class CaptureOutput

  # Use in test examples to run a command *once*, and then
  # have the stdout and/or stderr available for testing
  #
  # out = CaptureOutput.run do
  #         run_some_command
  #       end
  #
  # expect(out.stdout).to match(/some output/)
  # expect(out.stderr).to match(/some texts/)
  #
  # All of the output is recorded into files: "log/spec/output_NNNN_NAME.txt",
  # where NAME is either 'out' or 'err', and 'NNNNN' is the current process id.

  def self.run
    new.run { yield }
  end

  def initialize
    @orig_stdout, @orig_stderr = $stdout, $stderr
    reset
  end

  def reset
    @stdout, @stderr = StringIO.new, StringIO.new
  end

  def run
    $orig_stdout, $orig_stderr = $stdout, $stderr
    $stdout, $stderr = @stdout, @stderr
    begin
      yield
    ensure
      $stdout, $stderr = @orig_stdout, @orig_stderr
      record_all_output
    end
    self
  end

  def stdout
    @stdout.string
  end

  def stderr
    @stderr.string
  end

  private

  def record_all_output
    record_output('out', stdout)
    record_output('err', stderr)
  end

  def record_output(name, text)
    File.open(io_file_name(name), 'a+') {|file| file.write text}
  end

  def io_file_name(name)
    File.join(spec_log_dir, "output_#{$PID}_#{name}.txt")
  end

  def spec_log_dir
    File.join(Rails.root, 'log/spec').tap do |dir|
      Dir.exist?(dir) || FileUtils.mkdir_p(dir)
    end
  end
end