[FIX] calendar: recurring event around DST

Be in timezone Europe/Brussels, where, in 2018, the DST switch
happens on 2018-10-28.

Create a recurring event starting on 2018-10-27 until 2018-10-29
at say, 8am

For the days before the 28, the time of the event will be correct

Before this commit though, for the days after the 28,
the meeting time will be shifted by one hour
This is because the rrule took a TZ aware datetime, and generated recurring
event dates with that same TZ, i.e. the one in STD time

After, this commit, all the meetings are well scheduled at 8am,

OPW 1895315

closes odoo/odoo#27985

addons/calendar/models/calendar.py

@@ -609,13 +609,16 @@ class Meeting(models.Model):
             event_date = datetime.now()
 
         use_naive_datetime = self.allday and self.rrule and 'UNTIL' in self.rrule and 'Z' not in self.rrule
-        if use_naive_datetime:
-            rset1 = rrule.rrulestr(str(self.rrule), dtstart=event_date.replace(tzinfo=None), forceset=True, ignoretz=True)
-        else:
+        if not use_naive_datetime:
             # Convert the event date to saved timezone (or context tz) as it'll
             # define the correct hour/day asked by the user to repeat for recurrence.
-            event_date = event_date.astimezone(timezone)  # transform "+hh:mm" timezone
-            rset1 = rrule.rrulestr(str(self.rrule), dtstart=event_date, forceset=True, tzinfos={})
+            event_date = event_date.astimezone(timezone)
+
+        # The start date is naive
+        # the timezone will be applied, if necessary, at the very end of the process
+        # to allow for DST timezone reevaluation
+        rset1 = rrule.rrulestr(str(self.rrule), dtstart=event_date.replace(tzinfo=None), forceset=True, ignoretz=True)
+
         recurring_meetings = self.search([('recurrent_id', '=', self.id), '|', ('active', '=', False), ('active', '=', True)])
 
         # We handle a maximum of 50,000 meetings at a time, and clear the cache at each step to
@@ -629,12 +632,15 @@ class Meeting(models.Model):
                 if use_naive_datetime:
                     recurring_date = recurring_date.replace(tzinfo=None)
                 else:
-                    recurring_date = todate(meeting.recurrent_id_date)
+                    recurring_date = todate(meeting.recurrent_id_date).replace(tzinfo=None)
                 if date_field == "stop":
                     recurring_date += timedelta(hours=self.duration)
                 rset1.exdate(recurring_date)
             invalidate = True
-        return [d.astimezone(pytz.UTC) if d.tzinfo else d for d in rset1 if d.year < MAXYEAR]
+
+        def naive_tz_to_utc(d):
+            return timezone.localize(d).astimezone(pytz.UTC)
+        return [naive_tz_to_utc(d) if not use_naive_datetime else d for d in rset1 if d.year < MAXYEAR]
 
     @api.multi
     def _get_recurrency_end_date(self):

addons/calendar/tests/test_calendar.py

@@ -6,6 +6,7 @@ from datetime import datetime, timedelta
 
 from odoo import fields
 from odoo.tests.common import TransactionCase
+import pytz
 
 
 class TestCalendar(TransactionCase):
@@ -292,3 +293,24 @@ class TestCalendar(TransactionCase):
 
         self.assertEqual(event.start, '2018-10-16 08:00:00')
         self.assertEqual(event.stop, '2018-10-18 18:00:00')
+
+    def test_recurring_around_dst(self):
+        m = self.CalendarEvent.create({
+            'name': "wheee",
+            'start': '2018-10-27 14:30:00',
+            'allday': False,
+            'rrule': u'FREQ=DAILY;INTERVAL=1;COUNT=4',
+            'duration': 2,
+            'stop': '2018-10-27 16:30:00',
+        })
+
+        start_recurring_dates = m.with_context({'tz': 'Europe/Brussels'})._get_recurrent_date_by_event()
+        self.assertEqual(len(start_recurring_dates), 4)
+
+        for d in start_recurring_dates:
+            self.assertEqual(d.tzinfo, pytz.UTC)
+            if d.day < 28:  # DST switch happens between 2018-10-27 and 2018-10-28
+                self.assertEqual(d.hour, 14)
+            else:
+                self.assertEqual(d.hour, 15)
+            self.assertEqual(d.minute, 30)