Ajax form inside custom system.404 route controller doesn't work

Question

I need to display custom logic in 404 pages so I have overridden the route controller of the system.404 route like this:

class RouteSubscriber extends RouteSubscriberBase {

  protected function alterRoutes(RouteCollection $collection) {
    if ($route = $collection->get('system.404')) {
      $route->setDefault('_controller', '\Drupal\mymodule\Controller\myController::on404');
    }
  }
}

So let's say I am currently at the path /non-existing-path and the on404 callback renders my custom content just fine. Problem is I am also rendering a form that contains an Ajax element (a button), which doesn't work because apparently the Ajax request is made towards the following path:

/system/404?q=non-existing-path&destination=non-existing-path&_exception_statuscode=404&ajax_form=1&_wrapper_format=drupal_ajax

which, not unexpectedly, returns a Status Code: 404 Not Found

What am I missing?

--

So far I've tried intercepting the NotFoundHttpException with my own listener and changing it to a HttpException(200) in hopes I could turn a 404 page into a 200 but I am at a loss after that and it seems like an overkill.

Answer 1

Ajax form inside custom system.404 route controller does work

For example a node form returned from the controller:

<?php

namespace Drupal\mymodule\Controller;    

use Drupal\node\Entity\Node;

class MyController extends ControllerBase {

  public function on404() {
    $node = Node::create(['type' => 'article']);
    return $this->entityFormBuilder()->getForm($node);
  }

}

The node form is fully functional including complex Ajax operations like opening the media browser.

Status code 404 or 200

Problem is I am also rendering a form that contains an Ajax element (a button), which doesn't work because apparently the Ajax request is made towards the following path:

/system/404?q=non-existing-path&destination=non-existing-path&_exception_statuscode=404&ajax_form=1&_wrapper_format=drupal_ajax

which, not unexpectedly, returns a Status Code: 404 Not Found

This is not true, a request for /system/404 returns a 200 status code when called outside of an exception handler, for example in an ajax request. The system.404 route is a normal route which is executed by the exception handler in a sub-request. The 404 status is added later and not part of this route.