Problem with page cache and persistent captcha's

Currently, when captcha's are persisted in a user session, and the user has solved a captcha, and gets to the same page (e.g. contact form), the page will be cached after that. Which results in the captcha not being presented to other untrusted users.

To solve this, I think the code in captcha_form_alter that reads:

  if (!user_access('skip captcha challenges')) {
    // Visitor does not have permission to skip the captcha challenge

    // Do not present captcha if not captcha-persistent and user has already solved a captcha for this form
    if(!variable_get('captcha_persistence', TRUE) && ($_SESSION['captcha'][$form_id]['success'] === TRUE)) {
      return;
    }

    // Get captcha type and module for this form. Return if no captcha was set.
    $result = db_query("SELECT module, type FROM {captcha_points} WHERE form_id = '%s'", $form_id);
    if (!$result) {
      return;
    }
    $captcha_point = db_fetch_object($result);
    if (!$captcha_point->type) {
      return;
    }

Should be changed to something like:

  if (!user_access('skip captcha challenges')) {
    // Visitor does not have permission to skip the captcha challenge

    // Get captcha type and module for this form. Return if no captcha was set.
    $result = db_query("SELECT module, type FROM {captcha_points} WHERE form_id = '%s'", $form_id);
    if (!$result) {
      return;
    }
    $captcha_point = db_fetch_object($result);
    if (!$captcha_point->type) {
      return;
    }

    // Do not present captcha if not captcha-persistent and user has already solved a captcha for this form
    if(!variable_get('captcha_persistence', TRUE) && ($_SESSION['captcha'][$form_id]['success'] === TRUE)) {
      global $conf;
      $conf['cache'] = FALSE;
      return;
    }

I.e., I moved the check for persistent captcha to after the other checks, and made the page non-cacheable if the only reason the captcha is not shown is the fact that the user has already solved a captcha.