[ ghc-Bugs-1235728 ] unsafeInterleaveIO + Ctrl-C/killThread related segfault

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Bugs item #1235728, was opened at 2005-07-11 00:41
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Category: Runtime System
Group: None
Status: Open
Resolution: None
Priority: 5
Submitted By: Remi Turk (remit)
Assigned to: Nobody/Anonymous (nobody)
Summary: unsafeInterleaveIO + Ctrl-C/killThread related segfault

Initial Comment:
[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

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