command_runner.rb

require 'open3'

class Command
  def run(command, stdout_callback, stderr_callback)
    Open3.popen3(command) do |stdin_write_io, stdout_read_io, stderr_read_io, process_wait_thread|
      # We don't care about STDIN
      stdin_write_io.close

      stdout_reader = read_without_blocking_from(stdout_read_io, stdout_callback)
      stderr_reader = read_without_blocking_from(stderr_read_io, stderr_callback)

      exit_status = process_wait_thread.value

      stdout_reader.join
      stderr_reader.join

      exit_status # A Process::Status instance
    end
  end

  private

  def read_without_blocking_from(io, data_available_callback)
    Thread.new do
      begin
        data_available_callback.(io.read_nonblock(4096)) until io.closed?
      rescue IO::WaitReadable
        IO.select([io])
        retry
      rescue EOFError
        # Ignore end of file reached (EOFError)
      end
    end
  end
end


Command.new.run('echo 1; sleep 1; echo 2; foobar; sleep 1; echo 3', -> x { puts "STDOUT: #{x}" }, -> x { puts "STDERR: #{x}" })