gistfile1.php

/**
 * Implements hook_init().
 */
function foo_init() {
  $field = field_info_field('bar');
  $instance = field_info_instance('node', 'bar', 'baz');
  // This will call SelectionBase
  $handler = entity_reference_get_selection_handler($field, $instance);
  $ids = $handler->getReferencableEntities();
}


class SelectionBase implements SelectionInterface {

  /**
   * Returns a list of referencable entities.
   */
  public function getReferencableEntities($match = NULL, $match_operator = 'CONTAINS', $limit = 0) {
    // ...
    $query = $this->buildEntityFieldQuery($match, $match_operator);

  }

  /**
   * Builds an EntityFieldQuery to get referencable entities.
   */
  protected function buildEntityFieldQuery($match = NULL, $match_operator = 'CONTAINS') {
    //...

    // Add a generic entity access tag to the query.
    //$query->addMetaData('field', $this->field);
    //$query->addMetaData('entity_reference_selection_handler', $this);

    // ...
  }
}

/**
 * Implements hook_query_TAG_alter().
 */
function entity_reference_query_entity_reference_alter(AlterableInterface $query) {
  $handler = $query->getMetadata('entity_reference_selection_handler');
  // Alter the query to fix entity access per entity-type.
  $handler->entityFieldQueryAlter($query);
}