code: async function(filename, length) {
        if (await this.safeStat(filename) && length > 1) {
          var lastFile = null;
          for (var i = length; i > 0; i--) {
            var tmp = filename + "." + i;
            if (i == length) {
              await this.safeUnlink(tmp);
            } else if (await this.safeStat(tmp)) {
              await rename(tmp, lastFile);
            }
            lastFile = tmp;
          }
          await rename(filename, lastFile);
        }
      }
docs: Rotates files so that this file does not exist, by renaming the existing file to have a ".1"
  appended, and the ".1" to be renamed to ".2" etc, up to `length` versions
  
  @param filename {String} filename to rotate
  @param length {Integer} maximum number of files
  @async