;;;
;;; This is  called on  all workers. But  only rank-0 should  read the
;;; FIFO.  The reader will block for measurable time (minutes).  Other
;;; workers should not  be too eagerly waiting for  the result because
;;; of agressive polling used  by MPI_Bcast().  Otherwise everyone but
;;; rank-0 burns 100% CPU.
;;;
(define (read-fifo-v2 fifo)
  (let ((flag #f)
        (data #f)
        (mutex (make-mutex)))
    ;; This thread (created on  rank-0) blocks on reading a FIFO, when
    ;; done sets two globals secured by the mutex:
    (let ((helper (if (zero? (comm-rank))
                      (make-thread
                       (lambda ()
                         (let ((value (normalize (with-input-from-file fifo read))))
                           (with-mutex mutex
                                       (set! data value)
                                       (set! flag #t)))))
                      #f)))
      ;; Every worker checks the value of the flag on rank-0. If the
      ;; data has not yet arrived, then sleep for some time and
      ;; repeat. Access to flag needs to be secured by a mutex on
      ;; rank-0. FIXME: other workers dont need a mutex, actually.
      (let loop ()
        (unless (comm-bcast 0 (with-mutex mutex flag))
                (usleep 100000)         ; FIXME: literal 0.1 sec
                (loop)))
      (when helper (join-thread helper))
      (comm-bcast 0 data))))