class File
  # Write to a file atomically. Useful for situations where you don't
  # want other processes or threads to see half-written files.
  #
  #   File.atomic_write('important.file') do |file|
  #     file.write('hello')
  #   end
  #
  # If your temp directory is not on the same filesystem as the file you're
  # trying to write, you can provide a different temporary directory.
  #
  #   File.atomic_write('/data/something.important', '/data/tmp') do |file|
  #     file.write('hello')
  #   end
  def self.atomic_write(file_name, temp_dir = Dir.tmpdir)
    require 'tempfile' unless defined?(Tempfile)
    require 'fileutils' unless defined?(FileUtils)

    temp_file = Tempfile.new(basename(file_name), temp_dir)
    temp_file.binmode
    yield temp_file
    temp_file.close

    begin
      # Get original file permissions
      old_stat = stat(file_name)
    rescue Errno::ENOENT
      # No old permissions, write a temp file to determine the defaults
      temp_file_name = [
        '.permissions_check',
        Thread.current.object_id,
        Process.pid,
        rand(1000000)
      ].join('.')
      check_name = join(dirname(file_name), temp_file_name)
      open(check_name, 'w') { }
      old_stat = stat(check_name)
      unlink(check_name)
    end

    # Overwrite original file with temp file
    FileUtils.mv(temp_file.path, file_name)

    # Set correct permissions on new file
    chown(old_stat.uid, old_stat.gid, file_name)
    chmod(old_stat.mode, file_name)
  end
end