[backend] Lock splits which are currently being started

beat/web/backend/models.py

@@ -424,8 +424,9 @@ class Worker(models.Model):
         if settings.DEBUG: cmdline += ['-vv']
 
         # start newly assigned job splits
-        for split in JobSplit.objects.filter(worker=self, status=Job.QUEUED,
-            start_date__isnull=True, process_id__isnull=True):
+        for split in JobSplit.objects.select_for_update().filter(worker=self,
+            status=Job.QUEUED, start_date__isnull=True,
+            process_id__isnull=True):
             execute = pick_execute(split, environments)
             if execute is None:
                 message = "Environment `%s' is not available for split " \