#!/usr/bin/env ruby

# workaround-upstart-snafu
#
# When lied to about the behavior of a job's main process wrt. forking with the
# "expect" stanza, Upstart can get into a confused state where it's tracking a
# nonexistent pid.
#
# This hack creates new short-lived processes until one gets the pid in
# question, then has its parent die so that it's going to get reaped by pid 1
# (Upstart), which will get Upstart out of the confused state.
#
# When "status <jobname>" says "<jobname> stop/killed, process 12345" and
# there's no such process, run "workaround-upstart-snafu 12345" and wait until
# it exits.

# Copyright 2012 Johan Kiviniemi <devel@johan.kiviniemi.name>
#
# Permission to use, copy, modify, and/or distribute this software for any
# purpose with or without fee is hereby granted, provided that the above
# copyright notice and this permission notice appear in all copies.
#
# THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
# WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
# MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
# ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
# WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
# ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
# OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

class Workaround
  def initialize(target_pid)
    @target_pid = target_pid

    quit_if_process_exists
    first_child
  end

  def quit_if_process_exists
    warning = "There is already a process running with the PID #{@target_pid}, try killing it?"
    Process.kill(0, @target_pid)
    $stderr.puts(warning)
    exit 1
  rescue Errno::EPERM
    $stderr.puts(warning)
    exit 1
  rescue Errno::ESRCH
    puts 'No process with that PID currently running - attempting to obtain the PID.'
  end

  def first_child
    pid = fork do
      return true if Process.pid == @target_pid
      Process.setsid

      rio, wio = IO.pipe

      # Keep rio open
      print "\e[A" until second_child rio, wio
    end

    Process.wait pid
  end

  def second_child(parent_rio, parent_wio)
    rio, wio = IO.pipe

    pid = fork do
      rio.close
      parent_wio.close

      puts format('%20.20s', Process.pid)

      if Process.pid == @target_pid
        wio << 'a'
        wio.close

        parent_rio.read
      end
    end
    wio.close

    begin
      if rio.read == 'a'
        true
      else
        Process.wait pid
        false
      end
    ensure
      rio.close
    end
  end
end

if $PROGRAM_NAME == __FILE__
  pid = ARGV.shift
  raise 'USAGE: {$PROGRAM_NAME} pid' if pid.nil?
  Workaround.new Integer pid
end