class Gem::AtomicFileWriter

# File lib/rubygems/util/atomic_file_writer.rb, line 14
def self.open(file_name)
  require "securerandom" unless defined?(SecureRandom)

  old_stat = begin
               File.stat(file_name)
             rescue SystemCallError
               nil
             end

  # Names can't be longer than 255B
  tmp_suffix = ".tmp.#{SecureRandom.hex}"
  dirname = File.dirname(file_name)
  basename = File.basename(file_name)
  base_slice = basename.byteslice(0, 254 - tmp_suffix.bytesize)
  tmp_path = File.join(dirname, ".#{base_slice}#{tmp_suffix}")

  # The temporary name is longer than the final one, so on Windows a
  # writable destination can still map to a path beyond the 260-character
  # MAX_PATH limit. Trim the random suffix first, keeping at least 8 hex
  # characters to avoid collisions, then shorten the basename if that is not
  # enough. Recomputing the basename from the trimmed suffix would just let
  # it grow back, so trim the already-sliced basename instead.
  if tmp_path.length >= 260 && Gem.win_platform?
    overflow = tmp_path.length - 259
    trim = [tmp_suffix.bytesize - (".tmp.".bytesize + 8), overflow].min
    tmp_suffix = tmp_suffix.byteslice(0, tmp_suffix.bytesize - trim)
    overflow -= trim
    base_slice = base_slice.byteslice(0, [base_slice.bytesize - overflow, 0].max) if overflow > 0
    tmp_path = File.join(dirname, ".#{base_slice}#{tmp_suffix}")
  end

  flags = File::RDWR | File::CREAT | File::EXCL | File::BINARY
  flags |= File::SHARE_DELETE if defined?(File::SHARE_DELETE)

  File.open(tmp_path, flags) do |temp_file|
    temp_file.binmode
    if old_stat
      # Set correct permissions on new file
      begin
        File.chown(old_stat.uid, old_stat.gid, temp_file.path)
        # This operation will affect filesystem ACL's
        File.chmod(old_stat.mode, temp_file.path)
      rescue Errno::EPERM, Errno::EACCES
        # Changing file ownership failed, moving on.
      end
    end

    return_val = yield temp_file
  rescue StandardError => error
    begin
      temp_file.close
    rescue StandardError
      nil
    end

    begin
      File.unlink(temp_file.path)
    rescue StandardError
      nil
    end

    raise error
  else
    begin
      File.rename(temp_file.path, file_name)
    rescue StandardError
      begin
        File.unlink(temp_file.path)
      rescue StandardError
      end

      raise
    end

    return_val
  end
end