writeFile/readFile on multiple threads leads to error

[Neil Mitchell]

Hi,

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.

I'm happy to supply more details if you can think of anything that will help,

Thanks

Neil