Using `finally` here would seem like the wrong thing to me, since you don't want to `finally` kill subthreads, but only to kill them if you yourself are killed. Note that finally also only cleans up if interrupted *during* the computation of the first part -- it doesn't attach a handler to the thread as a whole.
The best I can think of at the moment is (in pseudocode, and probably not handling some corner cases)
withSafeFork :: (IO () -> IO ThreadId) -> IO a) -> IO a
{-
withSafeFork $ \safeFork -> do
safeFork something
safeFork somethingelse
-}
withSafeFork act = do
forkCleanup <- newMVar []
let safeFork x = withMVar forkCleanup $ \list -> do
tid <- forkIO x
return (tid, tid:list)
act safeFork `catchJustThreadKilled` const ((mapM_ (forkIO . killThread) =<< readMVar forkCleanup) >>
rethrowThreadKilled)
The unthoughthrough bit here being what happens when you catch a threadkilled in the middle of a safeFork call (at the moment, I suspect, deadlock)... but that should be easy to work out.
The way to use this of course would be to hide forkIO and only allow forking through withSafeFork.
Regards,
Sterl.
On Dec 18, 2008, at 4:43 PM, Conal Elliott wrote:
I realized in the shower this morning that there's a serious flaw in my unamb implementation as described in http://conal.net/blog/posts/functional-concurrency-with-unambiguous-choice. I'm looking for ideas for fixing the flaw. Here's the code for racing computations:
race :: IO a -> IO a -> IO a
a `race` b = do v <- newEmptyMVar
ta <- forkPut a v
tb <- forkPut b v
x <- takeMVar v
killThread ta
killThread tb
return x
forkPut :: IO a -> MVar a -> IO ThreadId
forkPut act v = forkIO ((act >>= putMVar v) `catch` uhandler `catch` bhandler)
where
uhandler (ErrorCall "Prelude.undefined") = return ()
uhandler err = throw err
bhandler BlockedOnDeadMVar = return ()
The problem is that each of the threads ta and tb may have spawned other threads, directly or indirectly. When I kill them, they don't get a chance to kill their sub-threads.
Perhaps I want some form of garbage collection of threads, perhaps akin to Henry Baker's paper "The Incremental Garbage Collection of Processes". As with memory GC, dropping one consumer would sometimes result is cascading de-allocations. That cascade is missing from my implementation.
Or maybe there's a simple and dependable manual solution, enhancing the method above.
Any ideas?
- Conal
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