Class: File

Inherits:
Object show all
Defined in:
lib/active_support/core_ext/file/atomic.rb

Class Method Summary collapse

Class 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 |file|
  file.write('hello')
end

Yields:

  • (temp_file)

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# File 'lib/active_support/core_ext/file/atomic.rb', line 17

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

  if File.exist?(file_name)    # Get original file permissions

    old_stat = stat(file_name)
  else
    # If not possible, probe which are the default permissions in the
    # destination directory.
    old_stat = probe_stat_in(dirname(file_name))
  end

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

  # Set correct permissions on new file
  begin
    chown(old_stat.uid, old_stat.gid, file_name)    # This operation will affect filesystem ACL's

    chmod(old_stat.mode, file_name)
  rescue Errno::EPERM    # Changing file ownership failed, moving on.

  end
end

.probe_stat_in(dir) ⇒ Object

Private utility method.


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# File 'lib/active_support/core_ext/file/atomic.rb', line 49

def self.probe_stat_in(dir) #:nodoc:
  basename = [
    '.permissions_check',
    Thread.current.object_id,
    Process.pid,
    rand(1000000)
  ].join('.')

  file_name = join(dir, basename)
  FileUtils.touch(file_name)
  stat(file_name)
ensure
  FileUtils.rm_f(file_name) if file_name
end