bug#14039: Bug in with-fluids semantics

[Stefan Israelsson Tampe <stefan.itampe@gmail.com>]

Consider this simple exmple with fluids and reodos via propmts,

(define (f x) 
  (let ((s (make-fluid 0))) 
     (with-fluids ((s 0)) 
        (let lp ((i 0)) 
           (cond ((>= i 100) (fluid-ref s)) 
                 ((= i 50) (abort-to-prompt 'tag) (lp (+ i 1))) 
                 (else (fluid-set! s (+ (fluid-ref s) i)) 
		       (lp (+ i 1))))))))

(define k (call-with-prompt 'tag (lambda () (f 1)) (lambda (k . l)
k)))

Then we will get in guile-2.0 pretty resent git version
scheme@(guile-user)> (k)
$1 = 4900
scheme@(guile-user)> (k)
$2 = 8575

The reason is that when the with-fluid returns normally it does a full
swap. It should only do half a swap e.g. restore the old value of the
fluid and not store the current which is of non use because it can not
be reached anymore and it contaminates the continuation k.

/Stefan