fix: handle renderer.run rejections (#17591)

* fix: handle renderer run rejections

* add test

* changeset

* simplify

* explanatory comment

---------

Co-authored-by: Antonio Bennett <abennett@mabelslabels.com>
Co-authored-by: Rich Harris <rich.harris@vercel.com>

.changeset/tidy-zebras-design.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: prevent unhandled exceptions arising from dangling promises in <script>

packages/svelte/src/internal/server/renderer.js

@@ -11,6 +11,7 @@ import { attributes } from './index.js';
 import { get_render_context, with_render_context, init_render_context } from './render-context.js';
 import { sha256 } from './crypto.js';
 import * as devalue from 'devalue';
+import { noop } from '../shared/utils.js';
 
 /** @typedef {'head' | 'body'} RendererType */
 /** @typedef {{ [key in RendererType]: string }} AccumulatedContent */
@@ -162,6 +163,11 @@ export class Renderer {
 			promises.push(promise);
 		}
 
+		// prevent unhandled rejections, and attach the promise to the renderer instance
+		// so that rejections correctly cause rendering to fail
+		promise.catch(noop);
+		this.promise = promise;
+
 		return promises;
 	}
 

packages/svelte/tests/runtime-runes/samples/async-error-in-script/_config.js

@@ -0,0 +1,6 @@
+import { test } from '../../test';
+
+export default test({
+	mode: ['async-server', 'hydrate'],
+	error: 'oops'
+});

packages/svelte/tests/runtime-runes/samples/async-error-in-script/main.svelte

@@ -0,0 +1,7 @@
+<script>
+	await 1;
+
+	throw new Error('oops');
+</script>
+
+<h1>hello</h1>