fix: error pages with discarded promises fail to load (#14722)

* test

* fix

* changeset

* Update .changeset/clear-vans-sniff.md

Co-authored-by: Tee Ming <[email protected]>

* move test

* format

* move test back

---------

Co-authored-by: Tee Ming <[email protected]>

Author: PatrickG

.changeset/clear-vans-sniff.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: avoid hanging when `error()` is used while streaming promises from a server `load` function

packages/kit/src/runtime/server/page/render.js

@@ -227,7 +227,6 @@ export async function render_response({
 					paths.reset();
 				}
 
-				// eslint-disable-next-line
 				const { head, html, css } = options.async ? await rendered : rendered;
 
 				return { head, html, css };
@@ -654,7 +653,7 @@ export async function render_response({
 					async start(controller) {
 						controller.enqueue(text_encoder.encode(transformed + '\n'));
 						for await (const chunk of chunks) {
-							controller.enqueue(text_encoder.encode(chunk));
+							if (chunk.length) controller.enqueue(text_encoder.encode(chunk));
 						}
 						controller.close();
 					},

packages/kit/test/apps/basics/src/routes/streaming/discarded-promise/+layout.server.js

@@ -0,0 +1,5 @@
+export function load() {
+	return {
+		promise: Promise.resolve(42)
+	};
+}

packages/kit/test/apps/basics/src/routes/streaming/discarded-promise/+page.server.js

@@ -0,0 +1,5 @@
+import { error } from '@sveltejs/kit';
+
+export function load() {
+	error(404);
+}

packages/kit/test/apps/basics/src/routes/streaming/discarded-promise/+page.svelte

@@ -1408,3 +1408,18 @@ test.describe('asset preload', () => {
 		});
 	}
 });
+
+test.describe('Streaming', () => {
+	test("Discarded promises from server load functions don't hang SSR request", async ({
+		request
+	}) => {
+		let error;
+		try {
+			await request.get('/streaming/discarded-promise');
+		} catch (e) {
+			error = e;
+		}
+
+		expect(error).toBeUndefined();
+	});
+});