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.

git-svn-id: https://develop.svn.wordpress.org/trunk@34270 602fd350-edb4-49c9-b593-d223f7449a82

src/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
 //

src/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 ) {

src/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 );
 

tests/phpunit/tests/comment/metaCache.php

@@ -81,61 +81,38 @@ class Tests_Comment_Meta_Cache extends WP_UnitTestCase {
 		$this->assertSame( $num_queries + 3, $wpdb->num_queries );
 	}
 
-	public function test_comment_meta_cache() {
-		$post_id = $this->factory->post->create( array(
-			'post_status' => 'publish'
-		) );
+	/**
+	 * @ticket 16894
+	 */
+	public function test_comment_meta_should_be_lazy_loaded_for_all_comments_in_comments_template() {
+		global $wpdb;
 
-		$comment_ids = $this->factory->comment->create_post_comments( $post_id, 10 );
+		$p = $this->factory->post->create( array( 'post_status' => 'publish' ) );
+		$comment_ids = $this->factory->comment->create_post_comments( $p, 3 );
 
 		foreach ( $comment_ids as $cid ) {
 			update_comment_meta( $cid, 'sauce', 'fire' );
 		}
 
-		$post = get_post( $post_id );
-
-		$this->assertEquals( $post->comment_count, count( $comment_ids ) );
-
-		$this->go_to( get_permalink( $post_id ) );
-
-		$this->assertTrue( is_single() );
-		$this->assertTrue( have_posts() );
-
-		global $wp_query;
+		$this->go_to( get_permalink( $p ) );
 
+		if ( have_posts() ) {
 			while ( have_posts() ) {
 				the_post();
 
-			$comment_args = array(
-				'order'   => 'ASC',
-				'orderby' => 'comment_date_gmt',
-				'status'  => 'approve',
-				'post_id' => get_the_ID(),
-			);
+				// Load comments with `comments_template()`.
+				$cform = get_echo( 'comments_template' );
 
-			$comments = get_comments( $comment_args );
+				// First request will hit the database.
+				$num_queries = $wpdb->num_queries;
+				get_comment_meta( $comment_ids[0], 'sauce' );
+				$this->assertSame( $num_queries + 1, $wpdb->num_queries );
 
-			// This is beyond awful
-			$wp_query->comments = $comments;
-
-			wp_list_comments( array(
-				'echo' => false,
-				'callback' => array( $this, '_comment_callback' )
-			) );
+				// Second and third requests should be in cache.
+				get_comment_meta( $comment_ids[1], 'sauce' );
+				get_comment_meta( $comment_ids[2], 'sauce' );
+				$this->assertSame( $num_queries + 1, $wpdb->num_queries );
 			}
 		}
-
-	public function _comment_callback( $comment ) {
-		global $wpdb;
-
-		get_comment_meta( $comment->comment_ID, 'sauce' );
-
-		if ( 0 === $this->i ) {
-			$this->queries = $wpdb->num_queries;
-		} else {
-			$this->assertEquals( $this->queries, $wpdb->num_queries );
-		}
-
-		$this->i++;
 	}
 }