Fix test failure on PostgreSQL

src/pretix/base/services/memberships.py

@@ -24,6 +24,7 @@ from typing import List, Optional
 
 from dateutil.relativedelta import relativedelta
 from django.core.exceptions import ValidationError
+from django.db import transaction
 from django.utils.formats import date_format
 from django.utils.translation import gettext_lazy as _
 
@@ -96,6 +97,8 @@ def validate_memberships_in_order(customer: Customer, positions: List[AbstractPo
     :param valid_from_not_chosen: Set to ``True`` to indicate that the customer is in an early step of the checkout flow
                                   where the valid_from date is not selected yet. In this case, the valid_from date is not checked.
     """
+    if lock and not transaction.get_connection().in_atomic_block:
+        raise Exception('validate_memberships_in_order(lock=True) should only be called in atomic transaction!')
     tz = event.timezone
     applicable_positions = [
         p for p in positions

src/tests/base/test_memberships.py

@@ -26,6 +26,7 @@ from zoneinfo import ZoneInfo
 import pytest
 from django.conf import settings
 from django.core.exceptions import ValidationError
+from django.db import transaction
 from django.utils.timezone import now
 from django_scopes import scope
 from freezegun import freeze_time
@@ -211,6 +212,7 @@ def test_validate_membership_required(event, customer, membership, requiring_tic
 
 
 @pytest.mark.django_db(transaction=True)
+@transaction.atomic
 def test_validate_membership_ensure_locking(event, customer, membership, requiring_ticket, membership_type, django_assert_num_queries):
     with django_assert_num_queries(4) as captured:
         validate_memberships_in_order(