writeFile/readFile on multiple threads leads to error

[Simon Marlow]

2009/4/22 Neil Mitchell <ndmitchell at gmail.com>:
> I've got a multi-threaded application which occasionally generates
> failures in openFile. I haven't been able to reproduce the errors
> reliably, the code is way too large to send over, and any small
> attempts at invoking the same problem don't seem to work. Despite the
> uselessness of the bug report, I thought I'd share what I've seen and
> how I fixed it.
>
> I have many threads, which read and write files. Every so often one
> thread will write a file, then another thread will read the same file
> - but fail during the open call. There are locks to ensure that the
> write call finishes before the read call begins. I modified the code
> to give:
>
> do print ("READ START",x) ; res <- readFile x ; print ("READ STOP",x)
> ; return res
>
> do print ("WRITE START",x); writeFile x src ; print ("WRITE STOP",x)
>
> I then get on the console:
>
> WRITE START foo
> WRITE STOP foo
> READ START foo
> openFile doesn't have permission to open foo.
>
> The writeFile/readFile are happening in different threads, and they
> usually succeed - but not always. The bug seems to go away when I add
> performGC just after writeFile. My guess is that something in the
> openFile/hClose pair isn't really closed until a garbage collection
> happens. All this is using GHC 6.10.2 on XP through Cygwin.

The hClose really does close the file descriptor.  The only thing left
is the finalizer, but it is just a no-op on an already-closed Handle.

I can't think of anything we're doing that could possibly cause this,
but I have seen rogue "permission denied" errors on Windows from time
to time, they're quite annoying.  Here's a possibly-related ticket:

http://hackage.haskell.org/trac/ghc/ticket/2924

You might want to run the process under ProcMon and see if you can
figure out what's going on (if you can bear to use ProcMon, it's a
very poor replacement for strace IMO).

Cheers,
   Simon