Bug in IO libraries when sending data through a pipe?

Manuel M. T. Chakravarty chak@cse.unsw.edu.au
Thu, 04 Apr 2002 23:45:03 +1000

Jens Petersen <juhp@01.246.ne.jp> wrote,

> Jens Petersen <petersen@redhat.com> writes:
> Also I just rediscovered Manuel Chakravarty's HPL (Haskell
> Ports Library), which provides a rather elegant,
> sophisticated approach.
>         http://www.cse.unsw.edu.au/~chak/haskell/ports/
> It compiles fine under ghc-5.02.2, and using the BufferMode
> patch included at the end output seems to be ok, 

Why LineBuffering?  Doesn't really change anything for me
(same GHC version).

> but input
> of more than 2048 bytes doesn't seem to be being handled
> reliably.
> mostly gives no output, but occasionally I see
>         Warning: Ports.listenToPort: Attempted to listen to a closed port!
> Needs some debugging I guess. :)

I guess, it needs more documentation.  Your test program is
wrong in two places.

The function `listenToPort' starts to listens at the given
port exactly when `listenToPort' is executed; ie, any data
that goes over the port earlier will not be received in the
stream that this call to `listenToPort' returns.  

You wrote

>        outpt <- newPort ' '
>        errpt <- newPort ' '
>        let p = proc cmd args
>        p inpt outpt errpt
>        putStrLn "output:"
>        out <- listenToPort outpt

As the processes `cmd' is fork()ed in a separate thread, it
may already have finshed its business until you get to
`listenToPort outpt', which means you have missed all the
data.  For similar reasons

>        errclosed <- isClosedPort errpt
>        unless errclosed $
> 	 do
> 	 err <- listenToPort errpt

isn't recommended.  

I changed your program to what I have appended.  It then
works for me for the

        % cat 4096 | test-processes cat

test...except that I sometimes get a "Broken Pipe".  So, I
guess, I have to do some debugging after all (plus improve
the documentation).


PS: I'll add your program to the ports/tests/ directory if
    you don't mind.


module Main
import Processes
import Ports
import IO (openFile, hGetContents, IOMode(..), hSetBuffering, BufferMode(..))
import Monad (unless)

main	:: IO()
main = do
     inpt <- getContents
     withPorts [] $ \ (cmd:args) ->
       outpt <- newPort ' '
       errpt <- newPort ' '
       out <- listenToPort outpt
       err <- listenToPort errpt
       let p = proc cmd args
       p inpt outpt errpt
       putStrLn "output:"
       mapM_ putStrLn $ lines out
       putStrLn "error:"
       putStr err
       putStrLn "test finished"