FSE: Pre-populate template with fallback upon creation

lib/compat/wordpress-6.1/block-template-utils.php

@@ -165,6 +165,22 @@ function gutenberg_get_block_templates( $query = array(), $template_type = 'wp_t
 	return apply_filters( 'get_block_templates', $query_result, $query, $template_type );
 }
 
+function gutenberg_get_template_slugs( $template ) {
+	$limit = 2;
+	if ( strpos( $template, 'single-' ) === 0 || strpos( $template, 'taxonomy-' ) === 0 ) {
+		// E.g. single-post-mypost or taxonomy-recipes-vegetarian.
+		$limit = 3;
+	}
+	$parts = explode( '-', $template, $limit );
+	$type  = array_shift( $parts );
+	$slugs = array( $type );
+
+	foreach ( $parts as $part ) {
+		array_unshift( $slugs, $slugs[0] . '-' . $part );
+	}
+	return $slugs;
+}
+
 /**
  * Retrieves a single unified template object using its id.
  *

lib/compat/wordpress-6.1/class-gutenberg-rest-templates-controller.php

@@ -194,10 +194,17 @@ protected function prepare_item_for_database( $request ) {
 			$changes->ID          = $template->wp_id;
 			$changes->post_status = 'publish';
 		}
-		if ( isset( $request['content'] ) ) {
+		if ( isset( $request['content'] ) && ! empty( $request['content'] ) ) {
 			$changes->post_content = $request['content'];
 		} elseif ( null !== $template && 'custom' !== $template->source ) {
 			$changes->post_content = $template->content;
+		} elseif ( null === $template && empty( $request['content'] ) ) {
+			$fallback_templates = gutenberg_get_template_slugs( $request['slug'] );
+			$fallback_template  = resolve_block_template( $request['slug'], $fallback_templates, '' );
+			if ( ! $fallback_template ) {
+				$fallback_template = resolve_block_template( 'index', array(), '' );
+			}
+			$changes->post_content = $fallback_template->content;
 		}
 		if ( isset( $request['title'] ) ) {
 			$changes->post_title = $request['title'];

phpunit/compat/wordpress-6.1/block-template-utils-test.php

@@ -0,0 +1,26 @@
+<?php
+/**
+ * Tests_Block_Template_Utils class
+ *
+ * @package gutenberg
+ */
+
+/**
+ * Tests for the Block Templates abstraction layer.
+ *
+ * @group block-templates
+ */
+class Tests_Block_Template_Utils_6_1 extends WP_UnitTestCase {
+	public function test_get_template_slugs() {
+		$templates = gutenberg_get_template_slugs( 'single-post-hello-world' );
+		$this->assertSame(
+			array(
+				'single-post-hello-world',
+				// Should *not* fall back to `single-post-hello`!
+				'single-post',
+				'single',
+			),
+			$templates
+		);
+	}
+}