UserPermissions should user select_for_update to keep cache reliable

src/c3nav/control/models.py

@@ -1,6 +1,6 @@
 from django.conf import settings
 from django.core.cache import cache
-from django.db import models
+from django.db import models, transaction
 from django.utils.functional import lazy
 from django.utils.translation import ugettext_lazy as _
 
@@ -53,14 +53,16 @@ class UserPermissions(models.Model):
                     break
         if result:
             return result
-        try:
-            result = user.permissions
-        except AttributeError:
+        with transaction.atomic():
+            result = cls.objects.filter(user=user).select_for_update().first()
+            if not result:
                 result = cls(user=user)
             cache.set(cache_key, result, 900)
         return result
 
     def save(self, *args, **kwargs):
+        with transaction.atomic():
+            UserPermissions.objects.filter(user_id=self.user_id).select_for_update()
             super().save(*args, **kwargs)
             cache_key = self.get_cache_key(self.pk)
             cache.set(cache_key, self, 900)