class system_handler_engine :This engine implements aUnixqueue.event_system ->[unit]system_handler_engine_type
Shell_sys.system_handler, and can
be used to watch an already started job_instance as created by
Shell_sys.run_job until the job is done. It is necessary to
Shell_sys.register_job at the system handler.
When the job terminates the status of the engine changes to
`Done (). The status of the engine does not reflect whether
the job was successful Job_ok or has failed Job_error.
The engine status `Error indicates an internal error. It
is also used when callback functions raise exceptions.
When the engine goes to `Error or `Aborted, the job
is terminated ( Shell_sys.abandon_job ).
