[FIX] website_shift: Wrong time when crossing DST

The issue: There was a time difference of one hour when the fictive
shifts cross the DST (Daylight Saving Time).

Explanation: Python works with two types of datetime. The datetime
without a timezone called *naive* and the one with a timezone called
*non-naive*. `timedelta()` is used to add a number of day to a datetime.
However, changing a non-naive datetime with `timedelta` just change the
date but leave the timezone unchanged. That's no correct because the new
date may not be in the same timezone because of the DST. If the new date
is after a change of time, due to DST, the timezone of the non-naive
datetime should be updated. Python don't do that because `timedelta`
doesn't realy add days but add the corresponding hours ! So when adding
hours it's consistent to not change the timezone (you can use
`normalize()` to get your datetime in the right timezone after a cross
of DST). To fix this, after adding a `timedelta` the timezone of the
datetime should be set to none and the relocalized with the `localize()`
function of the `pytz` corresponding timezone.

beesdoo_website_shift/controllers/main.py

@@ -7,10 +7,10 @@
 from ast import literal_eval
 from datetime import datetime, timedelta
 from itertools import groupby
+from pytz import timezone, utc
 
 from openerp import http, fields
 from openerp.http import request
-from openerp.tools import DEFAULT_SERVER_DATETIME_FORMAT as DATETIME_FORMAT
 
 from openerp.addons.beesdoo_shift.models.planning import float_to_time
 from openerp.addons.beesdoo_shift.models.cooperative_status import PERIOD
@@ -33,6 +33,34 @@ class WebsiteShiftController(http.Controller):
         working_mode = user.partner_id.working_mode
         return working_mode == 'exempt'
 
+    def add_days(self, datetime, days):
+        """
+        Add the number of days to datetime. This take the DST in
+        account, meaning that the UTC time will be correct even if the
+        new datetime has cross the DST boundary.
+
+        :param datetime: a naive datetime expressed in UTC
+        :return: a naive datetime expressed in UTC with the added days
+        """
+        # Ensure that the datetime given is without a timezone
+        assert datetime.tzinfo is None
+        # Get current user and user timezone
+        cur_user = request.env['res.users'].browse(request.uid)
+        user_tz = timezone(cur_user.tz)
+        # Convert to UTC
+        dt_utc = utc.localize(datetime, is_dst=False)
+        # Convert to user TZ
+        dt_local = dt_utc.astimezone(user_tz)
+        # Add the number of days
+        newdt_local = dt_local + timedelta(days=days)
+        # If the newdt_local has cross the DST boundary, its tzinfo is
+        # no longer correct. So it will be replaced by the correct one.
+        newdt_local = user_tz.localize(newdt_local.replace(tzinfo=None))
+        # Now the newdt_local has the right DST so it can be converted
+        # to UTC.
+        newdt_utc = newdt_local.astimezone(utc)
+        return newdt_utc.replace(tzinfo=None)
+
     @http.route('/my/shift', auth='user', website=True)
     def my_shift(self, **kw):
         """
@@ -283,11 +311,6 @@ class WebsiteShiftController(http.Controller):
                 'beesdoo_website_shift.regular_next_shift_limit'))
 
             for i in range(nb_subscribed_shifts, regular_next_shift_limit):
-                # Compute the new date for the created shift
-                start_time = fields.Datetime.from_string(main_shift.start_time)
-                start_time = (start_time + timedelta(days=i*PERIOD)).strftime(DATETIME_FORMAT)
-                end_time = fields.Datetime.from_string(main_shift.end_time)
-                end_time = (end_time + timedelta(days=i*PERIOD)).strftime(DATETIME_FORMAT)
                 # Create the fictive shift
                 shift = main_shift.new()
                 shift.name = main_shift.name
@@ -302,8 +325,14 @@ class WebsiteShiftController(http.Controller):
                 shift.replaced_id = main_shift.replaced_id
                 shift.revert_info = main_shift.revert_info
                 # Set new date
-                shift.start_time = start_time
-                shift.end_time = end_time
+                shift.start_time = self.add_days(
+                    fields.Datetime.from_string(main_shift.start_time),
+                    days=i*PERIOD
+                )
+                shift.end_time = self.add_days(
+                    fields.Datetime.from_string(main_shift.end_time),
+                    days=i*PERIOD
+                )
                 # Add the fictive shift to the list of shift
                 subscribed_shifts.append(shift)