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

Yes (I wrote this privately to Vincent too).

Also, it has the advantage of being clear that you're not sending any
meaningful values (for future maintainers of this code).

Robby


On Fri, May 3, 2013 at 11:06 AM, Vincent St-Amour <stamourv at ccs.neu.edu>wrote:

> Sounds good, I'll use a semaphore.
>
> Vincent
>
>
> At Fri, 3 May 2013 09:58:25 -0600,
> Matthew Flatt wrote:
> >
> > 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
> > > > _________________________
> > > >   Racket Developers list:
> > > >   http://lists.racket-lang.org/dev
> > > _________________________
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> > >   http://lists.racket-lang.org/dev
> _________________________
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>
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