RegisterBackgroundWorker does not actually start a bg worker process in 9.4.4

I am new to bg_workers so this may be my user error,
but when I build and run the  contrib/worker_spi
extension,    I find that :

  .   starting postgres with the extension named in shared_preload_libraries :
      its _PG_init is invoked as expected but no process is started -
      it is as though RegisterBackgroundWorker did nothing

  .  creating the extension and then
psql ...  "select worker_spi_launch(2);" :
    I see
 28409 28288 ?        463508       00:05 00:00:00  0.0 postgres: bgworker: worker 2   
as expected.

Is there maybe some bug in postmaster's processing of
    workers marked as start_at = BgWorkerStart_RecoveryFinished
in 9.4.4?

Cheers,   John