Fix the DB backend race condition when clients access the taskmeta at the same time as the server creates it

celery/backends/base.py

@@ -161,7 +161,10 @@ class BaseDictBackend(BaseBackend):
 
     def get_status(self, task_id):
         """Get the status of a task."""
-        return self.get_task_meta(task_id)["status"]
+        task_meta = self.get_task_meta(task_id)
+        if task_meta is None:
+            return states.PENDING
+        return task_meta['status']
 
     def get_traceback(self, task_id):
         """Get the traceback for a failed task."""
@@ -180,7 +183,7 @@ class BaseDictBackend(BaseBackend):
             return self._cache[task_id]
 
         meta = self._get_task_meta_for(task_id)
-        if cache and meta.get("status") == states.SUCCESS:
+        if cache and meta and meta.get("status") == states.SUCCESS:
             self._cache[task_id] = meta
         return meta
 

celery/backends/database.py

@@ -58,11 +58,11 @@ class DatabaseBackend(BaseDictBackend):
         session = self.ResultSession()
         try:
             task = session.query(Task).filter(Task.task_id == task_id).first()
-            if not task:
+            if task is None:
+                from celery import states
                 task = Task(task_id)
-                session.add(task)
-                session.flush()
-                session.commit()
+                task.status = states.PENDING
+
             return task.to_dict()
         finally:
             session.close()

celery/tests/test_backends/test_database.py

@@ -32,6 +32,15 @@ class test_DatabaseBackend(unittest.TestCase):
         tb = DatabaseBackend()
         self.assertEqual(tb.get_status("xxx-does-not-exist"), states.PENDING)
 
+    def test_missing_task_meta_is_dict_with_pending(self):
+        tb = DatabaseBackend()
+        self.assertDictContainsSubset({
+            'status' : states.PENDING,
+            'task_id' : "xxx-does-not-exist-at-all",
+            'result' : None,
+            'traceback' : None,
+            }, tb.get_task_meta("xxx-does-not-exist-at-all"))
+
     def test_mark_as_done(self):
         tb = DatabaseBackend()