[Haskell-cafe] Faster timeout but is it correct?

Bertram Felgenhauer bertram.felgenhauer at googlemail.com
Wed Feb 23 10:59:03 CET 2011

> > 1. registerTimeout
> > 2. (fmap Just f) raises an exception, or the thread gets killed otherwise.
> > 3. We enter the `catch` handler, with the corresponding exception.
> > 4. The timeout expires, and the event Manager runs the IO action, i.e.
> >   throwTo myTid $ Timeout key
> > 5. And now we have a pending Timeout exception which escapes the 'timeout'.
> >   The unregTimeout will come too late.
> Bummer! You're right.
> But maybe we can catch and ignore a potential pending Timeout
> exception: (code not tested and profiled yet)

The trouble is that the throwTo action, even though we can no longer stop
it using unregisterTimeout, may be delayed arbitrarily. So we'd have to
wait for it to arrive. In that regard we have actually lost power compared
to spawning a separate thread: throwTo (and thus killThread) explicitely
guarantees that if two threads throw exceptions at one another
simultaneously, only one of those will arrive, and also that if throwTo
returns, the exception has actually been delivered. So if the killThread
succeeds, the Timeout exception will never arrive.

> Actually the event manager based implementation totally crashes on
> your example, so again: bummer! I get the following error:
> "gotcha: user error (Pattern match failure in do expression at
> libraries/base/System/Event/Thread.hs:208:9-16)!"
> Line 208:
> Just mgr <- readIORef eventManager

Yikes! I don't believe that's supposed to happen, but I have no clue
how the event manager is initially started - does the RTS invoke
ensureIOManagerIsRunning  before running  main?

Best regards,


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