Always make explicit which tables to lock (#3058)

Co-authored-by: Richard Schreiber <schreiber@rami.io>

src/pretix/plugins/sendmail/signals.py

@@ -51,6 +51,7 @@ from pretix.base.signals import (
     EventPluginSignal, event_copy_data, logentry_display, periodic_task,
 )
 from pretix.control.signals import nav_event
+from pretix.helpers import OF_SELF
 from pretix.plugins.sendmail.models import ScheduledMail
 from pretix.plugins.sendmail.views import OrderSendView, WaitinglistSendView
 
@@ -187,6 +188,7 @@ def sendmail_run_rules(sender, **kwargs):
 
             with transaction.atomic(durable=True):
                 m = ScheduledMail.objects.select_for_update(
+                    of=OF_SELF,
                     skip_locked=connection.features.has_select_for_update_skip_locked
                 ).filter(pk=m_id).first()
                 if not m or m.state not in (ScheduledMail.STATE_SCHEDULED, ScheduledMail.STATE_FAILED):