Patch for wpDiscuz bug where persistent cache is being used and 'Load more comments' button loads the same comments

wpDiscuz-peristent-cache-patch

<?php
/*
Plugin Name: FasterWebsite wpDiscuz Cache Patch
Plugin URI: https://fasterwebsite.com/
Description: Fixes bug 'Load More button loads the same comments'
Author: Maciej Klepaczewski
Author URI: https://fasterwebsite.com/
Version: 0.1
Requires PHP: 7.4
*/

/**
 * @author    Maciej Klepaczewski <matt@fasterwebsite.com>
 * @link      https://fasterwebsite.com/
 * @copyright Copyright (c) 2021, Maciej Klepaczewski FasterWebsite.com
 */
declare(strict_types=1);

namespace fasterwebsite\wpdiscuz\patch1;

use WP_Comment_Query;

/**
 *
 * Patch for bug where when persistent caching is enabled wpDiscuz will fail to load next batch of comments when
 * 'Load More Comments' button is clicked.
 *
 * Why it happens:
 * The problem is in caching done by WP_Comment_Query::get_comments(). The method generates cache key based only on
 * WP_Comment_Query::query_var_defaults, which does not hold some keys that are being injected into SQL query to fetch
 * comments by wpDiscuz WpdiscuzCore::commentsClauses() (which hooks into 'comments_clauses' hook fired by
 * WP_Comment_Query::get_comment_ids()). The result is that two requests with different "lastParentId" parameters have the
 * same cache key. When persistent cache is enabled (Redis, Memcached etc.) this results in retrieving incorrect comments
 * for the request.
 *
 * Solution:
 * WP_Comment_Query::get_comments() uses WP_Comment_Query::query_vars to generate the cache key
 * WP_Comment_Query::get_comments() considers only those parameters in WP_Comment_Query::query_vars that are defined
 * in WP_Comment_Query::default_query_vars. So, the solution is:
 * - hook into 'comments_pre_query' and inject into WP_Comment_Query::default_query_vars our custom variables so they
 *   are being taken into account while generating cache key. Also, inject actual values into WP_Comment_Query::query_vars
 * - let WP_Comment_Query generate the cache key
 * - remove our changes after cache key has been generated, as soon as possible. This happens by hooking into the
 *   'the_comments' hook and removing our changes from both WP_Comment_Query::default_query_vars and WP_Comment_Query::query_vars
 *
 * Note that wpDiscuz injects some variables into WP_Comment_Query::query_vars and we could add these using
 * WP_Comment_Query::default_query_vars, but it seems to be more intrusive and requires more knowledge.
 */
class Patch {
    public function __construct() {
        if(!$this->isApplicableToRequest()) {
            return;
        }

        /* This will inject our custom variable into WP_Comment_Query::query_var_defaults and query_vars */
        add_filter('comments_pre_query', function($comment_data, WP_Comment_Query $query) {
            $query->query_var_defaults['wpdiscuz'] = 'temporary_from_' . __CLASS__ . "::" . __METHOD__;
            $this->addWpDiscuzParams($query);

            /* Don't change $comment_data */
            return $comment_data;
        }, PHP_INT_MAX, 2);

        /* This will remove it */
        add_filter('the_comments', function($_comments, WP_Comment_Query $query) {
            unset($query->query_var_defaults['wpdiscuz']);
            return $_comments;
        }, PHP_INT_MIN, 2);
    }

    /**
     * Decides if this patch should actually run.
     *
     * We run it
     * @return bool
     */
    public function isApplicableToRequest() : bool {
        if(!wp_doing_ajax()) {
            return false;
        }

        if(!isset($_REQUEST['action']) || $_REQUEST['action'] !== 'wpdLoadMoreComments') {
            return false;
        }

        return true;
    }

    private function addWpDiscuzParams(WP_Comment_Query $query) : void {
        $query->query_vars['wpdiscuz'] = wp_array_slice_assoc($_REQUEST, $this->getWpDiscuzSpecificArgs());
    }

    /**
     * @return array<string>
     */
    private function getWpDiscuzSpecificArgs() : array {
        return ['lastParentId', 'isFirstLoad', "offset", "sorting"];
    }

}

new Patch();