process_updates() don't lock globally, but only on unprocessed updates

src/c3nav/mapdata/management/commands/processupdates.py

@@ -1,5 +1,6 @@
 from django.conf import settings
 from django.core.management.base import BaseCommand
+from django.db import DatabaseError
 from django.utils.translation import ugettext_lazy as _
 
 from c3nav.mapdata.tasks import process_map_updates
@@ -9,7 +10,10 @@ class Command(BaseCommand):
     help = 'process unprocessed map updates'
 
     def handle(self, *args, **options):
-        process_map_updates()
+        try:
+            process_map_updates()
+        except DatabaseError:
+            print(_('Error: There is already map update processing in progress.'))
 
         if not settings.HAS_REAL_CACHE:
             print(_('You have no external cache configured, so don\'t forget to restart your c3nav instance!'))

src/c3nav/mapdata/models/update.py

@@ -81,8 +81,8 @@ class MapUpdate(models.Model):
 
     @classmethod
     def process_updates(cls):
-        with cls.lock():
-            new_updates = tuple(cls.objects.filter(processed=False))
+        with transaction.atomic():
+            new_updates = tuple(cls.objects.filter(processed=False).select_for_update(nowait=True))
             if not new_updates:
                 return ()
 

src/c3nav/mapdata/tasks.py

@@ -1,3 +1,4 @@
+from django.db import DatabaseError
 from django.utils.formats import date_format
 from django.utils.translation import ugettext_lazy as _
 from django.utils.translation import ungettext_lazy
@@ -5,10 +6,15 @@ from django.utils.translation import ungettext_lazy
 from c3nav.celery import app
 
 
-@app.task(rate_limit='1/m')
-def process_map_updates():
+@app.task(bind=True, max_retries=10)
+def process_map_updates(self):
     from c3nav.mapdata.models import MapUpdate
-    updates = MapUpdate.process_updates()
+    try:
+        updates = MapUpdate.process_updates()
+    except DatabaseError:
+        if self.request.called_directly:
+            raise
+        raise self.retry(countdown=30)
 
     if updates:
         print()