Footnotes: store in revisions

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

@@ -59,7 +59,7 @@ function render_block_core_footnotes( $attributes, $content, $block ) {
  * Registers the `core/footnotes` block on the server.
  */
 function register_block_core_footnotes() {
-	foreach ( array( 'post', 'page' ) as $post_type ) {
+	foreach ( array( 'post', 'page', 'revision' ) as $post_type ) {
 		register_post_meta(
 			$post_type,
 			'footnotes',
@@ -78,3 +78,45 @@ function register_block_core_footnotes() {
 	);
 }
 add_action( 'init', 'register_block_core_footnotes' );
+
+// Revisions must be created after post meta is updated, otherwise we don't have
+// access to the meta.
+remove_action( 'post_updated', 'wp_save_post_revision', 10, 1 );
+add_action( 'wp_after_insert_post', 'wp_save_post_revision', 10, 1 );
+
+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' );
+
+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 );
+
+function gutenberg_revision_fields( $fields ) {
+    $fields['footnotes'] = __( 'Footnotes' );
+    return $fields;
+}
+add_filter( '_wp_post_revision_fields', 'gutenberg_revision_fields' );
+
+function gutenberg_revision_field_footnotes( $value, $field ) {
+    global $revision;
+    return get_metadata( 'post', $revision->ID, $field, true );
+}
+add_filter( 'wp_post_revision_field_footnotes', 'gutenberg_revision_field_footnotes', 10, 2 );