Unsafe hGetContents

[Florian Weimer]

* Simon Marlow:

>> What about handles from System.Process?  Do they count as well?
>
> Sure - we hopefully don't consider System.Process to be unsafe.

Here's a demonstration that lazy input has an observable effect.  It
needs the Perl helper script included below.

Of course, this example is constructed, but there are similar issues
to consider when network IO is involved.  For instance, not reading
the lazy structure to its end causes the server to keep the connection
open longer than necessary.

----------------------------------------------------------------------
-- Based on Oleg Kiselyov's example in:
-- <http://www.haskell.org/pipermail/haskell/2009-March/021064.html>

module Main where

import System.IO (hGetContents)
import System.Process (runInteractiveProcess)

f1, f2:: String -> String -> String

f1 e1 e2 = e1 `seq` e2 `seq` e1
f2 e1 e2 = e2 `seq` e1 `seq` e1

f = head . tail . lines

spawn :: () -> IO String
spawn () = do
  (inp,out,err,pid) <-
      runInteractiveProcess "perl" ["magic.pl"] Nothing Nothing
  hGetContents out

main = do
       s1 <- spawn ()
       s2 <- spawn ()
       print $ f1 (f s1) (f s2)
       -- print $ f2 (f s1) (f s2)
----------------------------------------------------------------------

#!/usr/bin/perl

# Magic program to demonstrate that lazy I/O leads to observable
# differences in behavior.

use strict;
use warnings;

use Fcntl ':flock';

open my $self, '<', $0 or die "opening $0: $!\n"; # use this file as lock
flock($self, LOCK_SH) or die "flock(LOCK_SH): $!\n";
print "x" x 100_000 . "\n"; # blocks if reader blocks
print flock($self, LOCK_EX | LOCK_NB) ? "locked\n" : "failed\n";
  # only succeeds if the other process has exited