diff -r 490d5cc509ed -r cf61fcea0001 wp/wp-includes/class-wp-metadata-lazyloader.php
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/wp/wp-includes/class-wp-metadata-lazyloader.php	Mon Oct 14 17:39:30 2019 +0200
@@ -0,0 +1,170 @@
+<?php
+/**
+ * Meta API: WP_Metadata_Lazyloader class
+ *
+ * @package WordPress
+ * @subpackage Meta
+ * @since 4.5.0
+ */
+
+/**
+ * Core class used for lazy-loading object metadata.
+ *
+ * When loading many objects of a given type, such as posts in a WP_Query loop, it often makes
+ * sense to prime various metadata caches at the beginning of the loop. This means fetching all
+ * relevant metadata with a single database query, a technique that has the potential to improve
+ * performance dramatically in some cases.
+ *
+ * In cases where the given metadata may not even be used in the loop, we can improve performance
+ * even more by only priming the metadata cache for affected items the first time a piece of metadata
+ * is requested - ie, by lazy-loading it. So, for example, comment meta may not be loaded into the
+ * cache in the comments section of a post until the first time get_comment_meta() is called in the
+ * context of the comment loop.
+ *
+ * WP uses the WP_Metadata_Lazyloader class to queue objects for metadata cache priming. The class
+ * then detects the relevant get_*_meta() function call, and queries the metadata of all queued objects.
+ *
+ * Do not access this class directly. Use the wp_metadata_lazyloader() function.
+ *
+ * @since 4.5.0
+ */
+class WP_Metadata_Lazyloader {
+	/**
+	 * Pending objects queue.
+	 *
+	 * @since 4.5.0
+	 * @var array
+	 */
+	protected $pending_objects;
+
+	/**
+	 * Settings for supported object types.
+	 *
+	 * @since 4.5.0
+	 * @var array
+	 */
+	protected $settings = array();
+
+	/**
+	 * Constructor.
+	 *
+	 * @since 4.5.0
+	 */
+	public function __construct() {
+		$this->settings = array(
+			'term' => array(
+				'filter'   => 'get_term_metadata',
+				'callback' => array( $this, 'lazyload_term_meta' ),
+			),
+			'comment' => array(
+				'filter'   => 'get_comment_metadata',
+				'callback' => array( $this, 'lazyload_comment_meta' ),
+			),
+		);
+	}
+
+	/**
+	 * Adds objects to the metadata lazy-load queue.
+	 *
+	 * @since 4.5.0
+	 *
+	 * @param string $object_type Type of object whose meta is to be lazy-loaded. Accepts 'term' or 'comment'.
+	 * @param array  $object_ids  Array of object IDs.
+	 * @return bool|WP_Error True on success, WP_Error on failure.
+	 */
+	public function queue_objects( $object_type, $object_ids ) {
+		if ( ! isset( $this->settings[ $object_type ] ) ) {
+			return new WP_Error( 'invalid_object_type', __( 'Invalid object type' ) );
+		}
+
+		$type_settings = $this->settings[ $object_type ];
+
+		if ( ! isset( $this->pending_objects[ $object_type ] ) ) {
+			$this->pending_objects[ $object_type ] = array();
+		}
+
+		foreach ( $object_ids as $object_id ) {
+			// Keyed by ID for faster lookup.
+			if ( ! isset( $this->pending_objects[ $object_type ][ $object_id ] ) ) {
+				$this->pending_objects[ $object_type ][ $object_id ] = 1;
+			}
+		}
+
+		add_filter( $type_settings['filter'], $type_settings['callback'] );
+
+		/**
+		 * Fires after objects are added to the metadata lazy-load queue.
+		 *
+		 * @since 4.5.0
+		 *
+		 * @param array                  $object_ids  Object IDs.
+		 * @param string                 $object_type Type of object being queued.
+		 * @param WP_Metadata_Lazyloader $lazyloader  The lazy-loader object.
+		 */
+		do_action( 'metadata_lazyloader_queued_objects', $object_ids, $object_type, $this );
+	}
+
+	/**
+	 * Resets lazy-load queue for a given object type.
+	 *
+	 * @since 4.5.0
+	 *
+	 * @param string $object_type Object type. Accepts 'comment' or 'term'.
+	 * @return bool|WP_Error True on success, WP_Error on failure.
+	 */
+	public function reset_queue( $object_type ) {
+		if ( ! isset( $this->settings[ $object_type ] ) ) {
+			return new WP_Error( 'invalid_object_type', __( 'Invalid object type' ) );
+		}
+
+		$type_settings = $this->settings[ $object_type ];
+
+		$this->pending_objects[ $object_type ] = array();
+		remove_filter( $type_settings['filter'], $type_settings['callback'] );
+	}
+
+	/**
+	 * Lazy-loads term meta for queued terms.
+	 *
+	 * This method is public so that it can be used as a filter callback. As a rule, there
+	 * is no need to invoke it directly.
+	 *
+	 * @since 4.5.0
+	 *
+	 * @param mixed $check The `$check` param passed from the 'get_term_metadata' hook.
+	 * @return mixed In order not to short-circuit `get_metadata()`. Generally, this is `null`, but it could be
+	 *               another value if filtered by a plugin.
+	 */
+	public function lazyload_term_meta( $check ) {
+		if ( ! empty( $this->pending_objects['term'] ) ) {
+			update_termmeta_cache( array_keys( $this->pending_objects['term'] ) );
+
+			// No need to run again for this set of terms.
+			$this->reset_queue( 'term' );
+		}
+
+		return $check;
+	}
+
+	/**
+	 * Lazy-loads comment meta for queued comments.
+	 *
+	 * This method is public so that it can be used as a filter callback. As a rule, there is no need to invoke it
+	 * directly, from either inside or outside the `WP_Query` object.
+	 *
+	 * @since 4.5.0
+	 *
+	 * @param mixed $check The `$check` param passed from the {@see 'get_comment_metadata'} hook.
+	 * @return mixed The original value of `$check`, so as not to short-circuit `get_comment_metadata()`.
+	 */
+	public function lazyload_comment_meta( $check ) {
+		if ( ! empty( $this->pending_objects['comment'] ) ) {
+			update_meta_cache( 'comment', array_keys( $this->pending_objects['comment'] ) );
+
+			// No need to run again for this set of comments.
+			$this->reset_queue( 'comment' );
+		}
+
+		return $check;
+	}
+}