Place the two attached scripts in Moodle's root. From the CLI run: php createbackup.php then use the ID it returns to run: php failbackup.php [ID HERE]

You'll see that the controller is loaded, but the call to set its status as failed never completes.

The following code however, is able to mark a backup as failed:

$bc = new backup_controller(backup::TYPE_1COURSE, 1, backup::FORMAT_MOODLE, backup::INTERACTIVE_NO, backup::MODE_COPY, 2, backup::RELEASESESSION_YES);

$bc->set_status(backup::STATUS_FINISHED_ERR);

i.e., if the controller is marked as failed in the same session it's created in, everything is fine. But when it's loaded from the DB in a different session, it can't be marked as failed. It appears that there's some infinite recursion problem when it tries to calculate the checksum.