Class: File

Inherits:
Object show all
Defined in:
activesupport/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 'activesupport/lib/active_support/core_ext/file/atomic.rb', line 21

def self.atomic_write(file_name, temp_dir = dirname(file_name))
  old_stat = begin
    File.stat(file_name)
  rescue SystemCallError
    nil
  end

  # Names can't be longer than 255B
  tmp_suffix = ".tmp.#{SecureRandom.hex}"
  tmp_name = ".#{basename(file_name).byteslice(0, 254 - tmp_suffix.bytesize)}#{tmp_suffix}"
  tmp_path = File.join(temp_dir, tmp_name)
  open(tmp_path, RDWR | CREAT | EXCL | SHARE_DELETE | BINARY) do |temp_file|
    temp_file.binmode

    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

    return_val = yield temp_file
  rescue => error
    temp_file.close rescue nil
    unlink(temp_file.path) rescue nil
    raise error
  else
    rename(temp_file.path, file_name)
    return_val
  end
end