Symlinking a module breaks HookCollectorPass

This test is not actually working at the moment so I'm pulling it out for a cleaner commit:

<?php

declare(strict_types=1);

namespace Drupal\KernelTests\Core\Hook;

use Drupal\Core\Hook\HookCollectorPass;
use Drupal\KernelTests\KernelTestBase;
use Symfony\Component\DependencyInjection\ContainerBuilder;

/**
 * @coversDefaultClass \Drupal\Core\Hook\HookCollectorPass
 * @group Hook
 */
class HookCollectorPassTest extends KernelTestBase {

  protected function setUpFilesystem(): void {
    // VFS does not and can not support symlinks.
  }

  public function testSymlink(): void {
    mkdir($this->siteDirectory);
    symlink(realpath("core/modules/user/config"), "$this->siteDirectory/config");
    $container = new ContainerBuilder();
    $module_filenames = [
      'test' => ['pathname' => "$this->siteDirectory/test.info.yml"],
    ];
    $container->setParameter('container.modules', $module_filenames);
    (new HookCollectorPass())->process($container);
    unlink("$this->siteDirectory/config");
    rmdir($this->siteDirectory);
  }

}

Yes we removed the test because the symlink part is dependent on the env, so in order to keep this fix clean for now it's just the fix.

If you can review the test attached in 8 and provide advice we can readd it.

This issue is specifically to how gitlab runs tests, it's treating the symlinks as directories.

I updated the IS but I used this criteria:

Cause tests to fail in HEAD on the automated testing platform for any supported environment (including random failures), since this blocks all other work.

I think I might have a test.

Ok there is a test.

I have a positive test coming soon.

Ok there is now a test confirming a notice is not thrown when a symlink is encountered and a positive confirmation that hooks inside the symlinks are picked up.

I removed the negative test per @alexpott and @elc's comments. There is a positive assertion now too to confirm it read the hooks.

My only concern is the symlink failure was silent so ensuring that notice isn't appearing is probably worth it, but we have the history if we want to add it back.

For credit purposes other than the people chiming in on this issue.

@chi and @claudiu.cristea both alerted me to the issue here: #3442009: OOP hooks using attributes and event dispatcher and helped investigate in slack.

@fjgarlin pointed out how to patch core in a contrib test so we could verify the fix there and @hestenet helped loop in @fjgarlin.

@elc also provided a review, and I used a module he maintains as the test ground.

@ghostofdrupalpast identified the root cause and fix.

We probably want to get a cleanup phase in the test as well since this is not in vfs:

foreach (scandir($this->siteDirectory) as $item) {
      $target = "$this->siteDirectory/$item";
      if (is_link($target)) {
        unlink($target);
      }
    }

I removed it locally when I was confirming symlink creation, but didn't add it back.