Capture input and output and get the exit status of arbitrary Ruby code, variant of Open4::popen4

pfork.rb

# coding: utf-8

module PFork
  extend self

  # Call function +fun+ in a child process, giving access to the child process
  # id and capturing STDIN, STDOUT, and STDERR of the function call.
  #
  # Adapted from stdlib's Open3::popen3 and
  # {Open4::popen4}[https://github.com/ahoward/open4] by Ara T. Howard, but
  # without using double forking and fork-exec'ing.
  def pfork(fun, &block)
    pw, pr, pe, ps = IO.pipe, IO.pipe, IO.pipe, IO.pipe

    verbose = $VERBOSE

    begin
      $VERBOSE = nil

      cid = fork do
        pw.last.close
        STDIN.reopen pw.first
        pw.first.close

        pr.first.close
        STDOUT.reopen pr.last
        pr.last.close

        pe.first.close
        STDERR.reopen pe.last
        pe.last.close

        ps.first.close

        STDOUT.sync = STDERR.sync = true

        begin
          fun.call
          exit_status = 0
        rescue SystemExit => e
          # Make it seem to the caller that calling Kernel#exit in +fun+ kills
          # the child process normally. Kernel#exit! bypasses this rescue
          # block.
          exit_status = e.status
        rescue Exception => e
          Marshal.dump e, ps.last
          ps.last.flush
          exit_status = 42          # won't be seen by the caller
        ensure
          ps.last.close unless ps.last.closed?
          exit! exit_status
        end
      end
    ensure
      $VERBOSE = verbose
    end

    [ pw.first, pr.last, pe.last, ps.last ].each { |fd| fd.close }

    pw.last.sync = true

    pi = [ pw.last, pr.first, pe.first ]

    begin
      block.call(cid, *pi) if block_given?
    ensure
      pi.each { |fd| fd.close unless fd.closed? }
    end

    begin
      e = Marshal.load ps.first
      raise Exception === e ? e : "unknown failure!"
    rescue EOFError
      # Calling +fun+ did not raise exception.
    ensure
      ps.first.close
      exit_status = Process.waitpid2(cid).last
    end

    exit_status
  end
end

if $0 == __FILE__
  require 'minitest/autorun'

  class PForkTest < MiniTest::Unit::TestCase
    include PFork

    def test_fun_successful_return
      fun = lambda { 'lucky me' }
      status = pfork fun
      assert_equal 0, status.exitstatus
    end

    def test_fun_raise_exception
      msg = 'oh noes'
      fun = lambda { raise msg }
      err = assert_raises(RuntimeError) { pfork fun }
      assert_equal msg, err.to_s
    end

    def test_fun_force_exit
      exit_code = 43
      fun = lambda { exit! exit_code }
      status = pfork fun
      assert_equal exit_code, status.exitstatus
    end

    def test_fun_normal_exit
      exit_code = 43
      fun = lambda { exit exit_code }
      status = pfork fun
      assert_equal exit_code, status.exitstatus
    end

    def test_io
      via_msg = 'foo'
      err_msg = 'bar'
      fun = lambda do
        $stdout.write $stdin.read
        $stderr.write err_msg
      end
      out_actual, err_actual = nil, nil
      status = pfork fun do |_, stdin, stdout, stderr|
        stdin.write via_msg
        stdin.close
        out_actual = stdout.read
        err_actual = stderr.read
      end
      assert_equal via_msg, out_actual
      assert_equal err_msg, err_actual
      assert_equal 0, status.exitstatus
    end
  end
end