Fix cronjob failure on MySQL

src/pretix/base/services/invoices.py

@@ -43,7 +43,7 @@ import vat_moss.exchange_rates
 from django.conf import settings
 from django.core.files.base import ContentFile
 from django.core.serializers.json import DjangoJSONEncoder
-from django.db import transaction
+from django.db import connection, transaction
 from django.db.models import Count
 from django.dispatch import receiver
 from django.utils import timezone
@@ -459,7 +459,7 @@ def send_invoices_to_organizer(sender, **kwargs):
         with transaction.atomic():
             qs = Invoice.objects.filter(
                 sent_to_organizer__isnull=True
-            ).prefetch_related('event').select_for_update(skip_locked=True)
+            ).prefetch_related('event').select_for_update(skip_locked=connection.features.has_select_for_update_skip_locked)
             for i in qs[:batch_size]:
                 if i.event.settings.invoice_email_organizer:
                     with language(i.event.settings.locale):

src/tests/base/test_runperiodic.py

@@ -0,0 +1,29 @@
+#
+# This file is part of pretix (Community Edition).
+#
+# Copyright (C) 2014-2020 Raphael Michel and contributors
+# Copyright (C) 2020-2021 rami.io GmbH and contributors
+#
+# This program is free software: you can redistribute it and/or modify it under the terms of the GNU Affero General
+# Public License as published by the Free Software Foundation in version 3 of the License.
+#
+# ADDITIONAL TERMS APPLY: Pursuant to Section 7 of the GNU Affero General Public License, additional terms are
+# applicable granting you additional permissions and placing additional restrictions on your usage of this software.
+# Please refer to the pretix LICENSE file to obtain the full terms applicable to this work. If you did not receive
+# this file, see <https://pretix.eu/about/en/license>.
+#
+# This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied
+# warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Affero General Public License for more
+# details.
+#
+# You should have received a copy of the GNU Affero General Public License along with this program.  If not, see
+# <https://www.gnu.org/licenses/>.
+#
+import pytest
+
+from pretix.base.signals import periodic_task
+
+
+@pytest.mark.django_db
+def test_all_periodic_tasks():
+    periodic_task.send(sender=None)  # to trigger sendmail_run_rules