[reactive] Re: black hole detection and concurrency

[Peter Verswyvelen]

I fail to understand this part of the code:
           case fromException e of
               Just ThreadKilled -> do
                   myThreadId >>= killThread
                   unblock (race a b)

So the current thread gets killed synchronously, then then the race function
is evaluated again? The latter I don't get.



On Sun, Dec 28, 2008 at 3:03 AM, Bertram Felgenhauer <
bertram.felgenhauer at googlemail.com> wrote:

> Sterling Clover wrote:
> > On Dec 27, 2008, at 9:02 AM, Bertram Felgenhauer wrote:
> >> In the above code, there is a small window between catching the
> >> ThreadKilled exception and throwing it again though, where other
> >> exceptions may creep in. The only way I see of fixing that is to use
> >> 'block' and 'unblock' directly.
> >
> > That certainly seems to do the trick for the simple example at least. One
> > way to reason about it better would be, instead of folding everything
> into
> > the race function, to simply modify ghc's bracket function to give us the
> > behavior we'd prefer (speaking of which, I recall there's something in
> the
> > works for 6.12 or so to improve rethrowing of asynchronous exceptions?)
> >
> > brackAsync before after thing =
> >   block (do
> >     a <- before
> >     r <- catch
> >            (unblock (thing a))
> >            (\_ -> after a >> myThreadId >>= killThread >>
> >                   brackAsync before after thing )
> >     after a
> >     return r
> >  )
> >     where threadKilled ThreadKilled = Just ()
> >           threadKilled _            = Nothing
>
> This code turns any exception into ThreadKilled further down the stack.
>
>  (\e -> do
>       after a
>       myThreadId >>= flip throwTo (e :: SomeException)
>       ...
>
> might do the trick.
>
> My assumption was that anything but 'ThreadKilled' would be a
> real error. This isn't really true, I guess - thanks to throwTo,
> any exception could be asynchronous.
>
> If an exception is thrown, 'after a' is run again after the computation
> has resumed.
>
> That's why I did the cleanup within the 'catch'.
>
> But there's no reason why you couldn't do that as well:
>
>  brackAsync before after thing =
>    block $ do
>      a <- before
>       catch  (unblock (thing a) >>= \r -> after a >> return r) $
>             \e -> do
>                    after a
>                    myThreadId >>= flip throwTo (e :: SomeException)
>                    brackAsync before after thing )
>
> > This brackAsync just drops in to the previous code where bracket was and
> > appears to perform correctly.
>
> Right. 'race' should also unblock exceptions in the worker threads,
>
>    withThread u v = brackAsync (forkIO (unblock u)) killThread (const v)
>
> but that's an independent change.
>
> > Further, if we place a trace after the
> > killThread, we se it gets executed once when the example is read (i.e. a
> > resumption) but it does not get executed if the (`seq` v) is removed from
> > the example So this gives me some hope that this is actually doing what
> > we'd like. I don't doubt it may have further kinks however.
>
> At least the GHC RTS has support for the hard part - unwinding the stack
> so that computations can be resumed seamlessly.
>
> I'm not sure which of the approaches I like better - it seems that we
> have a choice between turning async exceptions into sync ones or vice
> versa, and neither choice is strictly superior to the other.
>
> Enjoy,
>
> Bertram
>
> 'race' update:
> - Bugfix: Previously, only AsyncException-s would be caught.
>  Use 'fromException' to select the ThreadKilled exception.
> - I tried using a custom 'SuspendException' type, but this resulted in
>  'test: SuspendException' messages on the console, while ThreadKilled
>  is silently ignored... as documented:
>
> http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Concurrent.html#v%3AforkIO
>     (http://tinyurl.com/9t5pxs)
> - Tweak: Block exceptions while running 'cleanup' to avoid killing
>  threads twice.
> - Trick: takeMVar is a blocking operation, so exceptions can be
>  delivered while it's waiting - there's no need to use 'unblock' for
>  this. In other words,  unblock (takeMVar v)  and  takeMVar v  are
>  essentially equivalent for our purposes.
>
> race :: IO a -> IO a -> IO a
> race a b = block $ do
>    v <- newEmptyMVar
>    let t x = unblock (x >>= putMVar v)
>    ta <- forkIO (t a)
>    tb <- forkIO (t b)
>    let cleanup = killThread ta >> killThread tb
>    (do r <- takeMVar v; cleanup; return r) `catch`
>        \e -> cleanup >>
>            case fromException e of
>                Just ThreadKilled -> do
>                    myThreadId >>= killThread
>                    unblock (race a b)
>                _ -> throwIO e
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