a.rb

#! /usr/local/rbenv/versions/1.8.7-p352-redmine/bin/ruby

### file: ./script/passenger

##
# get busy!
#
  Background.logger.info("process #{ Process.pid } starting...")

  logger = Background.logger

  loop do
    cmd = "passenger start -p 4200"

    Background.logger.info("cmd: #{ cmd }")

    system(cmd)

    seconds = Rails.env == 'production' ? 42 : 2
    sleep(seconds) # NOTE - signals will wake this up!
  end


BEGIN {
  ENV['PATH'] = "/usr/local/rbenv/versions/1.8.7-p352-redmine/bin:#{ ENV['PATH'] }"

  script = __FILE__
  require(File.dirname(script) + '/../lib/background.rb')
  Background.run!(script)
}

b.rb

##
# background.rb is magic.  it let's you have background rails code running
# with
#
#    . your app's env loaded
#    . that restarts like magic on a re-deploy
#
# usage in, say, ./script/run_jobs_in_the_background.rb, looks like:
#
#    #! /usr/bin/env ruby
#  
#    Background.logger.info("process #{ Process.pid } starting...")
#  
#    logger = Background.logger
#  
#    loop do
#      begin
#        Job.each_that_needs_to_be_run do |job|
#          job.run!
#        end
#      rescue => e
#        Background.logger.error(e)
#        seconds = Rails.env == 'production' ? 42 : 2
#        sleep(seconds) # NOTE - signals will wake this up!
#      end
#    end
#  
#    BEGIN {
#      script = __FILE__
#      require(File.dirname(script) + '/../lib/background.rb')
#      Background.run!(script)
#    }
#
# it includes some basic daemon-y stuff including a lock that prevents the
# script from running twice so cron'ing it every one minute will NOT stack
# processes
#

module Background
## libs
#
  require 'fileutils'
  require 'ostruct'
  require 'rbconfig'

  SIGHUP = Signal.list['HUP']

  %w(

    script
    mode
    dirname
    basename
    script_dir
    rails_root
    public_system
    private_system
    basename_dir
    lock_file
    log_file
    pid_file
    restart_txt
    started_at
    signals

  ).each{|a| attr(a)}

  def run!(*args)
    setup(*args).run()
  end

## setup
#
  def setup(script)
    @script = File.expand_path(script)
    @exec = command_line
    @dirname = File.expand_path(File.dirname(@script))
    @basename = File.basename(@script)
    @script_dir = File.expand_path(File.dirname(@script))
    @rails_root = File.dirname(@script_dir)
    @public_system = File.expand_path(File.join(@rails_root, 'public', 'system'))
    @private_system = File.expand_path(File.join(@rails_root, 'private', 'system'))

    if test(?d, @private_system)
      @background_dir = File.join(@private_system, 'background')
    else
      @background_dir = File.join(@public_system, 'background')
    end

    @basename_dir = File.join(@background_dir, @basename)

    @lock_file = File.join(@basename_dir, 'lock')
    @log_file = File.join(@basename_dir, 'log')
    @pid_file = File.join(@basename_dir, 'pid')

    @mode = (ARGV.shift || 'RUN').upcase

    FileUtils.mkdir_p(@basename_dir) rescue nil
    FileUtils.touch(@lock_file) rescue nil
    FileUtils.touch(@log_file) rescue nil

    @signals = []
    @restart_txt = File.join(@rails_root, 'tmp', 'restart.txt')
    @started_at = Time.now
    @monitor = new_monitor

    self
  end

  def command_line
    [which_ruby, @script, *ARGV.map{|arg| arg.dup}]
  end

  def new_monitor
    Thread.new(self) do |bg|
      Thread.current.abort_on_exception = true
      loop do
        #if bg.updated?
          #bg.logger.info('RESTART - update') if bg.logger
          #bg.restart!
        #end
        if bg.redeployed?
          bg.logger.info('RESTART - deploy') if bg.logger
          bg.restart!
        end
        if bg.signaled?
          bg.logger.info('RESTART - signal') if bg.logger
          bg.restart!
        end
        sleep(42)
      end
    end
  end

  def restart!
    if fork
      unlock!
      Kernel.exec(*@exec)
    else
      exit(42)
    end
  end

  def updated?
    a = File.stat(__FILE__).mtime rescue @started_at
    b = File.stat(@script).mtime rescue @started_at
    a > @started_at or b > @started_at
  end

  def redeployed?
    t = File.stat(@restart_txt).mtime rescue @started_at
    t > @started_at
  end

  def signaled?
    !signals.empty?
  end

  def which_ruby
    c = ::Config::CONFIG
    ruby = File::join(c['bindir'], c['ruby_install_name']) << c['EXEEXT']
    raise "ruby @ #{ ruby } not executable!?" unless test(?e, ruby)
    ruby
  end

# run based on mode
#
  def run
    case @mode


      when /RUN/i
      # aquire the lock and run, otherwise exit with 42 (already running)
      #
        lock!(:complain => true)

      # record the pid
      #
        pid!

      # setup signal handling
      #
        trap!

      # boot the rails app
      #
        boot!

      # setup logging
      #
        $logger = Logging.logger(STDERR)

      when /RESTART/i
      # try hard to find the running process by pid and signal it to restart
      #
        pid = Integer(IO.read(@pid_file)) rescue nil
        Process.kill('HUP', pid) rescue nil if pid
        exit(0)

      when /START/i
      # aquire the lock and run, otherwise exit with 42 (already running)
      #
        lock!(:complain => true)

