lock_file

# File activesupport/lib/active_support/cache/file_store.rb, line 104
        def lock_file(file_name, &block) # :nodoc:
          if File.exist?(file_name)
            File.open(file_name, 'r+') do |f|
              begin
                f.flock File::LOCK_EX
                yield
              ensure
                f.flock File::LOCK_UN
              end
            end
          else
            yield
          end
        end