fix: handle rejected streamed data race condition

.changeset/quiet-masks-shop.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: handle rejected streamed server data after delayed loads

packages/kit/src/runtime/server/data/index.js

@@ -96,29 +96,25 @@ export async function render_data(
 			return fn();
 		});
 
-		let length = promises.length;
-		const nodes = await Promise.all(
-			promises.map((p, i) =>
-				p.catch(async (error) => {
+		const data_serializer = server_data_serializer_json(event, event_state, options);
+		await Promise.all(
+			promises.map(async (p, i) => {
+				const node = await p.catch(async (error) => {
 					if (error instanceof Redirect) {
 						throw error;
 					}
 
-					// Math.min because array isn't guaranteed to resolve in order
-					length = Math.min(length, i + 1);
-
 					const transformed = await handle_error_and_jsonify(event, event_state, options, error);
 
 					return /** @type {import('types').ServerErrorNode} */ ({
 						type: 'error',
 						error: transformed
 					});
-				})
-			)
-		);
+				});
 
-		const data_serializer = server_data_serializer_json(event, event_state, options);
-		for (let i = 0; i < nodes.length; i++) data_serializer.add_node(i, nodes[i]);
+				data_serializer.add_node(i, node);
+			})
+		);
 		const { data, chunks } = data_serializer.get_data();
 
 		if (!chunks) {

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

@@ -1,3 +1,4 @@
 <a href="/streaming/universal">Universal</a>
 <a href="/streaming/server">Server</a>
 <a href="/streaming/server-error">Server Error</a>
+<a href="/streaming/server/delayed-rejection">Server Delayed Rejection</a>

packages/kit/test/apps/basics/src/routes/streaming/server/delayed-rejection/+layout.server.js

@@ -0,0 +1,4 @@
+export async function load() {
+	await new Promise((resolve) => setTimeout(resolve, 100));
+	return {};
+}

packages/kit/test/apps/basics/src/routes/streaming/server/delayed-rejection/+page.server.js

@@ -0,0 +1,6 @@
+export function load() {
+	return {
+		eager: 'eager',
+		streamed: Promise.reject(new Error('delayed rejection'))
+	};
+}

packages/kit/test/apps/basics/src/routes/streaming/server/delayed-rejection/+page.svelte

@@ -0,0 +1,12 @@
+<script>
+	/** @type {import('./$types').PageData} */
+	export let data;
+</script>
+
+<p class="eager">{data.eager}</p>
+
+{#await data.streamed}
+	<p class="loading">Loading</p>
+{:catch error}
+	<p class="fail">{error.message}</p>
+{/await}

packages/kit/test/apps/basics/test/client.test.js

@@ -902,7 +902,7 @@
 		expect(responses.length).toEqual(1);
 	});
 
 	test('data-sveltekit-preload-data', async ({ page }) => {
 		/** @type {string[]} */
 		const requests = [];
 		page.on('request', (req) => {
@@ -1362,6 +1362,16 @@
 		expect(page.locator('p.fail')).toBeVisible();
 	});
 
+	test('Catches rejected streamed server data after another load delays data serialization', async ({
+		page
+	}) => {
+		await page.goto('/streaming');
+		await page.click('[href="/streaming/server/delayed-rejection"]');
+
+		await expect(page.locator('p.eager')).toHaveText('eager');
+		await expect(page.locator('p.fail')).toHaveText('delayed rejection (500 Internal Error)');
+	});
+
 	// TODO `vite preview` buffers responses, causing these tests to fail
 	if (process.env.DEV) {
 		test('Works for universal load functions (direct hit)', async ({ page }) => {