#!/usr/bin/perl

use strict;
use warnings;
use v5.10;
use Fcntl qw( LOCK_EX LOCK_UN );
use autodie qw( open flock close );

sub main
{
	# create a tempfile
	open my $h,'>>','tempfile.tmp';
	close $h;

	# start a background process that locks the tempfile for 10 seconds
	if ($^O eq 'MSWin32') {
		system 1,$^X,$0,'locker';
	} else {
		my $pid=fork() // die "Can't fork: $!";
		unless ($pid) {
			exec $^X,$0,'locker' or die "Can't exec: $!";
		}
	}
	sleep 1; # wait one second for the helper process;

	open $h,'>>','tempfile.tmp';
	$@='';
	my $start=time();
	eval {
		local $SIG{'ALRM'}=sub { die "timeout" };
		say 'main: alarm 5';
		alarm(5);
		say 'main: flock LOCK_EX';
		flock($h,LOCK_EX);
		say 'main: alarm 0';
		alarm(0);
	};
	my $err=$@ || 'successfully locked';
	my $stop=time();
	say "main: $err";
	say 'main: ',$stop-$start,' seconds have passed';
	close $h;
	# allow locker() to finish before returning to command prompt
	($^O eq 'MSWin32') ? sleep 1 : wait;
}

sub locker
{
	open my $h,'>>','tempfile.tmp';
	say 'locker: flock LOCK_EX';
	flock($h,LOCK_EX);
	say 'locker: locked';
	say 'locker: sleep 10';
	sleep 10;
	say 'locker: flock LOCK_UN';
	flock($h,LOCK_UN);
	say 'locker: unlocked';
	close $h;
}


@ARGV ? locker() : main();