Transactiontask (#268)

* Introduce TransactionAwareTask

As described in
https://blog.hypertrack.io/2016/10/08/dealing-with-database-transactions-in-django-celery/

* Use TransactionAwareTask instead of countdown

… to prevent race conditions when using a newly created object in a
task.

src/pretix/plugins/banktransfer/tasks.py

@@ -9,6 +9,7 @@ from django.utils.translation import ugettext_noop
 
 from pretix.base.i18n import language
 from pretix.base.models import Event, Order, Quota
+from pretix.base.services.async import TransactionAwareTask
 from pretix.base.services.mail import SendMailException
 from pretix.base.services.orders import mark_order_paid
 from pretix.celery import app
@@ -95,7 +96,7 @@ def _get_unknown_transactions(event: Event, job: BankImportJob, data: list):
     return transactions
 
 
-@app.task
+@app.task(base=TransactionAwareTask)
 def process_banktransfers(event: int, job: int, data: list) -> None:
     with language("en"):  # We'll translate error messages at display time
         event = Event.objects.get(pk=event)

src/pretix/plugins/banktransfer/views.py

@@ -337,7 +337,7 @@ class ImportView(EventPermissionRequiredMixin, ListView):
             'event': self.request.event.pk,
             'job': job.pk,
             'data': parsed
-        }, countdown=1)
+        })
         return redirect(reverse('plugins:banktransfer:import.job', kwargs={
             'event': self.request.event.slug,
             'organizer': self.request.event.organizer.slug,