fix: streamed promise not resolving (#14753)

* test

* fix

* format

* changeset

* dont' hang

* DRY

* simpler version

Author: PatrickG

.changeset/floppy-mails-sing.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: streamed promise not resolving when another load function returns a fast resolving promise

packages/kit/src/utils/streaming.js

@@ -1,18 +1,4 @@
-/**
- * @returns {import('types').Deferred & { promise: Promise<any> }}}
- */
-function defer() {
-	let fulfil;
-	let reject;
-
-	const promise = new Promise((f, r) => {
-		fulfil = f;
-		reject = r;
-	});
-
-	// @ts-expect-error
-	return { promise, fulfil, reject };
-}
+import { with_resolvers } from './promise.js';
 
 /**
  * Create an async iterator and a function to push values into it
@@ -23,42 +9,32 @@ function defer() {
  * }}
  */
 export function create_async_iterator() {
-	let count = 0;
+	let resolved = -1;
+	let returned = -1;
 
-	const deferred = [defer()];
+	/** @type {import('./promise.js').PromiseWithResolvers<T>[]} */
+	const deferred = [];
 
 	return {
 		iterate: (transform = (x) => x) => {
 			return {
 				[Symbol.asyncIterator]() {
 					return {
 						next: async () => {
-							const next = await deferred[0].promise;
+							const next = deferred[++returned];
+							if (!next) return { value: null, done: true };
 
-							if (!next.done) {
-								deferred.shift();
-								return { value: transform(next.value), done: false };
-							}
-
-							return next;
+							const value = await next.promise;
+							return { value: transform(value), done: false };
 						}
 					};
 				}
 			};
 		},
 		add: (promise) => {
-			count += 1;
-
+			deferred.push(with_resolvers());
 			void promise.then((value) => {
-				deferred[deferred.length - 1].fulfil({
-					value,
-					done: false
-				});
-				deferred.push(defer());
-
-				if (--count === 0) {
-					deferred[deferred.length - 1].fulfil({ done: true });
-				}
+				deferred[++resolved].resolve(value);
 			});
 		}
 	};

packages/kit/test/apps/basics/src/routes/streaming/server/fast-n-slow/+layout.server.js

@@ -0,0 +1,3 @@
+export function load() {
+	return { fast: Promise.resolve('fast') };
+}

packages/kit/test/apps/basics/src/routes/streaming/server/fast-n-slow/+layout.svelte

@@ -0,0 +1,12 @@
+<script>
+	/** @type {import('./$types').LayoutData} */
+	export let data;
+</script>
+
+{#await data.fast}
+	<p class="loadingfast">Loading fast</p>
+{:then result}
+	<p class="fast">{result}</p>
+{/await}
+
+<slot />

packages/kit/test/apps/basics/src/routes/streaming/server/fast-n-slow/+page.server.js

@@ -0,0 +1,5 @@
+export async function load() {
+	const ssrd = await new Promise((r) => setTimeout(() => r('ssrd'), 100));
+	const streamed = new Promise((r) => setTimeout(() => r('streamed'), 200));
+	return { ssrd, streamed };
+}

packages/kit/test/apps/basics/src/routes/streaming/server/fast-n-slow/+page.svelte

@@ -0,0 +1,12 @@
+<script>
+	/** @type {import('./$types').PageData} */
+	export let data;
+</script>
+
+<p class="ssrd">{data.ssrd}</p>
+
+{#await data.streamed}
+	<p class="loadingstreamed">Loading streamed</p>
+{:then result}
+	<p class="streamed">{result}</p>
+{/await}

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

@@ -1308,6 +1308,16 @@ test.describe('Streaming', () => {
 			expect(page.locator('p.loadingfail')).toBeHidden();
 		});
 
+		test('Works with a fast and a slow server load functions which (direct hit)', async ({
+			page
+		}) => {
+			await page.goto('/streaming/server/fast-n-slow');
+
+			expect(await page.locator('p.ssrd').textContent()).toBe('ssrd');
+			await expect(page.locator('p.fast')).toHaveText('fast');
+			await expect(page.locator('p.streamed')).toHaveText('streamed');
+		});
+
 		test('Catches fetch errors from server load functions (direct hit)', async ({ page }) => {
 			page.goto('/streaming/server-error');
 			await expect(page.locator('p.eager')).toHaveText('eager');