#!/usr/bin/perl -w

use Fcntl ':flock';
use POSIX "sys_wait_h";

my $file = '/tmp/test-file.txt';
my $try;

my $pid = fork();
if ($pid == 0) {
        #this is the child
        #Go to sleep and let the parent lock the file
        print "Child ($$) going to sleep for a bit...\n";
        sleep(3);
        print "Child ($$) woke up!\n";
        #Wake up and try to open and lock the file
        print "Child ($$) attempting to open for writing...\n";
        if (open CWRITER, ">$file"){
                print "Child ($$) completed open for writing\n";
                print "Child ($$) closing filehandle.\n";
                close CWRITER;
                print "Child ($$) closed filehandle.  Exiting.\n";
        } else {
                print "Child ($$) unable to open for appending: $!\n";
        }
        exit;
} else {
        #this is the parent
        #open a filehandle and lock it with LOCK_EX
        print "Parent ($$) attempting to open file\n";
        if (open PWRITER, ">$file"){
                print "Parent ($$) opened file, attempting LOCK_EX\n";
                my $try = flock(PWRITER, LOCK_EX) or warn "Parent ($$) can't flock: $!\n";
                print "Parent ($$) try for LOCK_EX is '$try'\n";
                print "Parent ($$) printing to file\n";
                print PWRITER "Parent says hello.\n";
                print "Parent ($$) done writing, closing filehandle.\n";
                close PWRITER;
                print "Parent ($$) closed filehandle, now reopening as reader.\n";
                if (open PREADER, "$file") {
                        print "Parent ($$) opened file for reading.  Requesting LOCK_EX.\n";
                        flock(PREADER, LOCK_EX) or warn "Parent ($$) unable to LOCK_EX: $!, $?\n";
                        print "Parent ($$) got LOCK_EX.\n";

                        print "Parent ($$) going to sleep.\n";
                        sleep(10);
                        print "Parent ($$) woke up!\n";

                        print "Parent ($$) reading now...\n";
                        my @lines = <PREADER>;
                        print "Parent ($$) read:\n@lines\n";
                        print "Parent ($$) closing PREADER\n";
                        close PREADER;
                        print "Parent ($$) closed PREADER.\n";
                } else {
                        print "Parent unable to open file for read: $!\n";
                }

        } else {
                warn "Parent ($$) couldn't open '$file' for reading: $!\n";
        }

        my $waiter = 1;
        while ($waiter > 0){
                #try to reap the child PID
                $waiter = waitpid($pid, &WNOHANG);
                if ($waiter == -1){
                        print "Waiter got -1: $?\n";
                } elsif ($waiter == 0) {
                        print "Waiter got 0, $pid still alive...\n";
                } elsif ($waiter > 0) {
                        print "Waiter got $waiter, $?\n";
                } else {
                        print "Waiter got something odd: '$waiter','$?'\n";
                }
                sleep(2);
        }
        print "Parent ($$) done reaping child $pid\n";
}

print "$$ Exiting\n";