#!/usr/bin/perl
use strict;

# make it impossible to run this script more than once at a time
use Cron::AvoidMultipleRuns;

print "Running... pid = $$\n";

# pretend this script actually does something here...
sleep 20;

##</code><code>##

package Cron::AvoidMultipleRuns;
use strict;

my $cleanup;

INIT {
  $cleanup = 0;
  (my $pid_file = "$0.pid") =~ s/\.pl//;
  if ( -e $pid_file ) {
    open my $fh, '<', $pid_file or 
      die "Can't open $pid_file for reading: $!\n";
    my $line = <$fh>;
    my (undef, $pid) = split(/\s+/, $line);
    if ($pid) {
      print "Found a pid file: pid = $pid.\n";
      my $status = kill(0, $pid);
      if ($status) {
        print "old job is still running.\n";
        exit;
      } else {
        print "The old job is no longer running.\n";
      }
    }
  }

  open my $fh, '>', $pid_file or 
    die "Can't open $pid_file for writing: $!\n";
  flock ($fh, 2) or 
    die "can't obtain exclusive lock on file $pid_file: $!\n";
  print $fh "pid: ", $$, "\n";
  close $fh;
  $cleanup = 1;
}

END {
  (my $pid_file = "$0.pid") =~ s/\.pl//;
  if ( -e $pid_file && $cleanup ) {
    unlink $pid_file or 
      die "can't unlink $pid_file : $! \n";
  }
}

1;