Deal with cancelling memberships (#2130)

src/pretix/base/models/memberships.py

@@ -118,6 +118,10 @@ class Membership(models.Model):
         verbose_name=_('Test mode'),
         default=False
     )
+    canceled = models.BooleanField(
+        verbose_name=_('Canceled'),
+        default=False
+    )
     customer = models.ForeignKey(
         Customer,
         related_name='memberships',

src/pretix/base/models/orders.py

@@ -599,6 +599,8 @@ class Order(LockModel, LoggedModel):
             for gc in op.issued_gift_cards.all():
                 if gc.value != op.price:
                     return False
+            if op.granted_memberships.with_usages().filter(usages__gt=0):
+                return False
         if self.user_cancel_deadline and now() > self.user_cancel_deadline:
             return False
         if self.status == Order.STATUS_PENDING: