Term entity type's "parent" field does not specify "target_bundles" setting: a term from *any* vocabulary can be chosen

This inaccuracy in Typed Data metadata results in the latest JSON API release being unable to normalize terms when there are multiple vocabularies on a site. So I ported #2 over to JSON API as a work-around until this lands: #2977879-5: Regression in #2940339: when multiple vocabularies exist, normalization of Terms fails.

This must be one of the most obscure failures I've ever seen, along with one of the silliest fixes.

This is why I like having assert()s all over, along with strictly typed objects. This is super painful to debug.

The #5 interdiff is wrong. This is the interdiff:

+++ b/core/modules/taxonomy/src/Entity/Term.php
@@ -179,6 +179,16 @@ public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
-    $fields['parent']->setSetting('handler_settings', ['target_bundles' => [$bundle]]);
+    $fields['parent']->setSetting('handler_settings', ['target_bundles' => [$bundle => $bundle]]);

Done! Failing test-only patch is also the interdiff.

🎉

@jibran AFAICT that's not true. I could reproduce the bug with Drupal core's tags vocabulary and simply applying this patch and clearing caches fixed it.

Looking at the caller of ::bundleFieldDefinitions() seems to confirm this — \Drupal\Core\Entity\EntityFieldManager::getFieldDefinitions() does this:

      $cid = 'entity_bundle_field_definitions:' . $entity_type_id . ':' . $bundle . ':' . $this->languageManager->getCurrentLanguage()->getId();
      if ($cache = $this->cacheGet($cid)) {
        $bundle_field_definitions = $cache->data;
      }
      else {
        // Rebuild the definitions and put it into the cache.
        $bundle_field_definitions = $this->buildBundleFieldDefinitions($entity_type_id, $bundle, $base_field_definitions);
        $this->cacheSet($cid, $bundle_field_definitions, Cache::PERMANENT, ['entity_types', 'entity_field_info']);
      }

And $this->buildBundleDefinitions() does this:

    // Allow the entity class to provide bundle fields and bundle-specific
    // overrides of base fields.
    $bundle_field_definitions = $class::bundleFieldDefinitions($entity_type, $bundle, $base_field_definitions);

Thanks, @amateescu! But also thanks, @jibran, for asking the right critical questions :)

At least we need an update hook to clear the cache.

Hm, do we? I forget, but I thought that every update of Drupal core already would invalidate all caches.

Should be backported since it breaks jsonapi

Besides, JSON API already backported this:

/**
 * Implements hook_entity_bundle_field_info().
 */
function jsonapi_entity_bundle_field_info(EntityTypeInterface $entity_type, $bundle, array $base_field_definitions) {
  $fields = [];

  if (floatval(\Drupal::VERSION) < 8.6 && $entity_type->id() == 'taxonomy_term') {
    // Only terms in the same bundle can be a parent.
    $fields['parent'] = clone $base_field_definitions['parent'];
    $fields['parent']->setSetting('handler_settings', ['target_bundles' => [$bundle]]);
    return $fields;
  }

  return $fields;
}

That's why I forgot this was even still open. Thanks for the attempt though, @pwolanin :)

@larowlan++