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

Bugs item #1235728, was opened at 2005-07-11 00:41
Message generated for change (Comment added) made by remit
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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

----------------------------------------------------------------------

>Comment By: Remi Turk (remit)
Date: 2005-07-11 00:49

Message:
Logged In: YES 
user_id=26642

A few new datapoints. Using -threaded, 6.2.1 compiled
executables still survive, while 6.4 dies.

% ghc-6.4 -no-recomp foo.hs                 
% ./a.out                                   
% ghc-6.2.1 -no-recomp foo.hs 
% ./a.out                    

% ghc-6.4 -no-recomp -threaded foo.hs
% ./a.out                            
zsh: segmentation fault  ./a.out
% ghc-6.2.1 -no-recomp -threaded foo.hs
% ./a.out                              
% 

Also, the first forkIO can be changed into forkOS without
changing (crashing)behaviour, the second cannot.

Oh, and I won't be upset if this one won't be fixed for 6.4.1 ;)

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