Fix floating-point issue on bills (Fix #31)

uncloud_pay/__init__.py

@@ -1,9 +1,12 @@
 from django.utils.translation import gettext_lazy as _
+import decimal
 
 # Define DecimalField properties, used to represent amounts of money.
 AMOUNT_MAX_DIGITS=10
 AMOUNT_DECIMALS=2
 
+decimal.getcontext().prec = AMOUNT_DECIMALS
+
 # http://xml.coverpages.org/country3166.html
 COUNTRIES = (
     ('AD', _('Andorra')),

uncloud_pay/models.py

@@ -494,6 +494,10 @@ class BillRecord():
         else:
             self.one_time_price = 0
 
+        # Set decimal context for amount computations.
+        # XXX: understand why we need +1 here.
+        decimal.getcontext().prec = AMOUNT_DECIMALS + 1
+
     @property
     def recurring_count(self):
         # Compute billing delta.
@@ -531,7 +535,7 @@ class BillRecord():
             (_, days_in_month) = monthrange(
                     self.bill.starting_date.year,
                     self.bill.starting_date.month)
-            return days / days_in_month
+            return round(days / days_in_month, AMOUNT_DECIMALS)
         elif self.recurring_period == RecurringPeriod.PER_WEEK:
             weeks = ceil(billed_delta / timedelta(week=1))
             return weeks