finalizers on handles

[Ganesh Sittampalam]

Hi,

I recently spent a while debugging a problem where a program deadlocked in 
the non-threaded runtime, but ran fine in the threaded runtime, despite 
the program having no blocking FFI calls, and boiled it down to the 
following test case:

module Main(main) where

import System
import System.IO
import System.Process

main = do
    (ih, oh, _, _) <- runInteractiveProcess "cat" [] Nothing Nothing
    comphszp <- hGetContents oh
    print (length comphszp)
    -- hClose ih -- with this line they both deadlock

The reason for the deadlock is fairly straightforward; since ih isn't 
closed before comphszp is fully demanded, no progress can be made. My 
guess is that the threaded runtime is fine because ih is dead at that 
point, and the finalizer for ih gets a chance to run, closing ih.

If I'm right, is it really sensible for every handle to have this 
finalizer? Closing a pipe has externally visible side 
effects beyond just the release of resources, so it doesn't seem like it 
should happen non-deterministically.

Cheers,

Ganesh