[plt-scheme] subprocess: double fork() to avoid zombies

I've committed the wait()-via-scheduler change in SVN.

At Tue, 21 Jul 2009 09:59:34 -0600, Matthew Flatt wrote:
> I think a better solution is to have MzScheme call wait() in the
> scheduler after any SIGCHLD. The check_child_done() function already
> handles any terminating child and records its exit status.
> 
> At Tue, 21 Jul 2009 17:42:00 +0200, Tom Schouten wrote:
> > Hello,
> > 
> > I'd like to pick up this thread:
> > http://list.cs.brown.edu/pipermail/plt-scheme/2008-July/025637.html
> > 
> > On Tue Jul 1 2008, Eli Barzilay wrote:
> > > My guess would be that the subprocess object gets released, but the
> > > process is still alive, which leads to the zombie.  (The main problem
> > > being that custodians manage memory & ports, but not processes.)
> > 
> > I was wondering if it is possible to extend `subprocess' with
> > a double fork() mechanism.  I had a look at the code in
> > plt/src/mzscheme/src/port.c and it seems it should be
> > straightforward.  Something like this:
> > 
> > [mzscheme]--fork()-+-------wait()-----------------+--------->
> >                    |                              |
> >                    +--cleanup()--fork()-+--exit()-+
> >                                         |
> >                                         +--exec()-[child]---> 
> > 
> > in addition to the current behaviour:
> > 
> > [mzscheme]--fork()-+---------------------------------------->
> >                    |                              
> >                    +--cleanup()------------exec()-[child]--->
> > 
> > 
> > The idea being that in a lot of practical cases you don't
> > care about a child's exit status.  Doing it like this still
> > allows you to shut down a child by closing its input port,
> > which by most tools is interpreted as "exit", without ever
> > having to care about zombie processes caused by not performing
> > wait().
> > 
> > I'd try this using the FFI but it seems the cleanup()
> > functionality in port.c would have to be duplicated.
> > 
> > Cheers,
> > Tom
> > 
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