[reactive] Re: black hole detection and concurrency

[Conal Elliott]

This is a neat trick indeed!  I'd appreciate an explanation of killing one's
own thread and then continuing (with a restart in this case).  How does the
post-kill resumption occur?  That is, how does control pass to the
tail-recursive call after the self-kill?

  - Conal


2008/12/28 Peter Verswyvelen <bugfact at gmail.com>

> 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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