Lazy-load comment meta on single post pages.

[34268] introduced cache priming for commentmeta, enabled by default. To
ensure performance on single post pages - where commentmeta is most likely
to cause performance issues - we disable up-front cache-priming. Instead, we
prime commentmeta caches for all comments in the loop the first time
`get_comment_meta()` is called on the page.

Props bradt, dd32, wonderboymusic, boonebgorges.
Fixes #16894.
Built from https://develop.svn.wordpress.org/trunk@34270


git-svn-id: http://core.svn.wordpress.org/trunk@34234 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/comment-functions.php

@@ -2376,6 +2376,30 @@ function update_comment_cache( $comments, $update_meta_cache = true ) {
 	}
 }
 
+/**
+ * Lazy load comment meta when inside of a `WP_Query` loop.
+ *
+ * @since 4.4.0
+ *
+ * @param null $check      The `$check` param passed from the 'pre_comment_metadata' hook.
+ * @param int  $comment_id ID of the comment whose metadata is being cached.
+ * @return null In order not to short-circuit `get_metadata()`.
+ */
+function wp_lazyload_comment_meta( $check, $comment_id ) {
+	global $wp_query;
+
+	if ( ! empty( $wp_query->comments ) ) {
+		// Don't use `wp_list_pluck()` to avoid by-reference manipulation.
+		$comment_ids = array();
+		foreach ( $wp_query->comments as $comment ) {
+			$comment_ids[] = $comment->comment_ID;
+		}
+		update_meta_cache( 'comment', $comment_ids );
+	}
+
+	return $check;
+}
+
 //
 // Internal
 //

wp-includes/comment-template.php

@@ -1211,6 +1211,7 @@ function comments_template( $file = '/comments.php', $separate_comments = false
 		'orderby' => 'comment_date_gmt',
 		'status'  => 'approve',
 		'post_id' => $post->ID,
+		'update_comment_meta_cache' => false, // We lazy-load comment meta for performance.
 	);
 
 	if ( $user_ID ) {

wp-includes/default-filters.php

@@ -200,6 +200,7 @@ add_filter( 'nav_menu_meta_box_object', '_wp_nav_menu_meta_box_object'        );
 add_filter( 'pingback_ping_source_uri', 'pingback_ping_source_uri'            );
 add_filter( 'xmlrpc_pingback_error',    'xmlrpc_pingback_error'               );
 add_filter( 'title_save_pre',           'trim'                                );
+add_filter( 'get_comment_metadata',     'wp_lazyload_comment_meta',     10, 2 );
 
 add_filter( 'http_request_host_is_external', 'allowed_http_request_hosts', 10, 2 );
 

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.4-alpha-34269';
+$wp_version = '4.4-alpha-34270';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.