mailsystem_get_classes with caching

/**
 * Returns a list of classes which implement MailSystemInterface.
 */
function &mailsystem_get_classes() {
  $mailsystem_classes = &drupal_static(__FUNCTION__);
  if (!isset($mailsystem_classes) && $cache = cache_get('mailsystem_get_classes')) {
      $mailsystem_classes = $cache->data;
  }
  if (!isset($mailsystem_classes)) {
    $mailsystem_classes = array();
    // @todo Is there a better way to find all mail-related classes?
    $declared_classes = get_declared_classes();
    $all_classes = array_combine(
      $declared_classes,
      array_fill(0, count($declared_classes), 0)
    );
    $mail_classes = db_select('registry', 'registry')
      ->distinct()
      ->fields('registry', array('name', 'filename'))
      ->where("type=:type AND ( filename like :filename OR name like :name )",
        // Making the HUGE assumption that all classes which implement
        // MailSystemInterface have filenames containing '.mail.' or
        // classnames ending in 'MailSystem'.
        array(
          ':type' => 'class',
          ':name' => '%MailSystem',
          ':filename' => '%.mail.%',
        )
      )
      ->execute()
      ->fetchAllKeyed();
    foreach ($mail_classes as $classname => $classfile) {
      if ( file_exists($classfile)
        && drupal_autoload_class($classname)
      ) {
        $all_classes[$classname] = 1;
      }
    }
    foreach ($all_classes as $classname => $autoload) {
      if ( ($autoload || preg_match('/MailSystem/', $classname))
        && ($object = new $classname)
        && ($object instanceof MailSystemInterface)
      ) {
        $mailsystem_classes[$classname] = $classname;
      }
      elseif ($autoload) {
        // Clear classes that are no longer available.
        db_delete('registry')
          ->condition('name', $classname)
          ->execute();
      }
    }
    foreach (array_unique(mailsystem_get()) as $classname) {
      if (class_exists($classname)) {
        $mailsystem_classes[$classname] = $classname;
      }
      else {
        mailsystem_clear(array(mailsystem_default_id() => $classname));
      }
    }
    ksort($mailsystem_classes);
    cache_set('mailsystem_get_classes', $mailsystem_classes, 'cache', time() + 3600);
  }
  return $mailsystem_classes;
}