When using the MongoDB backend, don't cleanup if result_expires is 0 or None. (celery#6462)

Fixes celery#6450.

Author: Omer Katz

celery/backends/mongodb.py

@@ -248,6 +248,9 @@ def _forget(self, task_id):
 
     def cleanup(self):
         """Delete expired meta-data."""
+        if not self.expires:
+            return
+
         self.collection.delete_many(
             {'date_done': {'$lt': self.app.now() - self.expires_delta}},
         )

t/unit/backends/test_mongodb.py

@@ -485,6 +485,12 @@ def test_cleanup(self, mock_get_database):
         mock_get_database.assert_called_once_with()
         mock_collection.delete_many.assert_called()
 
+        self.backend.collections = mock_collection = Mock()
+        self.backend.expires = None
+
+        self.backend.cleanup()
+        mock_collection.delete_many.assert_not_called()
+
     def test_get_database_authfailure(self):
         x = MongoBackend(app=self.app)
         x._get_connection = Mock()