In `wp_list_comments()`, update the comment meta cache when the comments derive from `WP_Query` and the new `->comment_meta_cached` prop is `false`.

There are no uses of `wp_list_comments()` in Core where `$comments` are passed as the 2nd argument.

Adds unit tests.

Props wonderboymusic, bradt.
Fixes #16894.

Built from https://develop.svn.wordpress.org/trunk@33925


git-svn-id: http://core.svn.wordpress.org/trunk@33894 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/comment-template.php

@@ -2116,6 +2116,12 @@ function wp_list_comments( $args = array(), $comments = null ) {
 		} else {
 			$_comments = $wp_query->comments;
 		}
+
+		if ( ! $wp_query->comment_meta_cached ) {
+			$comment_ids = wp_list_pluck( $_comments, 'comment_ID' );
+			update_meta_cache( 'comment', $comment_ids );
+			$wp_query->comment_meta_cached = true;
+		}
 	}
 
 	if ( '' === $r['per_page'] && get_option('page_comments') )

wp-includes/query.php

@@ -1292,6 +1292,15 @@ class WP_Query {
 	 */
 	 public $thumbnails_cached = false;
 
+	/**
+	 * Set if comment meta has already been cached
+	 *
+	 * @since 4.4.0
+	 * @access public
+	 * @var bool
+	 */
+	 public $comment_meta_cached = false;
+
 	/**
 	 * Cached list of search stopwords.
 	 *

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.4-alpha-33924';
+$wp_version = '4.4-alpha-33925';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.