Hook an action when a transient is deleted ? Can't get it to work!

Question

I'm working on a plugin, and it caches a remote page when requested. The file is created with tempnam(). I save the path of that cached file in a transient, using the transient API.

Once the transient is deleted by Wordpress, I would like to delete my cached page too. I did try to add an action on the hook deleted_transient, but it seems that it is not fired. Maybe that one is fired only when using delete_transient(), not when WP does automatically the job.

My other problem is that to delete my file, I need to retrieve the path to the cached file BEFORE the transient is deleted...

...But the deleted_transient hook is fired after the deletion. There is another hook before the deletion ('delete_transient_' . $transient), but... I need to know the name of the transient run it. And my transient have random names...

Any ideas about that ?

Answer 1

Transients are generally cleared when read. If you want to check for this common case, you could catch the pre_transient_transientname and see if it expired, the same way WP is doing:

function get_transient( $transient ) {

    /**
     * Filter the value of an existing transient.
     *
     * The dynamic portion of the hook name, `$transient`, refers to the transient name.
     *
     * Passing a truthy value to the filter will effectively short-circuit retrieval
     * of the transient, returning the passed value instead.
     *
     * @since 2.8.0
     *
     * @param mixed $pre_transient The default value to return if the transient does not exist.
     *                             Any value other than false will short-circuit the retrieval
     *                             of the transient, and return the returned value.
     */
    $pre = apply_filters( 'pre_transient_' . $transient, false );
    if ( false !== $pre )
        return $pre;

    if ( wp_using_ext_object_cache() ) {
        $value = wp_cache_get( $transient, 'transient' );
    } else {
        $transient_option = '_transient_' . $transient;
        if ( ! defined( 'WP_INSTALLING' ) ) {
            // If option is not in alloptions, it is not autoloaded and thus has a timeout
            $alloptions = wp_load_alloptions();
            if ( !isset( $alloptions[$transient_option] ) ) {
                $transient_timeout = '_transient_timeout_' . $transient;
                if ( get_option( $transient_timeout ) < time() ) {
                    delete_option( $transient_option  );
                    delete_option( $transient_timeout );
                    $value = false;
                }
            }
        }

        if ( ! isset( $value ) )
            $value = get_option( $transient_option );
    }

    /**
     * Filter an existing transient's value.
     *
     * The dynamic portion of the hook name, `$transient`, refers to the transient name.
     *
     * @since 2.8.0
     *
     * @param mixed $value Value of transient.
     */
    return apply_filters( 'transient_' . $transient, $value );
}

You could also hook the filter pre_option__transient_timeout_<transientname> to catch that get_option above that happens just before the delete, and check that condition to see if it expired. Just make sure you return the first argument, in the functions you use to filter these.