iter queue."
  (let/ec return
    (let* ((suspend
            (lambda (cont future-to-wait)
              ;; FUTURE wishes to wait for the completion of FUTURE-TO-WAIT.
              ;; At this point, FUTURE is unlocked and in `started' state,
              ;; and FUTURE-TO-WAIT is unlocked.
              (with-mutex %futures-mutex
                (with-mutex (future-mutex future)
                  (set-future-thunk! future cont)
                  (set-future-state! future 'queued))

                (with-mutex (future-mutex future-to-wait)
                  ;; If FUTURE-TO-WAIT completed in the meantime, then
                  ;; reschedule FUTURE directly; otherwise, add it to the
                  ;; waiter queue.
                  (if (eq? 'done (future-state future-to-wait))
                      (begin
                        (enq! %futures future)
                        (signal-condition-variable %futures-available))
                      (set! %futures-waiting
                            (alist-cons future-to-wait future
                                        %futures-waiting))))

                (return #f))))
           (thunk (lambda ()
                    (call-with-prompt %future-prompt
                                      (lambda ()
                                        (parameterize ((%within-future? #t))
                                          ((future-thunk future))))
                                      suspend))))
      (set-future-result! future
                          (catch #t
                            (lambda ()
                              (call-with-values thunk
                                (lambda results
                                  (lambda ()
                                    (apply values results)))))
                            (lambda args
                              (lambda ()
                                (apply throw args)))))
      #t)))

(define (process-one-future)
  "Attempt to pick one future from the queue and process it."
  ;; %FUTURES-MUTEX must be locked on entry, and is locked on exit.
  (or (q-empty? %futures)
      (let ((future (deq! %futures)))
        (lock-mutex (future-mutex future))
        (case (future-state future)
          ((done started)
           ;; Nothing to do.
           (unlock-mutex (future-mutex future)))
          (else
           ;; Do the actual work.

           ;; We want to release %FUTURES-MUTEX so that other workers can
           ;; progress.  However, to avoid deadlocks, we have to unlock
           ;; FUTURE as well, to preserve lock ordering.
           (unlock-mutex (future-mutex future))
           (unlock-mutex %futures-mutex)

           (lock-mutex (future-mutex future))
           (if (eq? (future-state future) 'queued) ; lost the race?
               (begin                          ; no, so let's process it
                 (set-future-state! future 'started)
                 (unlock-mutex (future-mutex future))

                 (let ((done? (process-future! future)))
                   (when done?
                     (with-mutex %futures-mutex
                       (with-mutex (future-mutex future)
                         (set-future-state! future 'done)
                         (notify-completion future))))))
               (unlock-mutex (future-mutex future))) ; yes

           (lock-mutex %futures-mutex))))))

(define (process-futures)
  "Continuously process futures from the queue."
  (lock-mutex %futures-mutex)
  (let loop ()
    (when (q-empty? %futures)
      (wait-condition-variable %futures-available
                               %futures-mutex))

    (process-one-future)
    (loop)))

(define (notify-completion future)
  "Notify futures and callers waiting that FUTURE completed."
  ;; FUTURE and %FUTURES-MUTEX are locked.
  (broadcast-condition-variable (future-completion future))
  (let-values (((waiting remaining)
                (partition (match-lambda          ; TODO: optimize
                            ((waitee . _)
                             (eq? waitee future)))
                           %futures-waiting)))
    (set! %futures-waiting remaining)
    (for-each (match-lambda
               ((_ . waiter)
                (enq! %futures waiter)))
              waiting)))

(define (touch future)
  "Return the result of FUTURE, computing it if not already done."
  (define (work)
    ;; Do some work while waiting for FUTURE to complete.
    (lock-mutex %futures-mutex)
    (if (q-empty? %futures)
        (begin
          (unlock-mutex %futures-mutex)
          (with-mutex (future-mutex future)
            (unless (eq? 'done (future-state future))
              (wait-condition-variable (future-completion future)
                                       (future-mutex future)))))
        (begin
          (process-one-future)
          (unlock-mutex %futures-mutex))))

  (let loop ()
    (lock-mutex (future-mutex future))
    (case (future-state future)
      ((done)
       (unlock-mutex (future-mutex future)))
      ((started)
       (unlock-mutex (future-mutex future))
       (if (%within-future?)
           (abort-to-prompt %future-prompt future)
           (begin
             (work)
             (loop))))
      (else
       (unlock-mutex (future-mutex future))
       (work)
       (loop))))
  ((future-result future)))