Change semantics of validate_cart hook

src/pretix/base/services/cart.py

@@ -12,7 +12,6 @@ from pretix.base.models import (
 )
 from pretix.base.services.async import ProfiledTask
 from pretix.base.services.locking import LockTimeoutException
-from pretix.base.signals import validate_cart
 from pretix.celery_app import app
 
 
@@ -208,11 +207,6 @@ def _add_items_to_cart(event: Event, items: List[dict], cart_id: str=None) -> No
             # TODO: i18n plurals
             raise CartError(error_messages['max_items'], (event.settings.max_items_per_order,))
 
-        validate_cart.send(
-            sender=event, positions=CartPosition.objects.filter(Q(cart_id=cart_id) & Q(event=event)),
-            requested_add=items, requested_delete=[]
-        )
-
         expiry = now_dt + timedelta(minutes=event.settings.get('reservation_time', as_type=int))
         _extend_existing(event, cart_id, expiry, now_dt)
 
@@ -246,11 +240,6 @@ def add_items_to_cart(self, event: int, items: List[dict], cart_id: str=None) ->
 
 def _remove_items_from_cart(event: Event, items: List[dict], cart_id: str) -> None:
     with event.lock():
-        validate_cart.send(
-            sender=event, positions=CartPosition.objects.filter(Q(cart_id=cart_id) & Q(event=event)),
-            requested_add=[], requested_delete=items
-        )
-
         for i in items:
             cw = Q(cart_id=cart_id) & Q(item_id=i['item']) & Q(event=event)
             if i['variation']:

src/pretix/base/signals.py

@@ -95,14 +95,13 @@ As with all event-plugin signals, the ``sender`` keyword argument will contain t
 """
 
 validate_cart = EventPluginSignal(
-    providing_args=["positions", "requested_add", "requested_delete"]
+    providing_args=["positions"]
 )
 """
-This signal is sent out every time a cart is about to be changed. It includes an iterable
-with the current CartPosition objects as well as lists of dictionaries of the cart items
-that the user wants to add. Those dictionaries can contain the keys ``item``, ``variation``,
-``count``, ``price`` and ``voucher``. The response of receivers will be ignored, but you can
-raise an OrderError with an appropriate exception message.
+This signal is sent out before the user starts checkout. It includes an iterable
+with the current CartPosition objects.
+The response of receivers will be ignored, but you can raise a CartError with an
+appropriate exception message.
 
 As with all event-plugin signals, the ``sender`` keyword argument will contain the event.
 """

src/pretix/presale/views/checkout.py

@@ -5,6 +5,8 @@ from django.utils.translation import ugettext_lazy as _
 from django.views.generic import View
 
 from pretix.base.models import CartPosition
+from pretix.base.services.cart import CartError
+from pretix.base.signals import validate_cart
 from pretix.multidomain.urlreverse import eventreverse
 from pretix.presale.checkoutflow import get_checkout_flow
 
@@ -12,12 +14,20 @@ from pretix.presale.checkoutflow import get_checkout_flow
 class CheckoutView(View):
     def dispatch(self, request, *args, **kwargs):
         self.request = request
-        has_cart = CartPosition.objects.filter(
-            cart_id=self.request.session.session_key, event=self.request.event).exists()
-        if not has_cart and "async_id" not in request.GET:
+        cart_pos = CartPosition.objects.filter(
+            cart_id=self.request.session.session_key, event=self.request.event
+        )
+
+        if not cart_pos.exists() and "async_id" not in request.GET:
             messages.error(request, _("Your cart is empty"))
             return redirect(eventreverse(self.request.event, 'presale:event.index'))
 
+        try:
+            validate_cart.send(sender=self.request.event, positions=cart_pos)
+        except CartError as e:
+            messages.error(request, str(e))
+            return redirect(eventreverse(self.request.event, 'presale:event.index'))
+
         flow = get_checkout_flow(self.request.event)
         for step in flow:
             if not step.is_applicable(request):