timetrigger/timeformats/calendar_time.py

@@ -11,7 +11,7 @@
 #    under the License.
 import os
 
-from datetime import timedelta
+from datetime import datetime
 from dateutil import rrule
 from icalendar import Calendar
 from oslo_serialization import jsonutils
@@ -121,23 +121,9 @@ class ICal(timeformats.TimeFormat):
         :return: int(seconds) or None
 
         """
-        gen = self.rrule_obj
-        kwargs = FREQ_TO_KWARGS[self.min_freq]
-        endtime = self.dtstart + timedelta(**kwargs)
-
-        deltas = []
-        t0 = None
-        for dt in gen:
-            if dt > endtime:
-                break
-            t1 = t0
-            t0 = dt
-            if t1 is None or t0 is None or dt <= self.dtstart:
-                continue
-            delta = timeutils.delta_seconds(t1, t0)
-            if delta:
-                deltas.append(delta)
-        if len(deltas):
-            return min(deltas)
-        else:
+        try:
+            t1 = self.compute_next_time(datetime.now())
+            t2 = self.compute_next_time(t1)
+            return timeutils.delta_seconds(t1, t2)
+        except Exception:
             return None

karbor/tests/unit/operationengine/engine/triggers/timetrigger/timeformats/test_calendar_time.py

@@ -173,3 +173,13 @@ class CalendarTimeTestCase(base.TestCase):
         dtstart = datetime(2016, 2, 20, 17, 0, 0)
         time_obj = calendar_time.ICal(dtstart, pattern)
         self.assertIsNone(time_obj.get_min_interval())
+
+    def test_get_min_interval_when_interval_is_bigger_than_default_rate(self):
+        pattern = (
+            "BEGIN:VEVENT\n"
+            "RRULE:FREQ=MINUTELY;INTERVAL=5;\n"
+            "END:VEVENT"
+        )
+        dtstart = datetime(2016, 2, 20, 17, 0, 0)
+        time_obj = calendar_time.ICal(dtstart, pattern)
+        self.assertEqual(300, time_obj.get_min_interval())