      # boot the rails app
      #
        boot!

      # daemonize
      #
        daemonize!

      # dump pid to file
      #
        pid!

      # setup signal handling
      #
        trap!

      # setup logging
      #
        number_rolled = 7
        megabytes     = 2 ** 20
        max_size      = 42 * megabytes
        options       = { :safe => true } # for multi-process safe rolling, needs lockfile gem!
        logger = ::Logging.logger(@log_file, number_rolled, max_size, options)
        $logger = logger # HACK

      # redirect io to log_file
      #
        open(@log_file, 'r+') do |fd|
          fd.sync = true
          STDOUT.reopen(fd)
          STDERR.reopen(fd)
        end
        STDOUT.sync = true
        STDERR.sync = true


      when /PID/i
      # report the pid of running instance iff possible
      #
        pid = Integer(IO.read(@pid_file)) rescue nil
        if pid
          puts(pid)
          exit(0)
        else
          exit(1)
        end
        exit(1)

      when /FUSER/i
      # try to use fuser to display processes holding the lock
      #
        exec("fuser #{ @lock_file.inspect }")


      when /STOP/i
      # try hard to find the running process by pid and kill it
      #
        pid = Integer(IO.read(@pid_file)) rescue nil
        if pid
          10.times do
            Process.kill('TERM', pid) rescue nil
            begin
              Process.kill(0, pid)
              sleep(rand)
            rescue Errno::ESRCH
              puts(pid)
              exit(0)
            end
          end
          Process.kill(-9, pid) rescue nil
          sleep(rand)
          begin
            Process.kill(0, pid)
          rescue Errno::ESRCH
            puts(pid)
            exit(0)
          end
        end
        exit(1)

      when /SIGNAL/i
      # signal a running process, if any 
      #
        pid = Integer(IO.read(@pid_file)) rescue nil
        if pid
          signal = ARGV.shift || 'HUP'
          Process.kill(signal, pid)
          puts(pid)
          exit(0)
        end
        exit(42)

      when /LOG/i
      # display the log file
      #
        puts(@log_file)
        exit(42)

      when /TAIL/i
      # tail the log file
      #
        system("tail -F #{ @log_file.inspect }")
        exit(42)

    end
  end

# we'll be needing these
#

  def boot!
    Dir.chdir(@rails_root)
    require File.join(@rails_root, 'config', 'boot')
    require File.join(@rails_root, 'config', 'environment')
  end

  def lock!(options = {})
    complain = options['complain'] || options[:complain]
    fd = open(@lock_file, 'r+')
    status = fd.flock(File::LOCK_EX|File::LOCK_NB)

    unless status == 0
      if complain
        pid = Integer(IO.read(@pid_file)) rescue '?'
        warn("instance(#{ pid }) is already running!")
      end
      exit(42)
    end
    @lock = fd # prevent garbage collection from closing the file!
  end

  def unlock!
    @lock.flock(File::LOCK_UN|File::LOCK_NB) if @lock
  end

  def pid!
    open(@pid_file, 'w+') do |fd|
      fd.puts(Process.pid)
    end
    at_exit{ FileUtils.rm_f(@pid_file) }
  end

  def trap!
    %w( SIGHUP SIGALRM SIGUSR1 SIGUSR2 ).each do |signal|
      trap(signal){|s| Background.signals.push(s) }
    end
    trap('SIGTERM'){ exit(0) }
    trap('SIGINT'){ exit(0) }
  end

  def logger
    $logger || (
      require 'logger' unless defined?(Logger)
      Logger.new(STDERR)
    )
  end

  def logging_errors(&block)
    begin
      block.call()
    rescue SignalException => e
      logger.info(e)
      exit(0)
    rescue => e
      logger.error(e)
    end
  end

  def daemonize!(options = {})
  # setup
  #
    chdir = options[:chdir] || options['chdir'] || '.'
    umask = options[:umask] || options['umask'] || 0

  # setup pipes we'll use for IPC
  #
    ra, wa = IO.pipe
    rb, wb = IO.pipe

    pid = fork
    parent = !!pid
    child = !parent

  # in the parent we wait for the child to send back the grandchild's pid
  #
    if parent
      at_exit{ exit! }
      wa.close
      r = ra
      rb.close
      w = wb
      pid = r.gets
      w.puts(pid)
      exit(0)
    end

  # in the child start a grandchild.  handshake the pid with the parent.
  #
    if child
      ra.close
      w = wa
      wb.close
      r = rb

      open("/dev/null", "r+") do |fd|
        #STDIN.reopen(fd)
        #STDOUT.reopen(fd)
        #STDERR.reopen(fd)
      end

      Process::setsid rescue nil

      pid = fork

      if pid
        w.puts(pid)
        r.gets
        exit!
      else
        Dir::chdir(chdir)
        File::umask(umask)
        $DAEMON = true
        at_exit{ exit! }
      end
    end
  end

  extend(self)
end

1. this works on any machine, heroku, aws, home-spun, localhost....
2. upstart is stupid with restarts, this hooks into the 'normal' rails' restart mechanism that passenger, et al, use - ./tmp/restart.txt
3. this requires zero config: deploy, forget about it.
4. i like having all background logs/pids, etc centralized in my app dir, not scatter around on some unix fs layout