code: function _bufferStreamCallbackWrapper(err) {
      const buffer = options.objectMode
        ? _this._bufferStreamBuffer
        : Buffer.concat(_this._bufferStreamBuffer);
  
      err = err || null;
      _this._cb(err, buffer, (err2, buf) => {
        setImmediate(() => {
          _this.removeListener('error', _bufferStreamError);
          if (err2) {
            _this.emit('error', err2);
          }
          _this._bufferStreamBuffer = options.objectMode ? buf || [] : [buf];
          _this._bufferStreamFinished = true;
          _this._read();
        });
      });
    }
docs: Internal logic