Force a 503 error when database server not available

Question

I'm trying to replicate a solution I had working on Drupal 7 into Drupal 10. I was able to change the http response code to 504 such that Cloudflare would return a cached site copy in the instance of the database server being upgraded. I've got this far - using an Exception event subscriber - but my code never gets called. I've added the Request debug method - and that fires - I see those logs. However, when I turn the DB off - I just get the core FinalExceptionSubscriber firing. Could it be that my module never gets loaded during the bootstrap process?

<?php

namespace Drupal\neway_platform\EventSubscriber;

use Drupal\Core\Database\DatabaseConnectionRefusedException;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\HttpKernel\Event\ExceptionEvent;
use Symfony\Component\HttpKernel\KernelEvents;


class NewayExceptionSubscriber implements EventSubscriberInterface {

  /**
   * Handles exceptions for this subscriber.
   *
   * @param \Symfony\Component\HttpKernel\Event\ExceptionEvent $event
   *   The event to process.
   */
  public function onException(ExceptionEvent $event) {
    //
    error_log('Error here: ');
    $exception = $event->getThrowable();
    //$this->logger->get('php')->error('Custom logging message' . $exception->getMessage());
    if ($exception instanceof DatabaseConnectionRefusedException) {
      //$this->logger->get('php')->error('Custom logging message');
      $response = new Response('Site is being upgraded - please check back soon', 504);
      $event->setResponse($response);
    }
  }

  public function onRequest() {

    error_log('Your message here2: ');

  }

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents(): array {
    // Listen on 4xx exceptions late, but before the final exception handler.
    $events[KernelEvents::EXCEPTION][] = ['onException', 1000];
    $events[KernelEvents::REQUEST][] = ['onRequest', 1000];
    return $events;
  }

mymodule.services.yml

services:
  cron_fire_forget:
    class: '\Drupal\neway_platform\Service\CronFireForget'
  neway_exception:
    class: Drupal\neway_platform\EventSubscriber\NewayExceptionSubscriber
    tags:
      - { name: 'event_subscriber' }

logs:

    [02-Oct-2024 14:08:01 Europe/Berlin] Your message here2:
    [02-Oct-2024 14:08:01 Europe/Berlin] Your message here2:
    [02-Oct-2024 14:08:01 Europe/Berlin] Your message here2:
    [02-Oct-2024 14:08:20 Europe/Berlin] Drupal\Core\Database\DatabaseConnectionRefusedException: SQLST (edited) 

Answer 1

This does not seem possible using an EventSubscriber. Alternative is to change the core errors.inc file (ideally with a patch file if we have to got this route) - as even the core catch all Event Subscriber is not called.