Limit EntityResource to content entities, rather than them causing fatal errors

Just a rough start so far.

Re-roll of #1 to get it to apply again.

+++ b/core/modules/rest/src/Plugin/Deriver/EntityDeriver.php
@@ -70,11 +72,17 @@ public function getDerivativeDefinitions($base_plugin_definition) {
+        // @todo Add support for config entities later, see
+        //   https://www.drupal.org/node/2300677.
+        if (!$entity_type instanceof ContentEntityType) {
+          continue;
+        }

Only this part really makes sense to me: just only handle content entities. Then config entity types won't get REST routes created for them. Which is all that is necessary I think? Why all the other stuff?

Attempting to improve the title based on my understanding of this issue. I'm certain it's not entirely accurate. Please clarify.

My understanding:

• #2300677: JSON:API POST/PATCH support for fully validatable config entities is about exposing Config Entities via REST.
• This issue is about making sure things don't break in crazy ways when you try to access Config Entities via REST, until that issue lands. (That's why there's a @todo in this patch pointing to #2300677).

I think we can just close this one and fix the other :) (because it makes to me have rest support for config entities)

Hmm, k, that's fair. The other one will never land for 8.0.x indeed.

Yep. But then we first need to understand how the current code can actually cause breakage. Or is this solely for the purpose of allowing the REST UI contrib module to have slightly cleaner code?

Can't we add some kind of config validation to never allow this kind of setting?

Can't we add some kind of config validation to never allow this kind of setting?

Configuration doesn't have the concept of validation at all? We only have validation for importing: \Drupal\Core\Config\ConfigEvents::IMPORT_VALIDATE.

#2724823: EntityResource: read-only (GET) support for configuration entities already fixed Limit EntityResource to content entities, rather than them causing fatal errors.

It added this:

  /**
   * {@inheritdoc}
   */
  public function availableMethods() {
    $methods = parent::availableMethods();
    if ($this->isConfigEntityResource()) {
      // Currently only GET is supported for Config Entities.
      // @todo Remove when supported https://www.drupal.org/node/2300677
      $unsupported_methods = ['POST', 'PUT', 'DELETE', 'PATCH'];
      $methods = array_diff($methods, $unsupported_methods);
    }
    return $methods;
  }

  /**
   * Checks if this resource is for a Config Entity.
   *
   * @return bool
   *   TRUE if the entity is a Config Entity, FALSE otherwise.
   */
  protected function isConfigEntityResource() {
    return $this->entityType instanceof ConfigEntityType;
  }