Capture stderr data on EventMachine.popen

external-command.rb

require 'rubygems'
require 'eventmachine'

$stdout.sync = true
$stderr.sync = true

EM.run {
  
  EM.add_periodic_timer(0.1) {
    $stdout.write "stdout\n"
  }

  EM.add_periodic_timer(0.1) {
    $stderr.write "stderr\n"
  }
  
  EM.add_timer(1) {
    $stdout.write "stopping from stdout...\n"
    $stderr.write "stopping from stderr...\n"
    EM.stop
    
  }
  
  
}

popen3.rb

require 'rubygems'
require 'eventmachine'

if RUBY_VERSION < "1.8.7"
  def __method__
    caller = /`(.*)'/
    Kernel.caller.first.split(caller).last.to_sym
  end
end

def log *message
  p [Time.now, *message]
end

module EventMachine
  def self.popen3(*args)
    new_stderr = $stderr.dup
    rd, wr = IO::pipe
    $stderr.reopen wr
    connection = EM.popen(*args)
    $stderr.reopen new_stderr
    EM.attach rd, Popen3StderrHandler, connection
    connection
  end
  
  class Popen3StderrHandler < EventMachine::Connection
    def initialize(connection)
      @connection = connection
    end
    
    def receive_data(data)
      @connection.receive_stderr(data)
    end
  end  
end

class MyProcess < EventMachine::Connection  
  def initialize *args
    log __method__, args
  end

  def receive_data data 
    log __method__, data
  end

  def receive_stderr data 
    log __method__, data
  end
  
  def unbind
    log __method__
  end
end

if __FILE__ == $0
  cmd = "ruby #{File.dirname(__FILE__)}/external-command.rb"
  
  EM.run do
    EM.next_tick do
      $stderr.puts "stderr before"
      EM.popen3 cmd, MyProcess
      $stderr.puts "stderr after"
    end
   
    EM.add_timer(2) do
      log :exit
      EM.stop
    end
  end
end

Thanks guys! I took the core idea here, ran with it and published http://rubygems.org/gems/em_pessimistic, which fixes this annoyance with EM.popen, and adds proper error handling to DeferrableChildProcess