alarm handler

[Ed Kademan <kademan@phz.com>]

I am trying to write a scsh program that executes a process in such a
way that if it doesn't complete in a certain time period the program
gives up on it.  I've never been entirely successful in getting this
to work and things seem to have gotten worse in scsh-0.6.0.

I have a procedure called `execute/timeout' that takes two
arguments---a subprocedure and a number of seconds---and attempts to
execute the subprocedure.  The intended behavior is that if the
subprocedure finishes in the alloted number of seconds then
execute/timeout returns its value.  If it doesn't finish the return
value is 'timeout.  The actual behavior is something else.  (I have
appended the definition of execute/timeout and a transcript of a
sample run below.)

I would not be happy with the function even if it somehow worked.  It
seems to me that the alarm interrupt handler should clean up after the
incomplete subprocedure, but I don't see how to do that.  I am
prepared to believe that there is an entirely better way of writing
the function using threads but I don't see how to do that either.

My purpose in sending this message is to ask if anyone has already
written a procedure that does what I am describing, or to ask if they
have pointers on how to do so.  Also, I want to describe how my
procedure behaves in case it demonstrates a bug in scsh---but the more
I think about it the more I believe this is my fault.

Thanks for reading this.
-----------------------------------------------------------------
;;; This is scsh-0.6.0 on Solaris2.6.

  (define (execute/timeout proc sec)
    (let ((alrm-handler (interrupt-handler interrupt/alrm))
          (result 
           (call-with-current-continuation
            (lambda (break)
              (set-interrupt-handler interrupt/alrm
                                     (lambda (i) (break 'timeout)))
              (itimer sec)
              (with-enabled-interrupts (interrupt-set interrupt/alrm)
                                       (proc))))))
      (set-interrupt-handler interrupt/alrm alrm-handler)
      (itimer 0)
      result))

  > (execute/timeout
   (lambda () (run (sleep 10)) 'done)
   5)
  'timeout
  'done
  > 
  Interrupt: post-gc
  4> 
  Interrupt: post-gc
  8> 
  Interrupt: post-gc
  15> 
  Interrupt: post-gc
  20> 
  Interrupt: post-gc
  28> Scheme48 heap overflow
-----------------------------------------------------------------
-- 
Ed Kademan              508.651.3700
PHZ Capital Partners    508.653.1745 (fax)
321 Commonwealth Road   <kademan@phz.com>
Wayland, MA 01778