Re: ghci and ghc -threaded broken with pipes & forking

28 Feb 2007

      Hello,

Here is a simplified example that seems to exhibit the same behaviour,
unless I screwed up:

--->

module Main where

import System.Posix
import System.IO
import System.Exit

main =
    do putStrLn "running..."
       (stdinr, stdinw) <- createPipe
       (stdoutr, stdoutw) <- createPipe
       pid <- forkProcess $ do hw <- fdToHandle stdoutw
                               hr <- fdToHandle stdinr
                               closeFd stdinw
                               hGetContents hr >>= hPutStr hw
                               hClose hr
                               hClose hw
                               exitImmediately ExitSuccess
       closeFd stdoutw
       closeFd stdinw
       hr2 <- fdToHandle stdoutr
       hGetContents hr2 >>= putStr
       getProcessStatus True False pid >>= print

<---

Compiling with:

ghc --make -no-recomp test3.hs -o test3 && ./test3

works. But compiling with:

ghc --make -no-recomp -threaded test3.hs -o test3 && ./test3

results in a hang. If you comment out the "hGetContents hr >>=" and
change 'hPutStr hw' to 'hPutStr hw "hi"', then it seems to work ok.

As you suggested, it seems that hGetContents is not ever seeing the
EOF when -threaded is enabled. I think it gets 'Resource temporarily
unavailable' instead. So, it keeps retrying.

Assuming I have recreated the same bug, we at least have a simpiler
test case now...

j.

At Wed, 28 Feb 2007 11:15:04 -0600,
John Goerzen wrote:
...
Hi,
I've been hitting my head against a wall for the past couple of days
trying to figure out why my shell-like pipeline code kept hanging.  I
found fd leakage (file descriptors not being closed), which disrupts EOF
detection and can lead to deadlocks.  I just couldn't find the problem.
I finally tried compiling my test with ghc instead of running it in
ghci.
And poof, it worked fine the first time.
I tried asking on #haskell, and got the suggestion that ghci uses
-threaded.  I tried compiling my test program with ghc -threaded, and
again, same deadlock.  My program never calls forkIO or forkOS or any
other threading code.
You can see my test case with:
darcs get '--tag=glasgow ml' http://darcs.complete.org/hsh
ghc -fglasgow-exts --make -o test2 test2.hs
That'll run fine.  If you add -threaded, it will hang.
Ideas?
Thanks,
-- John
_______________________________________________
Glasgow-haskell-users mailing list
Glasgow-haskell-users@haskell.org
http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

Re: ghci and ghc -threaded broken with pipes & forking

Jeremy Shaw