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

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
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