use redis lock in process_updates if possible

src/c3nav/mapdata/models/update.py

@@ -2,6 +2,7 @@ import logging
 import os
 import pickle
 from contextlib import contextmanager, suppress
+from sqlite3 import DatabaseError
 
 from django.conf import settings
 from django.core.cache import cache
@@ -84,12 +85,37 @@ class MapUpdate(models.Model):
     def _changed_geometries_filename(self):
         return os.path.join(settings.CACHE_ROOT, 'changed_geometries', 'update_%d.pickle' % self.pk)
 
+    class ProcessUpdatesAlreadyRunning(Exception):
+        pass
+
+    @classmethod
+    @contextmanager
+    def get_updates_to_process(cls):
+        queryset = cls.objects.filter(processed=False)
+        with transaction.atomic():
+            if settings.HAS_REDIS:
+                import redis
+                lock_aquired = None
+                lock = redis.Redis().lock('mapupdate:process_updates_lock')
+                try:
+                    have_lock = lock.acquire(blocking=False, timeout=1800)
+                    if not have_lock:
+                        raise cls.ProcessUpdatesAlreadyRunning
+                    yield tuple(queryset)
+                finally:
+                    if lock_aquired:
+                        lock.release()
+            else:
+                try:
+                    yield tuple(queryset.select_for_update(nowait=True))
+                except DatabaseError:
+                    raise cls.ProcessUpdatesAlreadyRunning
+
     @classmethod
     def process_updates(cls):
         logger = logging.getLogger('c3nav')
 
-        with transaction.atomic():
-            new_updates = tuple(cls.objects.filter(processed=False).select_for_update(nowait=True))
+        with cls.get_updates_to_process() as new_updates:
             if not new_updates:
                 return ()