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/base/services/async.py

@@ -0,0 +1,28 @@
+"""
+This code has been taken from
+https://blog.hypertrack.io/2016/10/08/dealing-with-database-transactions-in-django-celery/
+
+Usage:
+    from pretix.base.services.async import TransactionAwareTask
+    @task(base=TransactionAwareTask)
+    def task_…():
+"""
+from celery import Task
+from django.db import transaction
+
+
+class TransactionAwareTask(Task):
+    """
+    Task class which is aware of django db transactions and only executes tasks
+    after transaction has been committed
+    """
+    abstract = True
+
+    def apply_async(self, *args, **kwargs):
+        """
+        Unlike the default task in celery, this task does not return an async
+        result
+        """
+        transaction.on_commit(
+            lambda: super(TransactionAwareTask, self).apply_async(*args, **kwargs)
+        )