class FileAccessControlHandler

class FileAccessControlHandler extends EntityAccessControlHandler {
    
    /**
     * {@inheritdoc}
     */
    protected function checkAccess(EntityInterface $entity, $operation, AccountInterface $account) {
        
        /** @var \Drupal\file\FileInterface $entity */
        if ($operation == 'download' || $operation == 'view') {
            if (\Drupal::service('stream_wrapper_manager')->getScheme($entity->getFileUri()) === 'public') {
                if ($operation === 'download') {
                    return AccessResult::allowed();
                }
                else {
                    return AccessResult::allowedIfHasPermission($account, 'access content');
                }
            }
            elseif ($references = $this->getFileReferences($entity)) {
                foreach ($references as $field_name => $entity_map) {
                    foreach ($entity_map as $referencing_entity_type => $referencing_entities) {
                        
                        /** @var \Drupal\Core\Entity\EntityInterface $referencing_entity */
                        foreach ($referencing_entities as $referencing_entity) {
                            $entity_and_field_access = $referencing_entity->access('view', $account, TRUE)
                                ->andIf($referencing_entity->{$field_name}
                                ->access('view', $account, TRUE));
                            if ($entity_and_field_access->isAllowed()) {
                                return $entity_and_field_access;
                            }
                        }
                    }
                }
            }
            elseif ($entity->getOwnerId() == $account->id()) {
                // This case handles new nodes, or detached files. The user who uploaded
                // the file can access it even if it's not yet used.
                if ($account->isAnonymous()) {
                    // For anonymous users, only the browser session that uploaded the
                    // file is positively allowed access to it. See file_save_upload().
                    // @todo Implement \Drupal\Core\Entity\EntityHandlerInterface so that
                    //   services can be more properly injected.
                    $allowed_fids = \Drupal::service('session')->get('anonymous_allowed_file_ids', []);
                    if (!empty($allowed_fids[$entity->id()])) {
                        return AccessResult::allowed()->addCacheContexts([
                            'session',
                            'user',
                        ]);
                    }
                }
                else {
                    return AccessResult::allowed()->addCacheContexts([
                        'user',
                    ]);
                }
            }
        }
        if ($operation == 'delete' || $operation == 'update') {
            $account = $this->prepareUser($account);
            $file_uid = $entity->get('uid')
                ->getValue();
            // Only the file owner can update or delete the file entity.
            if ($account->id() == $file_uid[0]['target_id']) {
                return AccessResult::allowed();
            }
            return AccessResult::forbidden('Only the file owner can update or delete the file entity.');
        }
        // No opinion.
        return AccessResult::neutral();
    }
    
    /**
     * Wrapper for file_get_file_references().
     *
     * @param \Drupal\file\FileInterface $file
     *   The file object for which to get references.
     *
     * @return array
     *   A multidimensional array. The keys are field_name, entity_type,
     *   entity_id and the value is an entity referencing this file.
     *
     * @see file_get_file_references()
     */
    protected function getFileReferences(FileInterface $file) {
        return file_get_file_references($file, NULL, EntityStorageInterface::FIELD_LOAD_REVISION, NULL);
    }
    
    /**
     * {@inheritdoc}
     */
    protected function checkFieldAccess($operation, FieldDefinitionInterface $field_definition, AccountInterface $account, FieldItemListInterface $items = NULL) {
        // Deny access to fields that should only be set on file creation, and
        // "status" which should only be changed based on a file's usage.
        $create_only_fields = [
            'uri',
            'filemime',
            'filesize',
        ];
        // The operation is 'edit' when the entity is being created or updated.
        // Determine if the entity is being updated by checking if it is new.
        $field_name = $field_definition->getName();
        if ($operation === 'edit' && $items && ($entity = $items->getEntity()) && !$entity->isNew() && in_array($field_name, $create_only_fields, TRUE)) {
            return AccessResult::forbidden();
        }
        // Regardless of whether the entity exists access should be denied to the
        // status field as this is managed via other APIs, for example:
        // - \Drupal\file\FileUsage\FileUsageBase::add()
        // - \Drupal\file\Plugin\EntityReferenceSelection\FileSelection::createNewEntity()
        if ($operation === 'edit' && $field_name === 'status') {
            return AccessResult::forbidden();
        }
        return parent::checkFieldAccess($operation, $field_definition, $account, $items);
    }
    
    /**
     * {@inheritdoc}
     */
    protected function checkCreateAccess(AccountInterface $account, array $context, $entity_bundle = NULL) {
        // The file entity has no "create" permission because by default Drupal core
        // does not allow creating file entities independently. It allows you to
        // create file entities that are referenced from another entity
        // (e.g. an image for a article). A contributed module is free to alter
        // this to allow file entities to be created directly.
        return AccessResult::neutral();
    }

}