[FIX] calendar: avoid memory overflow

On a DB with thousands of events including recurring ones, fetching the
complete list of events might lead to a memory overflow due to the
prefetching of all virtual events.

We reduce the memory footprint by handling no more than 50,000 records
at a time and clearing the cache at each step. This limit is purely
arbitrary, but seems to be a good compromise between performance and
memory usage.

opw-1832482

addons/calendar/models/calendar.py

@@ -617,13 +617,21 @@ class Meeting(models.Model):
             event_date = event_date.astimezone(timezone)  # transform "+hh:mm" timezone
             rset1 = rrule.rrulestr(str(self.rrule), dtstart=event_date, forceset=True, tzinfos={})
         recurring_meetings = self.search([('recurrent_id', '=', self.id), '|', ('active', '=', False), ('active', '=', True)])
-        for meeting in recurring_meetings:
-            recurring_date = fields.Datetime.from_string(meeting.recurrent_id_date)
-            if use_naive_datetime:
-                recurring_date = recurring_date.replace(tzinfo=None)
-            else:
-                recurring_date = todate(meeting.recurrent_id_date)
-            rset1.exdate(recurring_date)
+
+        # We handle a maximum of 50,000 meetings at a time, and clear the cache at each step to
+        # control the memory usage.
+        invalidate = False
+        for meetings in self.env.cr.split_for_in_conditions(recurring_meetings, size=50000):
+            if invalidate:
+                self.invalidate_cache()
+            for meeting in meetings:
+                recurring_date = fields.Datetime.from_string(meeting.recurrent_id_date)
+                if use_naive_datetime:
+                    recurring_date = recurring_date.replace(tzinfo=None)
+                else:
+                    recurring_date = todate(meeting.recurrent_id_date)
+                rset1.exdate(recurring_date)
+            invalidate = True
         return [d.astimezone(pytz.UTC) if d.tzinfo else d for d in rset1]
 
     @api.multi