Add a custom context manager for rollbacks (#282)

src/pretix/base/services/invoices.py

@@ -26,6 +26,7 @@ from pretix.base.models import Invoice, InvoiceAddress, InvoiceLine, Order
 from pretix.base.services.async import TransactionAwareTask
 from pretix.base.signals import register_payment_providers
 from pretix.celery import app
+from pretix.helpers.database import rolledback_transaction
 
 
 @transaction.atomic
@@ -408,12 +409,10 @@ class DummyRollbackException(Exception):
 
 def build_preview_invoice_pdf(event):
     locale = event.settings.invoice_language
-    pdf = None
     if not locale or locale == '__user__':
         locale = event.settings.locale
 
-    try:
-        with transaction.atomic(), language(locale):
+    with rolledback_transaction(), language(locale):
         order = event.orders.create(status=Order.STATUS_PENDING, datetime=now(),
                                     expires=now(), code="PREVIEW", total=119)
         invoice = Invoice(
@@ -444,9 +443,4 @@ def build_preview_invoice_pdf(event):
         with tempfile.NamedTemporaryFile(suffix=".pdf") as f:
             _invoice_generate_german(invoice, f)
             f.seek(0)
-                pdf = f.read()
-            raise DummyRollbackException()
-    except DummyRollbackException:
-        return pdf
-    else:
-        raise Exception('Invalid state, should have rolled back.')
+            return f.read()

src/pretix/base/services/tickets.py

@@ -11,6 +11,7 @@ from pretix.base.models import (
 )
 from pretix.base.signals import register_ticket_outputs
 from pretix.celery import app
+from pretix.helpers.database import rolledback_transaction
 
 
 @app.task
@@ -41,10 +42,8 @@ class DummyRollbackException(Exception):
 
 def preview(event: int, provider: str):
     event = Event.objects.get(id=event)
-    res = None
 
-    try:
-        with transaction.atomic(), language(event.settings.locale):
+    with rolledback_transaction(), language(event.settings.locale):
         item = event.items.create(name=_("Sample product"), default_price=42.23)
 
         order = event.orders.create(status=Order.STATUS_PENDING, datetime=now(),
@@ -56,9 +55,4 @@ def preview(event: int, provider: str):
         for receiver, response in responses:
             prov = response(event)
             if prov.identifier == provider:
-                    res = prov.generate(order)
-            raise DummyRollbackException()
-    except DummyRollbackException:
-        return res
-    else:
-        raise Exception('Invalid state, should have rolled back.')
+                return prov.generate(order)

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.')