Add signal validate_cart_addons

src/pretix/base/services/cart.py

@@ -26,6 +26,7 @@ from pretix.base.services.locking import LockTimeoutException, NoLockManager
 from pretix.base.services.pricing import get_price
 from pretix.base.services.tasks import ProfiledEventTask
 from pretix.base.settings import PERSON_NAME_SCHEMES
+from pretix.base.signals import validate_cart_addons
 from pretix.base.templatetags.rich_text import rich_text
 from pretix.celery_app import app
 from pretix.presale.signals import (
@@ -643,6 +644,15 @@ class CartManager:
                             'cat': str(iao.addon_category.name),
                         }
                     )
+                validate_cart_addons.send(
+                    sender=self.event,
+                    addons={
+                        (self._items_cache[s[0]], self._variations_cache[s[1]] if s[1] else None)
+                        for s in selected
+                    },
+                    base_position=cp,
+                    iao=iao
+                )
 
         # Detect removed add-ons and create RemoveOperations
         for cp, al in current_addons.items():

src/pretix/base/signals.py

@@ -265,6 +265,21 @@ appropriate exception message.
 As with all event-plugin signals, the ``sender`` keyword argument will contain the event.
 """
 
+validate_cart_addons = EventPluginSignal(
+    providing_args=["addons", "base_position", "iao"]
+)
+"""
+This signal is sent when a user tries to select a combination of addons. In contrast to
+ ``validate_cart``, this is executed before the cart is actually modified. You are passed
+an argument ``addons`` containing a set of ``(item, variation or None)`` tuples as well
+as the ``ItemAddOn`` object as the argument ``iao`` and the base cart position as
+``base_position``.
+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.
+"""
+
 order_placed = EventPluginSignal(
     providing_args=["order"]
 )

src/pretix/presale/checkoutflow.py

@@ -239,6 +239,7 @@ class AddOnsStep(CartMixin, AsyncAction, TemplateFlowStep):
                     'form': AddOnsForm(
                         event=self.request.event,
                         prefix='{}_{}'.format(cartpos.pk, iao.addon_category.pk),
+                        base_position=cartpos,
                         iao=iao,
                         price_included=iao.price_included,
                         initial=current_addon_products,

src/pretix/presale/forms/checkout.py

@@ -14,7 +14,8 @@ from pretix.base.forms.questions import (
 )
 from pretix.base.models import ItemVariation
 from pretix.base.models.tax import TAXED_ZERO
-from pretix.base.services.cart import error_messages
+from pretix.base.services.cart import CartError, error_messages
+from pretix.base.signals import validate_cart_addons
 from pretix.base.templatetags.money import money_filter
 from pretix.base.templatetags.rich_text import rich_text
 from pretix.base.validators import EmailBlacklistValidator
@@ -205,13 +206,14 @@ class AddOnsForm(forms.Form):
         """
         self.iao = kwargs.pop('iao')
         category = self.iao.addon_category
-        event = kwargs.pop('event')
+        self.event = kwargs.pop('event')
         subevent = kwargs.pop('subevent')
         current_addons = kwargs.pop('initial')
         quota_cache = kwargs.pop('quota_cache')
         item_cache = kwargs.pop('item_cache')
         self.price_included = kwargs.pop('price_included')
         self.sales_channel = kwargs.pop('sales_channel')
+        self.base_position = kwargs.pop('base_position')
 
         super().__init__(*args, **kwargs)
 
@@ -231,12 +233,12 @@ class AddOnsForm(forms.Form):
             ).select_related('tax_rule').prefetch_related(
                 Prefetch('quotas',
                          to_attr='_subevent_quotas',
-                         queryset=event.quotas.filter(subevent=subevent)),
+                         queryset=self.event.quotas.filter(subevent=subevent)),
                 Prefetch('variations', to_attr='available_variations',
                          queryset=ItemVariation.objects.filter(active=True, quotas__isnull=False).prefetch_related(
                              Prefetch('quotas',
                                       to_attr='_subevent_quotas',
-                                      queryset=event.quotas.filter(subevent=subevent))
+                                      queryset=self.event.quotas.filter(subevent=subevent))
                          ).distinct()),
                 'event'
             ).annotate(
@@ -260,7 +262,7 @@ class AddOnsForm(forms.Form):
                         self.vars_cache[v.pk] = v
                         choices.append(
                             (v.pk,
-                             self._label(event, v, cached_availability,
+                             self._label(self.event, v, cached_availability,
                                          override_price=var_price_override.get(v.pk)),
                              v.description)
                         )
@@ -294,7 +296,7 @@ class AddOnsForm(forms.Form):
                     continue
                 cached_availability = i.check_quotas(subevent=subevent, _cache=quota_cache)
                 field = forms.BooleanField(
-                    label=self._label(event, i, cached_availability,
+                    label=self._label(self.event, i, cached_availability,
                                       override_price=item_price_override.get(i.pk)),
                     required=False,
                     initial=i.pk in current_addons,
@@ -333,3 +335,8 @@ class AddOnsForm(forms.Form):
                     'cat': str(self.iao.addon_category.name),
                 }
             )
+        try:
+            validate_cart_addons.send(sender=self.event, addons=selected, base_position=self.base_position,
+                                      iao=self.iao)
+        except CartError as e:
+            raise ValidationError(str(e))