Lock periodic task meta table

celery/managers.py

@@ -1,5 +1,6 @@
 """celery.managers"""
 from django.db import models
+from django.db import connection
 from celery.registry import tasks
 from datetime import datetime, timedelta
 import random
@@ -55,6 +56,22 @@ class TaskManager(models.Manager):
 class PeriodicTaskManager(models.Manager):
     """Manager for :class:`celery.models.PeriodicTask` models."""
 
+    def lock(self):
+        """Lock the periodic task table for reading."""
+        cursor = connection.cursor()
+        table = self.model._meta.db_table
+        cursor.execute("LOCK TABLES %s READ" % table)
+        row = cursor.fetchone()
+        return row
+
+    def unlock(self):
+        """Unlock the periodic task table."""
+        cursor = connection.cursor()
+        table = self.model._meta.db_table
+        cursor.execute("UNLOCK TABLES")
+        row = cursor.fetchone()
+        return row
+
     def get_waiting_tasks(self):
         """Get all waiting periodic tasks.
 
@@ -65,10 +82,12 @@ class PeriodicTaskManager(models.Manager):
         # XXX This will become a lot of queries. Maybe just only create
         # the rows at init, and then select all later.
         for task_name, task in periodic_tasks.items():
+            self.lock()
             task_meta, created = self.get_or_create(name=task_name)
             # task_run.every must be a timedelta object.
             run_every_drifted = task.run_every + SERVER_DRIFT
             run_at = task_meta.last_run_at + run_every_drifted
             if datetime.now() > run_at:
                 waiting.append(task_meta)
+            self.unlock()
         return waiting