Site Editor: Resolve homepage template on server-side

Backports change from Gutenberg to support server-side home template resolution in the Site Editor. Original PR https://github.com/WordPress/gutenberg/pull/38817.

Props Mamaduka.
See #55505.




git-svn-id: https://develop.svn.wordpress.org/trunk@53093 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/block-template.php

@@ -331,3 +331,35 @@ function _resolve_template_for_new_post( $wp_query ) {
 		$wp_query->set( 'post_status', 'auto-draft' );
 	}
 }
+
+/**
+ * Returns the correct template for the site's home page.
+ *
+ * @access private
+ * @since 6.0.0
+ *
+ * @return array|null A template object, or null if none could be found.
+ */
+function _resolve_home_block_template() {
+	$show_on_front = get_option( 'show_on_front' );
+	$front_page_id = get_option( 'page_on_front' );
+
+	if ( 'page' === $show_on_front && $front_page_id ) {
+		return array(
+			'postType' => 'page',
+			'postId'   => $front_page_id,
+		);
+	}
+
+	$hierarchy = array( 'front-page', 'home', 'index' );
+	$template  = resolve_block_template( 'home', $hierarchy, '' );
+
+	if ( ! $template ) {
+		return null;
+	}
+
+	return array(
+		'postType' => 'wp_template',
+		'postId'   => $template->id,
+	);
+}