fix: don't wait for remote functions that are not awaited in the template

Fixes #15200

Author: dummdidumm

.changeset/large-squids-pull.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: don't wait for remote functions that are not awaited in the template

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

@@ -493,7 +493,22 @@ export async function render_response({
 				if (!info.id) continue;
 
 				for (const key in cache) {
-					remote[create_remote_key(info.id, key)] = await cache[key];
+					// Don't block the response on pending remote data — if a query
+					// hasn't settled yet (e.g. caught by a svelte:boundary), skip it
+					// and let the client fetch it instead via the remote function stub.
+					const result = await Promise.race([
+						Promise.resolve(cache[key]).then(
+							(v) => /** @type {const} */ ({ settled: true, value: v }),
+							() => /** @type {const} */ ({ settled: false })
+						),
+						new Promise((resolve) => {
+							queueMicrotask(() => resolve(/** @type {const} */ ({ settled: false })));
+						})
+					]);
+
+					if (result.settled) {
+						remote[create_remote_key(info.id, key)] = result.value;
+					}
 				}
 			}
 

packages/kit/test/apps/async/src/routes/remote/query-boundary/+page.svelte

@@ -0,0 +1,14 @@
+<script>
+	import { get_delayed_data, get_fast_data } from './data.remote.js';
+
+	// shouldn't block SSR since requested immediately but awaited inside pending boundary
+	const delayed = get_delayed_data();
+</script>
+
+<p id="fast-result">{await get_fast_data()}</p>
+<svelte:boundary>
+	{#snippet pending()}
+		<p id="delayed-pending">loading delayed</p>
+	{/snippet}
+	<p id="delayed-result">{await delayed}</p>
+</svelte:boundary>

packages/kit/test/apps/async/src/routes/remote/query-boundary/data.remote.js

@@ -0,0 +1,10 @@
+import { query } from '$app/server';
+
+export const get_delayed_data = query(async () => {
+	await new Promise((resolve) => setTimeout(resolve, 2000));
+	return 'delayed data';
+});
+
+export const get_fast_data = query(() => {
+	return 'fast data';
+});

packages/kit/test/apps/async/test/test.js

@@ -482,4 +482,19 @@ test.describe('remote functions', () => {
 			})
 		);
 	});
+
+	test('query stored as variable does not block SSR inside boundary', async ({
+		page,
+		javaScriptEnabled
+	}) => {
+		await page.goto('/remote/query-boundary');
+
+		await expect(page.locator('#delayed-pending')).toHaveText('loading delayed');
+
+		if (javaScriptEnabled) {
+			await expect(page.locator('#delayed-result')).toHaveText('delayed data', {
+				timeout: 5000
+			});
+		}
+	});
 });