More performance improvements to metadata lazyloading.

Comment and term meta lazyloading for `WP_Query` loops, introduced in 4.4,
depended on filter callback methods belonging to `WP_Query` objects. This meant
storing `WP_Query` objects in the `$wp_filter` global (via `add_filter()`),
requiring that PHP retain the objects in memory, even when the local variables
would typically be expunged during normal garbage collection. In cases where a
large number of `WP_Query` objects were instantiated on a single pageload,
and/or where the contents of the `WP_Query` objects were quite large, serious
performance issues could result.

We skirt this problem by moving metadata lazyloading out of `WP_Query`. The
new `WP_Metadata_Lazyloader` class acts as a lazyload queue. Query instances
register items whose metadata should be lazyloaded - such as post terms, or
comments - and a `WP_Metadata_Lazyloader` method will intercept comment and
term meta requests to perform the cache priming. Since `WP_Metadata_Lazyloader`
instances are far smaller than `WP_Query` (containing only object IDs), and
clean up after themselves far better than the previous `WP_Query` methods (bp
only running their callbacks a single time for a given set of queued objects),
the resource use is decreased dramatically.

See [36525] for an earlier step in this direction.

Props lpawlik, stevegrunwell, boonebgorges.
Fixes #35816.

git-svn-id: https://develop.svn.wordpress.org/trunk@36566 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/post.php

@@ -5947,6 +5947,43 @@ function wp_delete_auto_drafts() {
 	}
 }
 
+/**
+ * Queue posts for lazyloading of term meta.
+ *
+ * @since 4.5.0
+ *
+ * @param array $posts Array of WP_Post objects.
+ */
+function wp_queue_posts_for_term_meta_lazyload( $posts ) {
+	$post_type_taxonomies = $term_ids = array();
+	foreach ( $posts as $post ) {
+		if ( ! ( $post instanceof WP_Post ) ) {
+			continue;
+		}
+
+		if ( ! isset( $post_type_taxonomies[ $post->post_type ] ) ) {
+			$post_type_taxonomies[ $post->post_type ] = get_object_taxonomies( $post->post_type );
+		}
+
+		foreach ( $post_type_taxonomies[ $post->post_type ] as $taxonomy ) {
+			// Term cache should already be primed by `update_post_term_cache()`.
+			$terms = get_object_term_cache( $post->ID, $taxonomy );
+			if ( false !== $terms ) {
+				foreach ( $terms as $term ) {
+					if ( ! isset( $term_ids[ $term->term_id ] ) ) {
+						$term_ids[] = $term->term_id;
+					}
+				}
+			}
+		}
+	}
+
+	if ( $term_ids ) {
+		$lazyloader = wp_metadata_lazyloader();
+		$lazyloader->queue_objects( 'term', $term_ids );
+	}
+}
+
 /**
  * Update the custom taxonomies' term counts when a post's status is changed.
  *