called via @code{with-continuation-barrier}.

When @var{handler} is specified, then @var{thunk} is called from within
a @code{catch} with tag @code{#t} that has @var{handler} as its handler.
This catch is established inside the continuation barrier.

Once @var{thunk} or @var{handler} returns, the return value is made the
@emph{exit value} of the thread and the thread is terminated."
  (let ((cv (make-condition-variable))
        (mutex (make-mutex))
        (thunk (if handler
                   (lambda () (catch #t thunk handler))
                   thunk))
        (thread #f))
    (define (call-with-backtrace thunk)
      (let ((err (current-error-port)))
        (catch #t
          (lambda () (%start-stack 'thread thunk))
          (lambda _ (values))
          (lambda (key . args)
            ;; Narrow by three: the dispatch-exception,
            ;; this thunk, and make-stack.
            (let ((stack (make-stack #t 3)))
              (false-if-exception
               (begin
                 (when stack
                   (display-backtrace stack err))
                 (let ((frame (and stack (stack-ref stack 0))))
                   (print-exception err frame key args)))))))))
    (with-mutex mutex
      (%call-with-new-thread
       (lambda ()
         (call-with-values
             (lambda ()
               (call-with-prompt cancel-tag
                 (lambda ()
                   (lock-mutex mutex)
                   (set! thread (current-thread))
                   ;; Rather than use the 'set!' syntax here, we use the
                   ;; underlying 'setter' generic function to set the
                   ;; 'thread-join-data' property on 'thread'.  This is
                   ;; because 'set!' will try to resolve 'setter' in the
                   ;; '(guile)' module, which means acquiring the
                   ;; 'autoload' mutex.  If the calling thread is
                   ;; already holding that mutex, this will result in
                   ;; deadlock.  See <https://bugs.gnu.org/62691>.
                   ((setter thread-join-data) thread (cons cv mutex))
                   (signal-condition-variable cv)
                   (unlock-mutex mutex)
                   (call-with-unblocked-asyncs
                    (lambda () (call-with-backtrace thunk))))
                 (lambda (k . args)
                   (apply values args))))
           (lambda vals
             (lock-mutex mutex)
             ;; Probably now you're wondering why we are going to use
             ;; the cond variable as the key into the thread results
             ;; object property.  It's because there is a possibility
             ;; that the thread object itself ends up as part of the
             ;; result, and if that happens we create a cycle whereby
             ;; the strong reference to a thread in the value of the
             ;; weak-key hash table used by the object property prevents
             ;; the thread from ever being collected.  So instead we use
             ;; the cv as the key.  Weak-key hash tables, amirite?
             (set! (%thread-results cv) vals)
             (broadcast-condition-variable cv)
             (unlock-mutex mutex)
             (apply values vals)))))
      (let lp ()
        (unless thread
          (wait-condition-variable cv mutex)
          (lp))))
    thread))

(define* (join-thread thread #:optional timeout timeoutval)
  "Suspend execution of the calling thread until the target @var{thread}
terminates, unless the target @var{thread} has already terminated."
  (match (thread-join-data thread)
    (#f (error "foreign thread cannot be joined" thread))
    ((cv . mutex)
     (lock-mutex mutex)
     (let lp ()
       (cond
        ((%thread-results cv)
         => (lambda (results)
              (unlock-mutex mutex)
              (apply values results)))
        ((if timeout
             (wait-condition-variable cv mutex timeout)
             (wait-condition-variable cv mutex))
         (lp))
        (else
         (unlock-mutex mutex)
         timeoutval))))))

(define* (try-mutex mutex)
  "Try to lock @var{mutex}.  If the mutex is already locked, return
@code{#f}.  Otherwise lock the mutex and return @code{#t}."
  (lock-mutex mutex 0))