How to validate a REST submitted node?

Problem/Motivation

I want to validate my business rules when submitting nodes through either form edit of REST POST/PATCH

When using REST hook_node_validate is not invoked as that is form tied.

Using hook_entity_presave seems the only possible hook to validate a REST submission. But that would lead to weird code checking whether submission is through REST or Form API throwing exception.

core/modules/node/node.api.php:410
function hook_node_validate(\Drupal\node\NodeInterface $node, $form, \Drupal\Core\Form\FormStateInterface $form_state) {

core/modules/system/entity.api.php:894
function hook_entity_presave(Drupal\Core\Entity\EntityInterface $entity) {

Validation is done in core/modules/rest/src/Plugin/rest/resource/EntityResource.php using \Drupal\rest\Plugin\rest\resource\EntityResource::validate which is not part of \Drupal\Core\Entity\EntityInterface but of core/lib/Drupal/Core/Entity/ContentEntityBase.php:22

This is probably strongly related to #1696648: [META] Untie content entity validation from form validation

Example

The example below only accepts only article titles less then 4 chars long leading to schizo code using the form validation to tell the form user and REST PATCH validation. It needs hook_node_validate AND hook_entity_presave() which is weird and inconsistent.

/**
 * Implements hook_node_submit().
 *
 * Article titles may no exceed 3 chars.
 */
function example_rest_validate_node_validate(\Drupal\node\NodeInterface $node, $form, \Drupal\Core\Form\FormStateInterface $form_state) {
  $node_type = $node->type->target_id;
  if ($node_type == 'article') {
    if (strlen($node->getTitle()) > 3) {
     $form_state->setErrorByName('title', t('Title must contain 3 or less characters.'));
    }
  }
}

/**
 * Implements hook_ENTITY_presave().
 *
 * Article titles may no exceed 3 chars.
 * Only trigger on node:article:PATCH method as Form validation
 * is done through example_rest_validate_node_submit()
 *
 * @see example_rest_validate_node_submit()
 */
function example_rest_validate_node_presave(Drupal\Core\Entity\EntityInterface $entity) {
  $method = \Drupal::request()->getMethod();
  if ($method == 'PATCH') {
    $entity_type = $entity->getEntityTypeId();
    if ($entity_type == 'node') {
      $entity_bundle = $entity->type->target_id;
      if ($entity_bundle == 'article') {
        if (strlen($entity->getTitle()) > 3) {
          // $form_state->setErrorByName('title', t('Title must contain 3 or less characters.'));
          // really throw an exception?
          throw new HttpException(422, t('Title must contain 3 or less characters.'));
        }
      }
    }
  }
}

This could be rewritten by a helper function accepting an $entity listing validation errors same as ContentEntityBase::validate() does but the need for two interception functions to either set form errors of throw an exception feels bad.

Proposed resolution

Remaining tasks

• How to validate transparently using either Form API or REST?
• How to report validation errors through REST?

User interface changes

API changes