Tag: daemon

I have a lot of scripts running in a lot of places, doing various little jobs, mostly shuffling data files around and feeding them into pipelines and suchlike. I also use Jenkins CI to automatically run my tests and build deb packages for Debian/Ubuntu Linux. Unfortunately, being a lazy programmer I haven’t read up about all the great things deb and apt can do so I don’t know how to fire shell commands like “service x reload” or “/etc/init.d/x restart” once a package has been deployed. Kicking a script to pick up changes is quite a common thing to do.

Instead I have a little trick that makes use of the build process changing timestamps on files when it rolls up the package. So when the script wakes up, and starts the next iteration of its event loop, the first thing it does is check the timestamp of itself and if it’s different from the last iteration it executes itself, replacing the running process with a fresh one.

One added gotcha is that if you want to run in taint mode you need to satisfy a bunch of extra requirements such as detainting $ENV{PATH} and all commandline arguments before any re-execing occurs.

#!/usr/local/bin/perl
# -*- mode: cperl; tab-width: 8; indent-tabs-mode: nil; basic-offset: 2 -*-
# vim:ts=8:sw=2:et:sta:sts=2
#########
# Author: rpettett
# Last Modified: $Date$
# Id: $Id$
# $HeadURL$
#
use strict;
use warnings;
use Readonly;
use Carp;
use English qw(-no_match_vars);
our $VERSION = q[1.0];

Readonly::Scalar our $SLEEP_LONG  => 600;
Readonly::Scalar our $SLEEP_SHORT => 30;

$OUTPUT_AUTOFLUSH++;

my @original_argv = @ARGV;

#########

# handle SIGHUP restarts
#
local $SIG{HUP} = sub {
  carp q[caught SIGHUP];
  exec $PROGRAM_NAME, @original_argv;
};

my $last_modtime;

while(1) {
  #########
  # handle software-deployment restarts
  #
  my $modtime = -M $PROGRAM_NAME;

  if($last_modtime && $last_modtime ne $modtime) {
    carp q[re-execing];
    exec $PROGRAM_NAME, @original_argv;
  }
  $last_modtime = $modtime;

  my $did_work_flag;
  eval {
    $did_work_flag = do_stuff();
    1;
  } or do {
    $did_work_flag = 0;
  };

  local $SIG{ALRM} = sub {
    carp q[rudely awoken by SIGALRM];
  };

  my $sleep = $did_work_flag ? $SLEEP_SHORT : $SLEEP_LONG;
  carp qq[sleeping for $sleep];
  sleep $sleep;
}