[racket] Callback on shutdown

I've pushed a second attempt as v6.0.1.8. The `custodian-tidy-all`,
etc., functions are gone, replaced by `plumber-flush-all`,
`make-plumber`, `current-plumber`, `plumber-add-flush!`, etc.

The `racket/sandbox` library (by default) and DrRacket isolate
sandboxed programs by creating a new plumber. In the case of
`racket/sandbox`, a flush callback is installed in the current plumber
to propagate the flush request to the sandboxed plumber; the propagated
flush request happens in the sandbox's thread.

At Thu, 8 May 2014 11:30:59 -0600, Matthew Flatt wrote:
> At Wed, 7 May 2014 12:07:28 -0400, Eli Barzilay wrote:
> > Two hours ago, Matthew Flatt wrote:
> > > I've added `custodian-tidy-all` and related functions to
> > > [un]register a "tidy callback".
> > 
> > I was curious how it works, and on a quick look it sounded like
> > there's a potential problem if a sandbox is registering a bad tidier.
> 
> Oh, I see what you mean. The current design is wrong.
> 
> In adding this functionality, I wondered whether the job really belongs
> with custodians...
> 
> More generally, I had the wrong idea at the start. I initially thought
> that these callbacks were a kind of must-do-on-exit action. If that
> were the case, then the custodian hierarchy would help ensure that the
> callbacks are actually called. Consistent with that misunderstanding, I
> called the new functionality "exit" callbacks, at first.
> 
> But now I understand the new functionality as optional callbacks that
> don't "exit" anything. They try to put things into a nice state; that's
> a good thing to do just before exiting, but it can make sense at other
> times, too. Besides being not mandatory (anything mandatory has to be
> in the privileged world of custodian shut-down actions), they could use
> a slightly different hierarchy than custodians.
> 
> A sandbox should not only isolate a process so that tidying within the
> sandbox is confined, but it also isolate it so that tidying doesn't go
> directly into the sandbox, which means creating a new root for tidy
> callbacks. In response to tidying outside the sandbox, probably a
> sandbox should schedule tidy callbacks to happen within the sandbox's
> thread.
> 
> I'll try again, which probably means introducing a new thing with its
> own hierarchy, including the ability to create new roots.
> 
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