modify_value

# File activesupport/lib/active_support/cache/file_store.rb, line 187
        def modify_value(name, amount, options)
          file_name = normalize_key(name, options)

          lock_file(file_name) do
            options = merged_options(options)

            if num = read(name, options)
              num = num.to_i + amount
              write(name, num, options)
              num
            end
          end
        end