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"]
 )