writeFile/readFile on multiple threads leads to error

[Neil Mitchell]

>> 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:
>>
>> 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

I've added information from this thread to that ticket. It looks
suspiciously similar, and I have a feeling that some combination of
writeFile/readFile in a similar pattern might invoke the same issue -
it's certainly quite close to the behaviour of my application.

> 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).

I tried and the bug goes away, plus the computer grinds to a crunching
halt. The bug is very sensitive, and with the speed loss associated
with ProcMon probably disappears on its own. No useful information can
be obtained there.

Thanks

Neil