Disabling the default search page can make the entire site inaccessible

If #3086846: Deprecate ability to disable search pages (no reason for it and causes problems and code complexity) had landed, this would not be necessary. But that will land in Drupal 10 at the earliest per #3086846-16: Deprecate ability to disable search pages (no reason for it and causes problems and code complexity).

Patch explained, and next steps identified:

1. +++ b/core/modules/search/src/Form/SearchBlockForm.php
   @@ -94,8 +95,19 @@ public function buildForm(array $form, FormStateInterface $form_state, $entity_i
   +    $search_page = SearchPage::load($entity_id);
   

   Technically this needs to be using the entity storage service for accessing the search page.

   It's a pity the SearchPageRepositoryInterface is only so partially solving the problems here.

2. +++ b/core/modules/search/src/Form/SearchBlockForm.php
   @@ -94,8 +95,19 @@ public function buildForm(array $form, FormStateInterface $form_state, $entity_i
   +    $this->renderer->addCacheableDependency($form, $search_page);
   

   Yes … this has been wrong all along. This rendered form should be invalidated whenever the config entity it uses gets changed (e.g. from enabled to disabled or vice versa). That is not the case right now…

3. +++ b/core/modules/search/src/Form/SearchBlockForm.php
   @@ -94,8 +95,19 @@ public function buildForm(array $form, FormStateInterface $form_state, $entity_i
   +    // Because the block config entity does not have a config dependency on the
   +    // search page it is configured to use, it may not exist.
   +    if (!$search_page->status()) {
   

   The real problem to fix here is the missing config dependency.

#5.3 apparently already has an issue, yay! See #3086824: SearchBlock plugins need to add in dependencies on the search page they submit to.

Personally I find it strange to see this message, maybe instead of it it could hide the block to prevent display "not accessible" feature to visitors

I agree … but that's the pattern that already exists. Trying to minimize change here, and only fix the brokenness.

+++ b/core/modules/search/src/Form/SearchBlockForm.php
@@ -94,8 +95,19 @@ public function buildForm(array $form, FormStateInterface $form_state, $entity_i
-    $route = 'search.view_' . $entity_id;
...
+    $form['#action'] = $search_page->toUrl()->toString();

Apparently the search.view_* routes are 100% independent of the SearchPage config entity. My bad. But … super weird that the critical functionality that these config entities offer is not accessible in the entity type's link templates 🙃

I truly have no clue why this would trigger this particular failure. 😔

Note that this adversely affects migrations too — because quite a few D7 sites have this disabled, and having that migrated over to D9 causes … well … the problem you can see in the screenshot in the issue summary!