[racket] Callback on shutdown

I don't think the thing you want currently exists in general, although
it exists internally for flushing output ports.

Simply triggering the exit handler when the Racket process exits
doesn't seem like a good enough solution. It's awkward to add and
remove callbacks by changing the exit handler, and it doesn't
immediately address the problem of exiting sandbox-like nested
processes. I don't have a solution that immediately addresses the
problem of subprocesses, but if we have to revise sandbox-like things
to support a polite exit, then I think we can find a better interface.

Maybe the right solution is to add support for atexit()-like actions on
custodians, along with a new operation to exit a custodian (implying an
exit for all sub-custodians). Various exit modes, including normal exit
of the Racket process or calling the default exit handler, would use
the custodian-exit operation before existing. Sandboxing contexts can
be similarly improved to exit a custodian or add support for exiting,
depending on what makes sense for the context.

Unlike shutting down a custodian, which is analogous to SIGKILL,
exiting a custodian would be closer to SIGQUIT. Along those lines,
where shutting down a custodian implies mandatory and atomic actions,
exiting a custodian is a request and non-atomic --- so it would safe to
attach arbitrary exit procedures. Of course, there's no guarantee that
an exit procedure will be called if a custodian is first shut down.

I'm not sure why we haven't particularly needed this before, except in
the built-in case of flushing output ports. It's interesting that your
context is also about flushing.

At Sun, 04 May 2014 16:19:37 -0400, Tony Garnock-Jones wrote:
> On 05/04/2014 04:11 PM, Robby Findler wrote:
> > Sounds to me like you want to wire into the custodian facilities (via
> > the ffi I believe) but probably starting with just atexit() itself is
> > okay.
> 
> Via the FFI! Gosh. That sounds scary.
> 
> I did try register-custodian-shutdown from ffi/unsafe/custodian, but (a)
> it wasn't called when the program exited normally, and (b) when I called
> (custodian-shutdown (current-custodian)), it complained about a blocking
> operation in atomic mode or something like that, and abort()ed. I then
> explored use of exit-handler instead :-)
> 
> Would it make sense to change Racket to ensure the current exit-handler
> is called at the end of the main program?
> 
> Regards,
>   Tony
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