fdkiller in Ruby

.gitignore

.*.swp

fdkiller.rb

# Execute a code block while silencing chosen file descriptors. This can be
# used to silence stdout or stderr even in subprocesses, which inherit the
# parent process's stdout and stderr file descriptors.
#
# Author:: Ori Peleg (http://orip.org)
# License:: MIT, http://www.opensource.org/licenses/MIT

require 'fcntl'

# The file descriptors will be changed to write to /dev/null for the duration
# of the code block.
#
#     fdkiller(STDOUT.fileno) do
#         system "echo foo bar" # this won't print anything
#     end
def fdkiller(*target_fds)
  # simulate close(2)
  def fd_close(fd)
    IO.open(fd).close
  end

  # simulate dup(2)
  def fd_dup(fd)
    return IO.open(fd).fcntl(Fcntl::F_DUPFD)
  end

  # simulate dup2(2)
  def fd_dup2(fd, fd2)
    fd_close(fd2)

    # dup fd into fd2
    IO.open(fd).fcntl(Fcntl::F_DUPFD, fd2)
  end

  # duplicate each of the target fds
  duplicated_fds = target_fds.collect { |fd| fd_dup(fd) }

  null_fd = IO.sysopen("/dev/null", "w")

  # change target fds to write to /dev/null, by dup'ing the null fd into them
  target_fds.each { |fd| fd_dup2(null_fd, fd) }

  begin
    yield
  ensure
    target_fds.zip(duplicated_fds).each do |(original_fd, duplicated_fd)|
      # restore the original fd
      fd_dup2(duplicated_fd, original_fd)

      # clean up the duplicated fd
      fd_close(duplicated_fd)
    end

    fd_close(null_fd)
  end
end

def selftest
  def test_prints
    STDOUT.puts "same process stdout"
    STDERR.puts "same process stderr"
    system "echo child process stdout"
    system "echo child process stderr 1>&2"
  end

  def test_with_fds(name, *fds)
    puts "-- blocking #{name}"
    fdkiller(*fds) { test_prints }
    puts "-- finished blocking #{name}"
  end

  test_with_fds("stdout", STDOUT.fileno)
  puts
  test_with_fds("stderr", STDERR.fileno)
  puts
  test_with_fds("stdout + stderr", STDOUT.fileno, STDERR.fileno)
end

selftest if __FILE__ == $0