throwTo to a thread inside 'block'

[copy-pasting my original mail (http://www.haskell.org/pipermail/glasgow-haskell-bugs/2005-June/005235.html)]

Good evening,

I just stumbled across a segfault caused when running the following small program. (During an attempt to implement single-assignment variables.)

> module Main where
> 
> import Control.Concurrent
> import System.IO.Unsafe (unsafeInterleaveIO)
> 
> main = do
>     v <- newEmptyMVar
>     a <- unsafeInterleaveIO (readMVar v)
>     t <- forkIO (print a)
>     threadDelay (1000*1000)
>     killThread t
>     forkIO (print a)
>     putMVar v ()

The crucial part about it seems to be the interruption of the lazy IO. Typing Ctl-c while running the first "print a" by hand from ghci instead of the forkIO+killThread doesn't change behaviour:

 Prelude System.IO.Unsafe Control.Concurrent> v <- newEmptyMVar
 Prelude System.IO.Unsafe Control.Concurrent> a <- unsafeInterleaveIO (readMVar v)
 Prelude System.IO.Unsafe Control.Concurrent> print a
 Interrupted.
 Prelude System.IO.Unsafe Control.Concurrent> forkIO (print a)
 Prelude System.IO.Unsafe Control.Concurrent> putMVar v ()
 zsh: segmentation fault (core dumped)  ghci

Both 6.4 and 6.2.1 crash when running main from ghci. When running it as a compiled executable everything is fine.

Although I'm pretty sure I've seen 6.2.1 crashing on it when run with -e main, I cannot reproduce it anymore. 6.4 certainly happily runs it with -e main. (A serious lack of sleep the last week may play a role too.. :-/)

Whether the module is compiled before being loaded into ghci has no effect.

Core-dumps etc can of course be sent if necessary.

Good night, Remi