[racket-dev] TR tests sometimes fail with a promise error

Thanks for tracking this down!

In case you mean a channel in the sense of `make-channel', I recommend
a semaphore, instead, since a semaphore is the lightest-weight
synchronization construct.

At Fri, 03 May 2013 11:45:38 -0400, Vincent St-Amour wrote:
> Sam, Asumu and I found and fixed the bug. Here's the problem in a
> nutshell.
> 
> This is the implementation of `delay/thread' from racket/promise.
> 
>     (define (delay/thread thunk)
>       (let ()
>         (define (run)
>           (call-with-exception-handler
>            (lambda (e) (pset! p (make-reraise e)) (kill-thread 
> (current-thread)))
>            (lambda () (pset! p (call-with-values thunk list)))))
>         (define p
>           (make-promise/thread
>            (make-running-thread
>             (object-name thunk)
>             (thread run))))
>         p))
> 
> `run' depends on `p', and vice versa. The `run' thread may start
> executing *before* `p' is actually defined. This causes `pset!' (which
> is just promise mutation) to mutate `#<undefined>', which raises an
> exception[1]. The exception handler then calls `pset!' again, which
> raises an exception, and kills the thread without setting the promise's
> value.
> 
> Our solution is to have `run' block on a channel until `p' is fully
> initialized. I'll push the fix.
> 
> Vincent
> 
> 
> [1] Actually, since `pset!' uses `unsafe-struct-set!', there is no
>     actual exception. The thread gets killed some other way.
> 
> 
> At Thu, 2 May 2013 22:27:14 -0400,
> Asumu Takikawa wrote:
> > 
> > On 2013-05-02 22:14:44 -0400, Asumu Takikawa wrote:
> > > This produces a promise error on every third or fifth run or so for me.
> > > Commenting out the #:when line makes it work, very oddly.
> > 
> > Tried it on another machine, the #:when line didn't matter there. Also,
> > I can reproduce this more reliably on a machine with fewer cores.
> > 
> > Cheers,
> > Asumu
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