open() {
    // If we do not have a filename then we were passed a stream and
    // don't need to keep track of size.
    if (!this.filename) return;
    if (this._opening) return;

    this._opening = true;

    // Stat the target file to get the size and create the stream.
    this.stat((err, size) => {
      if (err) {
        return this.emit('error', err);
      }
      debug('stat done: %s { size: %s }', this.filename, size);
      this._size = size;
      this._dest = this._createStream(this._stream);
      this._opening = false;
      this.once('open', () => {
        if (!this._stream.emit('rotate')) {
          this._rotate = false;
        }
      });
    });
  }
Checks to see the filesize of.
@returns {undefined}