No, that's not supposed to happen. I've pushed a repair.
I was able to replicate the problem on Mac OS X, and it looks like the problem with filesystem events is specific to that platform. But the filesystem-event problem is due to a more general bug in the scheduler, and the same bug shows up with (sync (wrap-evt s (lambda (v) 'ok))) on all platforms. The problem is related to `thread-suspend`, a semaphore becoming ready while the thread is suspended, and having a wrapper (in the sense of `wrap-evt`) around the semaphore. That pattern happens internally with filesystem events on Mac OS X. I think `thread-suspend` probably isn't used that much, which would explain how this scheduler bug survived for so long. You can work around the bug by creating a dummy pipe and waiting for the input end of the pipe in addition to the filesystem event, like this: (define-values (dummy-i dummy-o) (make-pipe)) (sync dummy-i (wrap-evt s (lambda (v) 'ok))) Having a pipe in the set of events sends the scheduler into a path that doesn't have the bug. At Sun, 13 Nov 2016 17:43:43 -0800 (PST), Philip McGrath wrote: > Hi everyone, > > I've been trying to get to know Racket's threads and particularly > filesystem-change-evt, and I've come to a point of confusion. I thought that > a > filesystem-change-evt's synchronization result was supposed to be the event > itself, which it is most of the time, but in combination with thread-suspend > it seems to produce #f, and I'm not sure why. > > Here's a tiny example: > > > (define t > (thread (λ () > (println (sync (filesystem-change-evt "test")))))) > > (thread-suspend t) > ;; at this point I go and do something like "echo foo >test" > > (thread-resume t) > ;; prints #f > > Is this supposed to happen? > > What's particularly confusing is that the same thing happens (i.e. #f is > printed) if I instead define t to be: > > (thread > (λ () > (println > (sync (wrap-evt (filesystem-change-evt "test") > (λ (x) 'my-result)))))) > > Does anyone know what's going on? > > Thanks, > Philip > > -- > You received this message because you are subscribed to the Google Groups > "Racket Users" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > For more options, visit https://groups.google.com/d/optout. -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
