Add revisioning of post meta, including ‘footnotes’ by default

src/wp-admin/includes/post.php

@@ -1961,15 +1961,73 @@ function wp_create_post_autosave( $post_data ) {
 		 * @param array $new_autosave Post array - the autosave that is about to be saved.
 		 */
 		do_action( 'wp_creating_autosave', $new_autosave );
-
+		wp_autosave_post_revisioned_meta_fields( $new_autosave );
 		return wp_update_post( $new_autosave );
 	}
 
 	// _wp_put_post_revision() expects unescaped.
 	$post_data = wp_unslash( $post_data );
 
 	// Otherwise create the new autosave as a special post revision.
-	return _wp_put_post_revision( $post_data, true );
+	$revision = _wp_put_post_revision( $post_data, true );
+
+	// Update the revisioned meta data as well.
+	wp_autosave_post_revisioned_meta_fields( $revision );
+	return $revision;
+}
+
+/**
+ * Autosave the revisioned meta fields.
+ *
+ * Iterates thru the revisioned meta fields and checks each to see if they are set,
+ * and have a changed value. If so, the meta value is saved and attached to the autosave.
+ *
+ * @since 6.4.0
+ *
+ * @param Post object $new_autosave The new post being autosaved.
+ */
+function wp_autosave_post_revisioned_meta_fields( $new_autosave ) {
+	/*
+	 * The post data arrives as either $_POST['data']['wp_autosave'] or the $_POST
+	 * itself. This sets $posted_data to the correct variable.
+	 *
+	 * Ignoring sanitization to avoid altering meta. Ignoring the nonce check because
+	 * this is hooked on inner core hooks where a valid nonce was already checked.
+	 *
+	 * @phpcs:disable WordPress.Security
+	 */
+	$posted_data = isset( $_POST['data']['wp_autosave'] ) ? $_POST['data']['wp_autosave'] : $_POST;
+	// phpcs:enable
+
+	/*
+	 * Go thru the revisioned meta keys and save them as part of the autosave, if
+	 * the meta key is part of the posted data, the meta value is not blank and
+	 * the the meta value has changes from the last autosaved value.
+	 */
+	foreach ( wp_post_revision_meta_keys() as $meta_key ) {
+
+		if (
+		isset( $posted_data[ $meta_key ] ) &&
+		get_post_meta( $new_autosave['ID'], $meta_key, true ) !== wp_unslash( $posted_data[ $meta_key ] )
+		) {
+			/*
+			 * Use the underlying delete_metadata() and add_metadata() functions
+			 * vs delete_post_meta() and add_post_meta() to make sure we're working
+			 * with the actual revision meta.
+			 */
+			delete_metadata( 'post', $new_autosave['ID'], $meta_key );
+
+			/*
+			 * One last check to ensure meta value not empty().
+			 */
+			if ( ! empty( $posted_data[ $meta_key ] ) ) {
+				/*
+				 * Add the revisions meta data to the autosave.
+				 */
+				add_metadata( 'post', $new_autosave['ID'], $meta_key, $posted_data[ $meta_key ] );
+			}
+		}
+	}
 }
 
 /**

src/wp-includes/default-filters.php

@@ -719,4 +719,7 @@
 // CPT wp_block custom postmeta field.
 add_action( 'init', 'wp_create_initial_post_meta' );
 
+// Including revisioned meta when considering whether a post revision has changed.
+add_filter( 'wp_save_post_revision_post_has_changed', 'wp_check_revisioned_meta_fields_have_changed' ), 10, 3 );
+
 unset( $filter, $action );

src/wp-includes/revision.php

@@ -365,11 +365,34 @@ function _wp_put_post_revision( $post = null, $autosave = false ) {
 		 * @param int $revision_id Post revision ID.
 		 */
 		do_action( '_wp_put_post_revision', $revision_id );
+
+		// Save any revisioned meta fields.
+		wp_save_revisioned_meta_fields( $revision_id );
 	}
 
 	return $revision_id;
 }
 
+
+/**
+ * Save the revisioned meta fields.
+ *
+ * @param int $revision_id The ID of the revision to save the meta to.
+ *
+ * @since 6.4.0
+ */
+function wp_save_revisioned_meta_fields( $revision_id ) {
+	$revision = get_post( $revision_id );
+	$post_id  = $revision->post_parent;
+
+	// Save revisioned meta fields.
+	foreach ( wp_post_revision_meta_keys() as $meta_key ) {
+		if ( metadata_exists( 'post', $post_id, $meta_key ) ) {
+			_wp_copy_post_meta( $post_id, $revision_id, $meta_key );
+		}
+	}
+}
+
 /**
  * Gets a post revision.
  *
@@ -450,6 +473,9 @@ function wp_restore_post_revision( $revision, $fields = null ) {
 	// Update last edit user.
 	update_post_meta( $post_id, '_edit_last', get_current_user_id() );
 
