One-shot type condition variable

waiter.rkt

#lang racket/base

(require racket/async-channel
         racket/function)

(struct waiter (thd req-ch notify-ch))

(define (make-waiter)
  (define req-ch (make-channel))
  (define notify-ch (make-channel))
  (define (run clients)
    (sync (handle-evt req-ch
                      (lambda (ch) (run (cons ch clients))))
          (handle-evt notify-ch
                      (lambda (ignore)
                        (for ([c clients]) (async-channel-put c #t))))))
  (define thd
    (thread (lambda () (run null))))
  (waiter thd req-ch notify-ch))

(define (wait a-waiter)
  (define ch (make-async-channel))
  (channel-put (waiter-req-ch a-waiter) ch)
  (void (async-channel-get ch)))

(define (notify-all a-waiter)
  (channel-put (waiter-notify-ch a-waiter) #t)
  (thread-wait (waiter-thd a-waiter)))

(define w (make-waiter))
(define t1 (thread (thunk (wait w)
                          (displayln "first"))))
(define t2 (thread (thunk (wait w)
                          (displayln "second"))))

(sleep 1)
(notify-all w)
(thread-wait t1)
(thread-wait t2)