API: Make all mutating endpoints transactional (#6430)

All of our API endpoints that do something in the system do at least two
SQL queries, one for the actual change and one for the log entry. Often
many more. We want all of this to happen in a transaction so we know an
API call was executed or not at all, not half-way.

src/pretix/api/views/event.py

@@ -257,6 +257,7 @@ class EventViewSet(viewsets.ModelViewSet):
                 data=self.request.data
             )
 
+    @transaction.atomic()
     def perform_create(self, serializer):
         copy_from = None
         if 'clone_from' in self.request.GET:
@@ -320,6 +321,7 @@ class EventViewSet(viewsets.ModelViewSet):
             data=self.request.data
         )
 
+    @transaction.atomic()
     def perform_destroy(self, instance):
         if not instance.allow_delete():
             raise PermissionDenied('The event can not be deleted as it already contains orders. Please set \'live\''
@@ -355,6 +357,7 @@ class CloneEventViewSet(viewsets.ModelViewSet):
         ctx['organizer'] = self.request.organizer
         return ctx
 
+    @transaction.atomic()
     def perform_create(self, serializer):
         # Weird edge case: Requires settings permission on the event (to read) but also on the organizer (two write)
         perm_holder = (self.request.auth if isinstance(self.request.auth, (Device, TeamAPIToken))
@@ -513,6 +516,7 @@ class SubEventViewSet(ConditionalListView, viewsets.ModelViewSet):
         resp['X-Page-Generated'] = date
         return resp
 
+    @transaction.atomic()
     def perform_update(self, serializer):
         original_data = self.get_serializer(instance=serializer.instance).data
         super().perform_update(serializer)
@@ -529,6 +533,7 @@ class SubEventViewSet(ConditionalListView, viewsets.ModelViewSet):
             data=self.request.data
         )
 
+    @transaction.atomic()
     def perform_create(self, serializer):
         serializer.save(event=self.request.event)
         serializer.instance.log_action(
@@ -538,6 +543,7 @@ class SubEventViewSet(ConditionalListView, viewsets.ModelViewSet):
             data=self.request.data
         )
 
+    @transaction.atomic()
     def perform_destroy(self, instance):
         if not instance.allow_delete():
             raise PermissionDenied('The sub-event can not be deleted as it has already been used in orders. Please set'
@@ -566,6 +572,7 @@ class TaxRuleViewSet(ConditionalListView, viewsets.ModelViewSet):
     def get_queryset(self):
         return self.request.event.tax_rules.all()
 
+    @transaction.atomic()
     def perform_update(self, serializer):
         super().perform_update(serializer)
         serializer.instance.log_action(
@@ -575,6 +582,7 @@ class TaxRuleViewSet(ConditionalListView, viewsets.ModelViewSet):
             data=self.request.data
         )
 
+    @transaction.atomic()
     def perform_create(self, serializer):
         serializer.save(event=self.request.event)
         serializer.instance.log_action(
@@ -584,6 +592,7 @@ class TaxRuleViewSet(ConditionalListView, viewsets.ModelViewSet):
             data=self.request.data
         )
 
+    @transaction.atomic()
     def perform_destroy(self, instance):
         if not instance.allow_delete():
             raise PermissionDenied('This tax rule can not be deleted as it is currently in use.')
@@ -757,6 +766,7 @@ class SeatViewSet(ConditionalListView, UpdateModelMixin, viewsets.ReadOnlyModelV
         }
         return ctx
 
+    @transaction.atomic()
     def perform_update(self, serializer):
         super().perform_update(serializer)
         serializer.instance.event.log_action(
@@ -766,6 +776,7 @@ class SeatViewSet(ConditionalListView, UpdateModelMixin, viewsets.ReadOnlyModelV
             data={"seats": [serializer.instance.pk]},
         )
 
+    @transaction.atomic()
     def bulk_change_blocked(self, blocked):
         s = SeatBulkBlockInputSerializer(
             data=self.request.data,