+	// Restore any revisioned meta fields.
+	wp_restore_post_revision_meta( $post_id, $revision['ID'] );
+
 	/**
 	 * Fires after a post revision has been restored.
 	 *
@@ -463,6 +489,83 @@ function wp_restore_post_revision( $revision, $fields = null ) {
 	return $post_id;
 }
 
+/**
+ * Restore the revisioned meta values for a post.
+ *
+ * @param int $post_id     The ID of the post to restore the meta to.
+ * @param int $revision_id The ID of the revision to restore the meta from.
+ *
+ * @since 6.4.0
+ */
+function wp_restore_post_revision_meta( $post_id, $revision_id ) {
+
+	// Restore revisioned meta fields.
+	foreach ( (array) wp_post_revision_meta_keys() as $meta_key ) {
+
+		// Clear any existing meta.
+		delete_post_meta( $post_id, $meta_key );
+
+		_wp_copy_post_meta( $revision_id, $post_id, $meta_key );
+	}
+}
+
+/**
+ * Copy post meta for the given key from one post to another.
+ *
+ * @param int    $source_post_id Post ID to copy meta value(s) from.
+ * @param int    $target_post_id Post ID to copy meta value(s) to.
+ * @param string $meta_key       Meta key to copy.
+ *
+ * @since 6.4.0
+ */
+function _wp_copy_post_meta( $source_post_id, $target_post_id, $meta_key ) {
+
+	foreach ( get_post_meta( $source_post_id, $meta_key ) as $meta_value ) {
+		/**
+		 * We use add_metadata() function vs add_post_meta() here
+		 * to allow for a revision post target OR regular post.
+		 */
+		add_metadata( 'post', $target_post_id, $meta_key, wp_slash( $meta_value ) );
+	}
+}
+
+/**
+ * Determine which post meta fields should be revisioned.
+ *
+ * @since 6.4.0
+ *
+ * @return array An array of meta keys to be revisioned.
+ */
+function wp_post_revision_meta_keys() {
+	/**
+	 * Filter the list of post meta keys to be revisioned.
+	 *
+	 * @since 6.4.0
+	 *
+	 * @param array $keys An array of default meta fields to be revisioned.
+	 */
+	return apply_filters( 'wp_post_revision_meta_keys', array( 'footnotes' ) ); // phpcs:ignore WordPress.NamingConventions.PrefixAllGlobals
+}
+
+/**
+ * Check whether revisioned post meta fields have changed.
+ *
+ * @param bool    $post_has_changed Whether the post has changed.
+ * @param WP_Post $last_revision    The last revision post object.
+ * @param WP_Post $post             The post object.
+ *
+ * @since 6.4.0
+ */
+function wp_check_revisioned_meta_fields_have_changed( $post_has_changed, WP_Post $last_revision, WP_Post $post ) {
+	foreach ( wp_post_revision_meta_keys() as $meta_key ) {
+		if ( get_post_meta( $post->ID, $meta_key ) !== get_post_meta( $last_revision->ID, $meta_key ) ) {
+			$post_has_changed = true;
+			break;
+		}
+	}
+	return $post_has_changed;
+}
+
 /**
  * Deletes a revision.
  *
@@ -728,6 +831,7 @@ function _set_preview( $post ) {
 
 	add_filter( 'get_the_terms', '_wp_preview_terms_filter', 10, 3 );
 	add_filter( 'get_post_metadata', '_wp_preview_post_thumbnail_filter', 10, 3 );
+	add_filter( 'get_post_metadata', '_wp_preview_meta_filter', 10, 4 );
 
 	return $post;
 }
@@ -948,3 +1052,38 @@ function _wp_upgrade_revisions_of_post( $post, $revisions ) {
 
 	return true;
 }
+
+/**
+ * Filters preview post meta retrieval to get values from the autosave.
+ *
+ * Filters revisioned meta keys only.
+ *
+ * @since 6.4.0
+ *
+ * @param mixed  $value     Meta value to filter.
+ * @param int    $object_id Object ID.
+ * @param string $meta_key  Meta key to filter a value for.
+ * @param bool   $single    Whether to return a single value. Default false.
+ * @return mixed Original meta value if the meta key isn't revisioned, the object doesn't exist,
+ *               the post type is a revision or the post ID doesn't match the object ID.
+ *               Otherwise, the revisioned meta value is returned for the preview.
+ */
+function _wp_preview_meta_filter( $value, $object_id, $meta_key, $single ) {
+
+	$post = get_post();
+	if (
+		empty( $post ) ||
+		$post->ID !== $object_id ||
+		! in_array( $meta_key, wp_post_revision_meta_keys(), true ) ||
+		'revision' === $post->post_type
+	) {
+		return $value;
+	}
+
+	$preview = wp_get_post_autosave( $post->ID );
+	if ( ! is_object( $preview ) ) {
+		return $value;
+	}
+
+	return get_post_meta( $preview->ID, $meta_key, $single );
+}