Redirection from user login to another page for logged in users

Question

Due to requirements with our reverse proxies, I had to show the user login form on a different URL. Well, no problem, I thought, I'll just copy the relevant section from user.routing.yml and display it on a different path:

mymodule.user.login:
  path: '/ls/login'
  defaults:
    _form: '\Drupal\user\Form\UserLoginForm'
    _title: 'Log in'
  requirements:
    _user_is_logged_in: 'FALSE'
  options:
    _maintenance_access: TRUE

This works, but if an user accesses the login URL after having logged in (typically because they bookmarked the login page), they get an "Access denied" page. I can understand it – after all, there is the requirement that the user is NOT logged is. I was thinking of ways to circumvent this, but then I noticed that the original URL (still accessible if I access the server directly rather than through the proxies) does not have this problem: if I go to /usr/login it after logging in, I get redirected to the user page, /user/{{id}}.

I don't understand how the latter works, I looked at UserLoginForm.php in addition to user.login.form, but I found no clue. What is involved here?

Answer 1

I don't understand how the latter works, I looked at UserLoginForm.php in addition to user.login.form, but I found no clue. What is involved here?

In core there is an exception event subscriber catching the 403 and redirecting to the current user page:

web/core/modules/user/src/EventSubscriber/AccessDeniedSubscriber.php

...

  public function onException(ExceptionEvent $event) {
    $exception = $event->getThrowable();
    if ($exception instanceof AccessDeniedHttpException) {
      $route_name = RouteMatch::createFromRequest($event->getRequest())->getRouteName();
      $redirect_url = NULL;
      if ($this->account->isAuthenticated()) {
        switch ($route_name) {
          case 'user.login';
            // Redirect an authenticated user to the profile page.
            $redirect_url = Url::fromRoute('entity.user.canonical', ['user' => $this->account->id()], ['absolute' => TRUE]);
            break;

...

You can copy the class to your custom module to do the same thing for a custom route, together with the service definition (to mymodule.services.yml):

services:
  user_access_denied_subscriber:
    class: Drupal\user\EventSubscriber\AccessDeniedSubscriber
    arguments: ['@current_user']
    tags:
      - { name: event_subscriber }

and adjust both with custom names and namespace.

---

Different approach

A different approach might be to alter the existing route instead of creating a new one, so that dependencies on the route name, like this redirect code, still work. See Change '/user(/*)' URLs to 'profile(/*)'