Module: ActiveSupport::CoreExtensions::File::Atomic

Included in:
File
Defined in:
lib/active_support/core_ext/file/atomic.rb

Instance Method Summary collapse

Instance Method Details

#atomic_write(file_name, temp_dir = Dir.tmpdir) {|temp_file| ... } ⇒ Object

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

Yields:

  • (temp_file)

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# File 'lib/active_support/core_ext/file/atomic.rb', line 20

def atomic_write(file_name, temp_dir = Dir.tmpdir)
  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 = ".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