Am Sun, 8 Nov 2020 02:17:58 +0000
schrieb Chris Vine <[email protected]>:
> I wonder therefore why you think that this renders the proposed
> with-lock function inappropriate, if that is what you were implying?
> Perhaps you weren't and were making some other point. Macros for
> handling resources such as mutex invoking dynamic-wind are
> commonplace.
Sorry, short on time. Two experiments for you kind consideration in
the appended `test.scm`
Best
/Jörg
;; experiment 1
(define x (make-mutex))
(thread-start!
(make-thread
(lambda ()
(dynamic-wind
(lambda () (mutex-lock! x))
(lambda () (raise 2))
(lambda () (mutex-unlock! x))))))
(mutex-state x)
(mutex-lock! x)
;; experiment 2
(define (current-dynamic-extent)
;; a stripped down version of srfi-154
;;
;; Note: any unlucky combination of call/cc, e.g., from exception
;; handlers exception handler might result in an obfuscated version
;; of this:
((call-with-current-continuation
(lambda (unit)
((lambda (d) (lambda () d))
(lambda (thunk)
(call-with-current-continuation
(lambda (k)
(unit (lambda () (call-with-values thunk k)))))))))))
(define (dynamic f #!optional (dynamic-extent #f))
(let ((fixed (if dynamic-extent dynamic-extent (current-dynamic-extent))))
(lambda args (fixed (lambda () (apply f args))))))
;; Here we go
(define (with-mutex m t)
(dynamic-wind
(lambda () (mutex-lock! m))
t
(lambda () (mutex-unlock! m))))
(define x (make-mutex))
(mutex-specific-set! x #t)
(define invert-x!
(dynamic (lambda () (with-mutex x (lambda () (mutex-specific-set! x (not (mutex-specific x))))))))
(display
(with-mutex
x
(lambda ()
(let ((before (mutex-specific x)))
(invert-x!) ;; I bet you'd expect this to block, don't you?
(eq? before (mutex-specific x))))))
