~michaelforrest/use-case-mapper/trunk

module ActiveSupport #:nodoc:

  module CoreExtensions #:nodoc:

    module File #:nodoc:

      module Atomic

        # 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 |f|

        #     file.write("hello")

        #   end

        def atomic_write(file_name, temp_dir = Dir.tmpdir)

          require 'tempfile' unless defined?(Tempfile)

 

          temp_file = Tempfile.new(basename(file_name), temp_dir)

          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

            check_name = join(dirname(file_name), ".permissions_check.#{Thread.current.object_id}.#{Process.pid}.#{rand(1000000)}")

            open(check_name, "w") { }

            old_stat = stat(check_name)

            unlink(check_name)

          end

 

          # Overwrite original file with temp file

          rename(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

    end

  end

end