Add a custom context manager for rollbacks (#282)

src/pretix/helpers/database.py

@@ -0,0 +1,28 @@
+import contextlib
+
+from django.db import transaction
+
+
+class DummyRollbackException(Exception):
+    pass
+
+
+@contextlib.contextmanager
+def rolledback_transaction():
+    """
+    This context manager runs your code in a database transaction that will be rolled back in the end.
+    This can come in handy to simulate the effects of a database operation that you do not actually
+    want to perform.
+
+    Note that rollbacks are a very slow operation on most database backends. Also, long-running
+    transactions can slow down other operations currently running and you should not use this
+    in a place that is called frequently.
+    """
+    try:
+        with transaction.atomic():
+            yield
+            raise DummyRollbackException()
+    except DummyRollbackException:
+        pass
+    else:
+        raise Exception('Invalid state, should have rolled back.')