[FIX] project: calculate recurring tasks with interval

Steps to reproduce:
    - activate "Recurring Tasks" option inside Project settings;
    - go to "Field Service" app;
    - create a task;
    - check recurrence box;
    - make the following configuration in the recurrence tab:
       * Repeat Every 6 Months
       * Repeat On Date of the Month 1
       * Until End Date [your choice]

Issue:
    The dates calculated for the recurrence are not correct.

Cause:
    When we select an interval other than 1 for a frequency in months, we do not divide the number of months between the start and end date.
    And therefore the loop that calculates the recurrence dates goes too far into the dates.

Solution:
    Divide the number of months between the start date and the end date by the interval.

opw-3078084

closes odoo/odoo#107320

X-original-commit: 5d44d7d595dab866695945e5a6d2091ddd9df342
Signed-off-by: Xavier Bol (xbo) <xbo@odoo.com>
Signed-off-by: Lefebvre Thomas (thle) <thle@odoo.com>

addons/project/models/project_task_recurrence.py

@@ -8,6 +8,8 @@ from calendar import monthrange
 from dateutil.relativedelta import relativedelta
 from dateutil.rrule import rrule, rruleset, DAILY, WEEKLY, MONTHLY, YEARLY, MO, TU, WE, TH, FR, SA, SU
 
+import math
+
 MONTHS = {
     'january': 31,
     'february': 28,
@@ -179,7 +181,7 @@ class ProjectTaskRecurrence(models.Model):
                 start = date_start - relativedelta(days=1)
                 if repeat_type == 'until' and repeat_until > date_start:
                     delta = relativedelta(repeat_until, date_start)
-                    count = delta.years * 12 + delta.months
+                    count = math.floor((delta.years * 12 + delta.months) / repeat_interval)
                 for i in range(count):
                     start = start.replace(day=min(repeat_day, monthrange(start.year, start.month)[1]))
                     if i == 0 and start < date_start:

addons/project/tests/test_project_recurrence.py

@@ -437,6 +437,24 @@ class TestProjectrecurrence(TransactionCase):
         self.assertEqual(dates[3], datetime(2020, 4, 25))
         self.assertEqual(dates[4], datetime(2020, 5, 23))
 
+        dates = self.env['project.task.recurrence']._get_next_recurring_dates(
+            date_start=datetime(2020, 1, 10),
+            repeat_interval=6, # twice a year
+            repeat_unit='month',
+            repeat_type='until',
+            repeat_until=datetime(2021, 1, 11),
+            repeat_on_month='date',
+            repeat_on_year=False,
+            weekdays=[TH(+1)],
+            repeat_day='3', # the 3rd of the month
+            repeat_week=False,
+            repeat_month=False,
+            count=1)
+
+        self.assertEqual(len(dates), 2)
+        self.assertEqual(dates[0], datetime(2020, 7, 3))
+        self.assertEqual(dates[1], datetime(2021, 1, 3))
+
     def test_recurrence_next_dates_year(self):
         dates = self.env['project.task.recurrence']._get_next_recurring_dates(
             date_start=date(2020, 12, 1),