double_checked_lock_iterator.py

double_checked_lock_iterator.py

# refactor of https://lukeplant.me.uk/blog/posts/double-checked-locking-with-django-orm/
# untested

def double_checked_lock_iterator(queryset):
    for item_pk in queryset.values_list("pk", flat=True):
        with transaction.atomic():
            try:
                yield queryset.select_for_update(skip_locked=True).get(id=item_pk)
            except queryset.model.DoesNotExist:
                pass


def send_pending_order_shipped_emails():
    orders_to_email = Order.objects.filter(
        shipped_at__isnull=False, shipped_email_sent=False,
    )

    for order in double_checked_lock_iterator(orders_to_email):
        send_shipped_email(order)
        order.shipped_email_sent = True
        order.save()