Footnotes: store in revisions

packages/block-library/src/footnotes/index.php

@@ -78,3 +78,114 @@ function register_block_core_footnotes() {
 	);
 }
 add_action( 'init', 'register_block_core_footnotes' );
+
+/**
+ * Saves the footnotes meta value to the revision.
+ *
+ * @param int $revision_id The revision ID.
+ */
+function gutenberg_save_footnotes_meta( $revision_id ) {
+	$post_id = wp_is_post_revision( $revision_id );
+
+	if ( $post_id ) {
+		$footnotes = get_post_meta( $post_id, 'footnotes', true );
+
+		if ( $footnotes ) {
+			// Can't use update_post_meta() because it doesn't allow revisions.
+			update_metadata( 'post', $revision_id, 'footnotes', $footnotes );
+		}
+	}
+}
+add_action( 'wp_after_insert_post', 'gutenberg_save_footnotes_meta' );
+
+/**
+ * Keeps track of the revision ID for "rest_after_insert_{$post_type}".
+ *
+ * @param int $revision_id The revision ID.
+ */
+function gutenberg_wp_put_post_revision( $revision_id ) {
+	global $_gutenberg_revision_id;
+	$_gutenberg_revision_id = $revision_id;
+}
+
+add_action( '_wp_put_post_revision', 'gutenberg_wp_put_post_revision' );
+
+/**
+ * This is a specific fix for the REST API. The REST API doesn't update the post
+ * and post meta in one go (through `meta_input`). While it does fix the
+ * `wp_after_insert_post` hook to be called correctly after updating meta, it
+ * does NOT fix hooks such as post_updated and save_post, which are normally
+ * also fired after post meta is updated in `wp_insert_post()`. Unfortunately,
+ * `wp_save_post_revision` is added to the `post_updated` action, which means
+ * the meta is not available at the time, so we have to add it afterwards
+ * through the `"rest_after_insert_{$post_type}"` action.
+ *
+ * @param WP_Post $post The post object.
+ */
+function gutenberg_save_footnotes_meta_rest_api( $post ) {
+	global $_gutenberg_revision_id;
+
+	if ( $_gutenberg_revision_id ) {
+		$revision = get_post( $_gutenberg_revision_id );
+		$post_id  = $revision->post_parent;
+
+		// Just making sure we're updating the right revision.
+		if ( $post->ID === $post_id ) {
+			$footnotes = get_post_meta( $post_id, 'footnotes', true );
+
+			if ( $footnotes ) {
+				// Can't use update_post_meta() because it doesn't allow revisions.
+				update_metadata( 'post', $_gutenberg_revision_id, 'footnotes', $footnotes );
+			}
+		}
+	}
+}
+
+foreach ( array( 'post', 'page' ) as $post_type ) {
+	add_action( "rest_after_insert_{$post_type}", 'gutenberg_save_footnotes_meta_rest_api' );
+}
+
+/**
+ * Restores the footnotes meta value from the revision.
+ *
+ * @param int $post_id      The post ID.
+ * @param int $revision_id  The revision ID.
+ */
+function gutenberg_restore_footnotes_meta( $post_id, $revision_id ) {
+	$footnotes = get_post_meta( $revision_id, 'footnotes', true );
+
+	if ( $footnotes ) {
+		update_post_meta( $post_id, 'footnotes', $footnotes );
+	} else {
+		delete_post_meta( $post_id, 'footnotes' );
+	}
+}
+add_action( 'wp_restore_post_revision', 'gutenberg_restore_footnotes_meta', 10, 2 );
+
+/**
+ * Adds the footnotes field to the revision.
+ *
+ * @param array $fields The revision fields.
+ *
+ * @return array The revision fields.
+ */
+function gutenberg_revision_fields( $fields ) {
+	$fields['footnotes'] = __( 'Footnotes' );
+	return $fields;
+}
+add_filter( '_wp_post_revision_fields', 'gutenberg_revision_fields' );
+
+/**
+ * Gets the footnotes field from the revision.
+ *
+ * @param string $revision_field The field value, but $revision->$field
+ *                               (footnotes) does not exist.
+ * @param string $field          The field name, in this case "footnotes".
+ * @param object $revision       The revision object to compare against.
+ *
+ * @return string The field value.
+ */
+function gutenberg_revision_field_footnotes( $revision_field, $field, $revision ) {
+	return get_metadata( 'post', $revision->ID, $field, true );
+}
+add_filter( 'wp_post_revision_field_footnotes', 'gutenberg_revision_field_footnotes' );