Add validate_cart hook

doc/development/api/general.rst

@@ -19,7 +19,7 @@ Order events
 There are multiple signals that will be sent out in the ordering cycle:
 
 .. automodule:: pretix.base.signals
-   :members: order_paid, order_placed
+   :members: validate_cart, order_paid, order_placed
 
 Frontend
 --------

src/pretix/base/services/cart.py

@@ -12,6 +12,7 @@ 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
 
 
@@ -207,6 +208,11 @@ 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)
 
@@ -240,6 +246,11 @@ 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

@@ -94,6 +94,19 @@ subclass of pretix.base.exporter.BaseExporter
 As with all event-plugin signals, the ``sender`` keyword argument will contain the event.
 """
 
+validate_cart = EventPluginSignal(
+    providing_args=["positions", "requested_add", "requested_delete"]
+)
+"""
+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.
+
+As with all event-plugin signals, the ``sender`` keyword argument will contain the event.
+"""
+
 order_placed = EventPluginSignal(
     providing_args=["order"]
 )