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 = dirname(file_name)) ⇒ 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

This method needs to create a temporary file. By default it will create it in the same directory as the destination file. If you don’t like this behavior you can provide a different directory but it must be on the same physical filesystem as the file you’re trying to write.

File.atomic_write('/data/something.important', '/data/tmp') do |file|
  file.write('hello')
end



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# File 'lib/active_support/core_ext/file/atomic.rb', line 21

def self.atomic_write(file_name, temp_dir = dirname(file_name))
  require "tempfile" unless defined?(Tempfile)

  Tempfile.open(".#{basename(file_name)}", temp_dir) do |temp_file|
    temp_file.binmode
    return_val = yield temp_file
    temp_file.close

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

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

    # Overwrite original file with temp file
    rename(temp_file.path, file_name)
    return_val
  end
end

.probe_stat_in(dir) ⇒ Object

Private utility method.



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# File 'lib/active_support/core_ext/file/atomic.rb', line 56

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)
rescue Errno::ENOENT
  file_name = nil
ensure
  FileUtils.rm_f(file_name) if file_name
end