SessionManager.prototype.logIn = function(req, user, options, cb) {
  if (typeof options == 'function') {
    cb = options;
    options = {};
  }
  options = options || {};

  if (!req.session) { return cb(new Error('Login sessions require session support. Did you forget to use `express-session` middleware?')); }

  var self = this;
  var prevSession = req.session;

  // regenerate the session, which is good practice to help
  // guard against forms of session fixation
  req.session.regenerate(function(err) {
    if (err) {
      return cb(err);
    }

    self._serializeUser(user, req, function(err, obj) {
      if (err) {
        return cb(err);
      }
      if (options.keepSessionInfo) {
        merge(req.session, prevSession);
      }
      if (!req.session[self._key]) {
        req.session[self._key] = {};
      }
      // store user information in session, typically a user id
      req.session[self._key].user = obj;
      // save the session before redirection to ensure page
      // load does not happen before session is saved
      req.session.save(function(err) {
        if (err) {
          return cb(err);
        }
        cb();
      });
    });
  });
}

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