Process forms in finalizers (was: add-finalizer!)

I encountered an interesting problem when trying to run an external
process from a finalizer.  I'm writing this message mostly for
documenting my experience for others that might try to achieve the
same.

My first try was the obvious (see my previous e-mail for the whole
example):

   (define (finalize-foo! a-foo)
     (& (touch "/tmp/huhu")
        (> 1 "/dev/null")
        (= 2 1)))

This causes Scsh to crash when GCing:

   > ,collect
   Before: 10545979 words free in semispace
   After:  10645034 words free in semispace
   > 155 / 1548 <- 1580 <- 3864 <- 3871 <- 3880 <- 3915 <- 1607 <-
1108 <- 1604 <- 1108 <- 1314 <- 1284 <- 1108 <-
   Process scheme exited abnormally with code 123

(BTW, what is the meaning of the funny numbers above?)

After somme twiddling I finally came to something that works:

   (define (finalize-foo! a-foo)
     (call-with-output-file  "/dev/null"
       (lambda (null-port)
         (fork (lambda ()
                 (dup null-port 1)
                 (dup null-port 2)
                 (exec "/usr/bin/touch" "/tmp/huhu"))
               #t))))

The crucial factor here is the second argument to `fork', #t.  The manual says:

   The optional boolean argument continue-threads? specifies whether the
   currently active threads continue to run in the child or not. The
   default is #f.

I guess this has something to do with the collector running in its own
thread, but I can't really explain the problem.

-- 
Emilio C. Lopes <[email protected]>
Munich, Germany