[racket] Detecting that a place channel is unreachable

You're right: the implementation has that kind of information (for
non-distributed channels), but it's significant work to add a trigger.

At Fri, 6 Dec 2013 23:53:04 -0800, Eric Dobson wrote:
> Is there a way to determine if a place channel is unreachable? My
> experiments with will executors seem to show that they trigger even if
> the place channel is reachable from another place. Is there a way to
> determine that all other places don't have access to the value, I
> assume that it has to be possible because otherwise memory would leak,
> but maybe getting triggers based of of it would be hard.
>
> My goal is to have two places sharing a channel, and one stop sending
> messages across if the other one is provable not listening. I can make
> it so that it can turn it off manual, but I'm looking to see if I can
> get it to work if the listener just lets the channel get gc'd.
> 
> 
> Testing program:
> #lang racket/base
> 
> (require racket/place)
> 
> (define (holder)
>   (place chan
>     (let loop ([v #f])
>       (define v2 (sync chan))
>       (place-channel-put chan v)
>       (collect-garbage)
>       (loop v2))))
> 
> 
> (module+ main
>   (define other (holder))
>   (define exec (make-will-executor))
>   (define exec-thread
>     (thread
>       (lambda ()
>         (let loop ()
>           (will-execute exec)
>           (loop)))))
>   (define in
>     (let-values ([(in out) (place-channel)])
>       (place-channel-put other out)
>       (sync other)
>       (will-register exec out
>         (lambda (v) (printf "~a was deleted~n" v)))
>       in))
>   (collect-garbage)
>   ;; Let will execute
>   (sleep 1))
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