Hijack stdout and stderr for forks

hijack.rb

require 'logger'
require 'stringio'

def capture_output
  old_stderr, $stderr = $stderr, StringIO.new
  old_stdout, $stdout = $stdout, StringIO.new
  yield
  [$stderr.string, $stdout.string]
ensure
  $stderr = old_stderr
  $stdout = old_stdout
end

class Request
  def make_logger
    lg = Logger.new(STDERR)
    lg.level = Logger::DEBUG
    lg
  end

  def initialize(ident)
    @ident = ident
    @logger = make_logger
    original_formatter = Logger::Formatter.new
    @logger.formatter = proc do |severity, datetime, progname, msg|
      original_formatter.call(severity, datetime, @ident, msg.dump)
    end
  end

  def func_in_fork
    3.times do |i|
      puts "Regular puts message #{i}"
      $stdout.puts "#{i} on stdout"
      $stderr.puts "#{i} on stderr"
    end
  end

  def process
    stderr, stdout= capture_output do
      func_in_fork
    end
    stderr.each_line do |line|
      @logger.debug line
    end
    stdout.each_line do |line|
      @logger.debug line
    end
  end
end

pids = []

3.times do |i|
  r = Request.new(i)

  pids << Process.fork do
    sleep 0.5
    r.process
  end
end

pids.each do |pid|
  Thread.new do
    Process.waitpid(pid)
  end.join
end

puts "Regular done on stdout!"