Add signal quota_availability

doc/development/api/general.rst

@@ -12,7 +12,7 @@ Core
 
 .. automodule:: pretix.base.signals
    :members: periodic_task, event_live_issues, event_copy_data, email_filter, register_notification_types,
-      item_copy_data, register_sales_channels, register_global_settings
+      item_copy_data, register_sales_channels, register_global_settings, quota_availability
 
 Order events
 """"""""""""

src/pretix/base/models/items.py

@@ -23,6 +23,7 @@ from i18nfield.fields import I18nCharField, I18nTextField
 from pretix.base.models import fields
 from pretix.base.models.base import LoggedModel
 from pretix.base.models.tax import TaxedPrice
+from pretix.base.signals import quota_availability
 
 from .event import Event, SubEvent
 
@@ -1312,6 +1313,9 @@ class Quota(LoggedModel):
             return _cache[self.pk]
         now_dt = now_dt or now()
         res = self._availability(now_dt, count_waitinglist)
+        for recv, resp in quota_availability.send(sender=self.event, quota=self, result=res,
+                                                  count_waitinglist=count_waitinglist):
+            res = resp
 
         self.event.cache.delete('item_quota_cache')
         rewrite_cache = count_waitinglist and (

src/pretix/base/signals.py

@@ -524,3 +524,20 @@ a ``subevent`` argument which might be none and you are expected to return a lis
 ``pretix.base.timeline.TimelineEvent``, which is a ``namedtuple`` with the fields ``event``, ``subevent``,
 ``datetime``, ``description`` and ``edit_url``.
 """
+
+
+quota_availability = EventPluginSignal(
+    providing_args=['quota', 'result', 'count_waitinglist']
+)
+"""
+This signal allows you to modify the availability of a quota. You are passed the ``quota`` and an
+``availability`` result calculated by pretix code or other plugins. ``availability`` is a tuple
+with the first entry being one of the ``Quota.AVAILABILITY_*`` constants and the second entry being
+the number of available tickets (or ``None`` for unlimited). You are expected to return a value
+of the same time. The parameter ``count_waitinglists`` specifies whether waiting lists should be taken
+into account.
+
+**Warning: Use this signal with great caution, it allows you to screw up the performance of the
+system really bad.** Also, keep in mind that your response is subject to caching and out-of-date
+quotas might be used for display (not for actual order processing).
+"""

src/pretix/control/templates/pretixcontrol/items/quota.html

@@ -51,6 +51,13 @@
                     </div>
                 </div>
             {% endfor %}
+            {% if has_plugins > 0 %}
+                <div class="alert alert-warning">
+                    {% blocktrans trimmed with num=quota_overbooked %}
+                        A plugin is active that might modify the actual result of this quota from what you see here.
+                    {% endblocktrans %}
+                </div>
+            {% endif %}
             {% if quota_overbooked > 0 %}
                 <div class="alert alert-warning">
                     {% blocktrans trimmed with num=quota_overbooked %}

src/pretix/control/views/item.py

@@ -25,6 +25,7 @@ from pretix.base.models import (
 from pretix.base.models.event import SubEvent
 from pretix.base.models.items import ItemAddOn, ItemBundle
 from pretix.base.services.tickets import invalidate_cache
+from pretix.base.signals import quota_availability
 from pretix.control.forms.item import (
     CategoryForm, ItemAddOnForm, ItemAddOnsFormSet, ItemBundleForm,
     ItemBundleFormSet, ItemCreateForm, ItemUpdateForm, ItemVariationForm,
@@ -666,6 +667,17 @@ class QuotaView(ChartContainingView, DetailView):
         ctx['quota_table_rows'] = list(data)
         ctx['quota_overbooked'] = sum_values - self.object.size if self.object.size is not None else 0
 
+        ctx['has_plugins'] = False
+        res = (
+            Quota.AVAILABILITY_GONE if self.object.size is not None and self.object.size - sum_values <= 0 else
+            Quota.AVAILABILITY_OK,
+            self.object.size - sum_values if self.object.size is not None else None
+        )
+        for recv, resp in quota_availability.send(sender=self.request.event, quota=self.object, result=res,
+                                                  count_waitinglist=True):
+            if resp != res:
+                ctx['has_plugins'] = True
+
         ctx['has_ignore_vouchers'] = Voucher.objects.filter(
             Q(allow_ignore_quota=True) &
             Q(Q(valid_until__isnull=True) | Q(valid_until__gte=now())) &