MapUpdate.last_update() and MapUpdate.lock()

src/c3nav/mapdata/models/update.py

@@ -1,5 +1,8 @@
+from contextlib import contextmanager
+
 from django.conf import settings
-from django.db import models
+from django.core.cache import cache
+from django.db import models, transaction
 from django.utils.translation import ugettext_lazy as _
 
 
@@ -17,6 +20,24 @@ class MapUpdate(models.Model):
         default_related_name = 'mapupdates'
         get_latest_by = 'datetime'
 
+    @classmethod
+    def last_update(cls):
+        last_update = cache.get('mapdata:last_update', None)
+        if last_update is not None:
+            return last_update
+        with cls.lock():
+            last_update = cls.objects.latest().datetime
+            cache.set('mapdata:last_update', last_update, 900)
+        return last_update
+
+    @classmethod
+    @contextmanager
+    def lock(cls):
+        with transaction.atomic():
+            yield cls.objects.select_for_update().earliest()
+
     def save(self, **kwargs):
         if self.pk is not None:
             raise TypeError
+        super().save(**kwargs)
+        cache.set('mapdata:last_update', self.datetime, 900)