Add signal quota_availability

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+system really bad.** Also, keep in mind that your response is subject to caching and out-of-date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).
+"""