Moved locking out of the model layer

src/pretix/base/models.py

@@ -1559,8 +1559,6 @@ class Order(Versionable):
     def _is_still_available(self):
         error_messages = {
             'unavailable': _('Some of the ordered products were no longer available.'),
-            'busy': _('We were not able to process the request completely as the '
-                      'server was too busy.'),
         }
         positions = list(self.positions.all().select_related(
             'item', 'variation'
@@ -1570,7 +1568,6 @@ class Order(Versionable):
         ))
         quota_cache = {}
         try:
-            with self.event.lock():
             for i, op in enumerate(positions):
                 quotas = list(op.item.quotas.all()) if op.variation is None else list(op.variation.quotas.all())
                 if len(quotas) == 0:
@@ -1591,10 +1588,6 @@ class Order(Versionable):
                         raise Quota.QuotaExceededException(error_messages['unavailable'])
         except Quota.QuotaExceededException as e:
             return str(e)
-        except EventLock.LockTimeoutException:
-            # Is raised when there are too many threads asking for quota locks and we were
-            # unaible to get one
-            return error_messages['busy']
         return True
 
     @property

src/pretix/base/services/locking.py

@@ -52,7 +52,7 @@ def release_event(event):
     :raises EventLock.LockReleaseException: if we do not own the lock
     """
     if not hasattr(event, '_lock') or not event._lock:
-        raise EventLock.LockReleaseException('')
+        raise EventLock.LockReleaseException('Lock is not owned by this thread')
     if settings.HAS_REDIS:
         return release_event_redis(event)
     else:

src/pretix/control/views/orders.py

@@ -12,7 +12,9 @@ from django.utils.timezone import now
 from django.utils.translation import ugettext_lazy as _
 from django.views.generic import DetailView, ListView, TemplateView, View
 
-from pretix.base.models import CachedFile, CachedTicket, Item, Order, Quota
+from pretix.base.models import (
+    CachedFile, CachedTicket, EventLock, Item, Order, Quota,
+)
 from pretix.base.services.export import export
 from pretix.base.services.orders import mark_order_paid
 from pretix.base.services.stats import order_overview
@@ -247,12 +249,17 @@ class OrderExtend(OrderView):
             if oldvalue > now():
                 self.form.save()
             else:
+                try:
+                    with self.order.event.lock():
                         is_available = self.order._is_still_available()
                         if is_available is True:
                             self.form.save()
                             messages.success(self.request, _('The payment term has been changed.'))
                         else:
                             messages.error(self.request, is_available)
+                except EventLock.LockTimeoutException:
+                    messages.error(self.request, _('We were not able to process the request completely as the '
+                                                   'server was too busy.'))
             return self._redirect_back()
         else:
             return self.get(*args, **kwargs)