Sync product sizes

/**
 * Get products sizes
 */
function bis_get_sizes($continue_import, $post_id, $data, $import_id) {
    $transient_name = "bis_size_{$import_id}";

    // Get the product ID & size
    $product_id = $data['manufacturer_artno']; // A field that will be used for SKU
    $product_size = $data['size'];

    $sizes = (array) get_transient($transient_name);
    $sizes[$product_id][] = $product_size;
    set_transient($transient_name, $sizes);

    return $continue_import;
}
add_filter('wp_all_import_is_post_to_update', 'bis_get_sizes', 10, 4);

/**
 * Save sizes as product attributes
 */
function bis_save_sizes($import_id) {
    global $wpdb;

    $transient_name = "bis_size_{$import_id}";
    $all_sizes = (array) get_transient($transient_name);
    $taxonomy = 'pa_size';

    // Get products IDs from last import
    $product_ids = $wpdb->get_col("SELECT post_id FROM {$wpdb->prefix}pmxi_posts WHERE import_id = {$import_id}");

    // Remove empty items from the array with sizes
    $all_sizes = array_filter($all_sizes);

    if(!empty($all_sizes)) {
        foreach($all_sizes as $sku => $product_sizes) {
            // Find the product with exact SKU in the imported products list
            $product_id = $wpdb->get_var(sprintf("SELECT p.ID FROM {$wpdb->posts} AS p LEFT JOIN {$wpdb->postmeta} AS pm ON (p.ID = pm.post_ID) WHERE p.ID IN (%s) AND pm.meta_key = '_sku' AND pm.meta_value = '%s'", implode(", ", $product_ids), $sku));

            // Add product sizes again to the product
            if(!empty($product_id)) {
                $product_terms = array();

                foreach($product_sizes as $product_size) {
                    $term = term_exists((string) $product_size, $taxonomy);

                    if(empty($term)) {
                        $term = wp_insert_term((string) $product_size, $taxonomy);
                    }

                    $product_terms[] = (int) $term['term_id'];
                }

                wp_set_post_terms((int) $product_id, $product_terms, $taxonomy);
            }
        }
    }
}
add_action('after_xml_import', 'bis_save_sizes');

add_action('init', function() {
    if(isset($_GET['test']) && is_numeric($_GET['test'])) {
        $import_id = (int) $_GET['test'];

        bis_save_sizes($import_id);

        $transient_name = "bis_size_{$import_id}";
        $all_sizes = get_transient($transient_name);
        print_r($all_sizes);
        die();
    }
});