Example forking with signal handler and message pipe

fork.rb

#!/usr/bin/env ruby
#
# Forking example using pipes to communicate from child processes

def log(msg)
  case msg
  when Array then msg.each {|m| log("  #{m}") }
  else
    puts "[#{Time.now}][#{$$}] #{msg}"
  end
end

def listen
  return unless $$ == @master_pid
  @pids   ||= {}
  @pipes  ||= {}
  log "listening to #{@pipes.size} remaining pipes.."
  @pipes.each do |pid, pipe|
    loop do
      begin
        tmp = ''
        pipe.read_nonblock(1, tmp)
        tmp += pipe.gets.strip
        case tmp
        when /hi/i then
          log "Child #{pid} says hi.."
        when /hungry/i then
          log "Need to feed child.. #{pid}"
        else
          log "Child #{pid} wants something: #{tmp}"
        end
      rescue IO::WaitReadable
        # ok, nothing in the pipe.. 
        break
      rescue EOFError, IOError
        # this sucker's dead?
        pipe.close unless pipe.closed?
        @pipes.delete(pid)
        break
      rescue Object => err
        log "failed to catch #{err.class}:#{err} while reading pipe on pid #{pid}"
        break
      end
    end
  end
end

def monitor_pids
  return unless $$ == @master_pid
  @pids   ||= {}
  @pipes  ||= {}
  log "monitoring #{@pids.size} remaining pids.."
  @pids.each do |job, pid|
    listen
    found, status = Process.waitpid2(pid, Process::WNOHANG)
    if found
      log "found pid #{pid} exited with #{status ? status.exitstatus : 'no status'}"
      @pipes[pid].close if @pipes[pid] && !@pipes[pid].closed?
      @pipes.delete(pid)
      @pids.delete(job)
    end
  end
end

def cry(msg)
  if @to_master
    if @to_master.closed?
      log "master isn't listening, pipe's closed? re: '#{msg}'"
    else
      begin
        @to_master.puts msg
      rescue Errno::EPIPE
        log "failed to talk to master, broken pipe? re: '#{msg}'"
      end
    end
  else
    log msg
  end
end

def run
  @master_pid = $$
  @pids   = {}
  @pipes  = {}
  %w[TERM QUIT INT].each do |sig|
    Signal.trap(sig) do 
      cry("caught #{sig} signal")
      shutdown 3
    end
  end
  %w[a b c].each do |job|
    from_child,to_master = IO.pipe
    if pid = fork
      @pids[job] = pid
      @pipes[pid] = from_child
      to_master.close
      log "launching pid #{pid} for #{job}"
    else
      begin
        @to_master = to_master
        from_child.close
        cry("Hi!!")
        3.times do
          sleep rand(5)
          cry("*YAWN* I'm hungry...")
        end
        shutdown
      rescue SystemExit
        # we're outta here.. 
      rescue Interrupt,SignalException
        cry("Abort requested, exiting (#{$!.class}:#{$!})")
        shutdown 2
      rescue Object => err
        cry("OUCH: #{err.class}:#{err}")
        shutdown 1
      end
    end
  end
  log "Entering monitor_pids loop"
  loop do
    monitor_pids
    break if @pids.empty?
    sleep 1
  end
  log "All done!"
end

def shutdown(code=0)
  if $$ == @master_pid
    @pids   ||= {}
    @pipes  ||= {}
    until @pids.empty? 
      monitor_pids
      sleep 1
    end
  else
    cry("Shutting down..")
    exit! code
  end
end

if __FILE__ == $0
  run
end