[wp-trac] [WordPress Trac] #14713: Comment cache is never invalidated when new comments are approved or added

[WordPress Trac]

#14713: Comment cache is never invalidated when new comments are approved or added
--------------------------+-------------------------------------------------
 Reporter:  TomUK         |       Owner:                          
     Type:  defect (bug)  |      Status:  new                     
 Priority:  normal        |   Milestone:  Awaiting Review         
Component:  Comments      |     Version:  3.0.1                   
 Severity:  major         |    Keywords:  comments cache memcached
--------------------------+-------------------------------------------------
 I've recently started using memcached on my high traffic wordpress site.

 On line 214 of wp-includes/comment.php there is this block of code:

 {{{
         $last_changed = wp_cache_get('last_changed', 'comment');
         if ( !$last_changed ) {
                 $last_changed = time();
                 wp_cache_set('last_changed', $last_changed, 'comment');
         }
         $cache_key = "get_comments:$key:$last_changed";

         if ( $cache = wp_cache_get( $cache_key, 'comment' ) ) {
                 return $cache;
         }
 }}}

 Which seems to determine the cache key for caching the comments in the
 wordpress cache. The value of key last_changed is used as part of the
 comment cache key, and is set to be the current time if it is not set.

 However, last_changed is never updated - meaning when new comments are
 added the cache for the old comments is still used (as there is value in
 the key referenced by $cache_key which isn't invalidated).

 This wouldn't normally be a problem in the normal object cache, but using
 memcached means the cache is more persistent between page loads which
 causing the problem of comments not updating.

 The cache key "last_changed" needs to be deleted whenever comments are
 changed, something like:


 {{{
 /**
  * Fix for comment cache never getting invalidated. We need to remove the
 last updated cache
  */
 function updateLastCommentUpdated(){
     wp_cache_delete('last_changed', 'comment');
 }
 add_action('wp_set_comment_status', 'updateLastCommentUpdated');
 add_action('comment_post', 'updateLastCommentUpdated');
 }}}

 Which I have added to my functions.php file, but should presumably be in
 core code somewhere.

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Ticket URL: <http://core.trac.wordpress.org/ticket/14713>
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