delay initial async derived resolution if invalidated during init

packages/svelte/src/internal/client/reactivity/deriveds.js

@@ -30,6 +30,7 @@ import { tracing_mode_flag } from '../../flags/index.js';
 import { capture, suspend } from '../dom/blocks/boundary.js';
 import { component_context } from '../context.js';
 import { noop } from '../../shared/utils.js';
+import { UNINITIALIZED } from '../../../constants.js';
 
 /** @type {Effect | null} */
 export let from_async_derived = null;
@@ -99,23 +100,25 @@ export function async_derived(fn, detect_waterfall = true) {
 	}
 
 	var promise = /** @type {Promise<V>} */ (/** @type {unknown} */ (undefined));
-	var value = source(/** @type {V} */ (undefined));
+	var signal = source(/** @type {V} */ (UNINITIALIZED));
 
 	// only suspend in async deriveds created on initialisation
 	var should_suspend = !active_reaction;
 
+	/** @type {(() => void) | null} */
+	var unsuspend = null;
+
 	// TODO this isn't a block
-	block(async () => {
+	block(() => {
 		if (DEV) from_async_derived = active_effect;
 		var current = (promise = fn());
 		if (DEV) from_async_derived = null;
 
 		var restore = capture();
-		var unsuspend = should_suspend ? suspend() : noop;
+		if (should_suspend) unsuspend ??= suspend();
-
-		try {
-			var v = await promise;
 
+		promise.then(
+			(v) => {
 				if ((parent.f & DESTROYED) !== 0) {
 					return;
 				}
@@ -124,30 +127,38 @@ export function async_derived(fn, detect_waterfall = true) {
 					restore();
 					from_async_derived = null;
 
-				internal_set(value, v);
+					internal_set(signal, v);
 
 					if (DEV && detect_waterfall) {
-					recent_async_deriveds.add(value);
+						recent_async_deriveds.add(signal);
 
 						setTimeout(() => {
-						if (recent_async_deriveds.has(value)) {
+							if (recent_async_deriveds.has(signal)) {
 								w.await_waterfall();
-							recent_async_deriveds.delete(value);
+								recent_async_deriveds.delete(signal);
 							}
 						});
 					}
-			}
-		} catch (e) {
-			handle_error(e, parent, null, parent.ctx);
-		} finally {
-			unsuspend();
 
 					// TODO we should probably null out active effect here,
 					// rather than inside `restore()`
+					unsuspend?.();
+					unsuspend = null;
+				}
+			},
+			(e) => {
+				handle_error(e, parent, null, parent.ctx);
 			}
+		);
 	}, EFFECT_PRESERVED);
 
-	return Promise.resolve(promise).then(() => value);
+	return new Promise(async (fulfil) => {
+		// if the effect re-runs before the initial promise
+		// resolves, delay resolution until we have a value
+		var p;
+		while (p !== (p = promise)) await p;
+		fulfil(signal);
+	});
 }
 
 /**

packages/svelte/tests/runtime-runes/samples/async-derived-invalidation-during-init/Child.svelte

@@ -0,0 +1,9 @@
+<script>
+	let { promise } = $props();
+
+	let d = $derived({
+		value: await promise
+	});
+</script>
+
+<p>{(await d).value}</p>

packages/svelte/tests/runtime-runes/samples/async-derived-invalidation-during-init/_config.js

@@ -0,0 +1,43 @@
+import { flushSync, tick } from 'svelte';
+import { deferred } from '../../../../src/internal/shared/utils.js';
+import { test } from '../../test';
+
+/** @type {ReturnType<typeof deferred>} */
+let d1;
+
+export default test({
+	html: `<p>pending</p>`,
+
+	get props() {
+		d1 = deferred();
+
+		return {
+			promise: d1.promise
+		};
+	},
+
+	async test({ assert, target, component, errors }) {
+		await Promise.resolve();
+		var d2 = deferred();
+		component.promise = d2.promise;
+
+		d1.resolve('unused');
+		await Promise.resolve();
+		await Promise.resolve();
+		d2.resolve('hello');
+
+		await Promise.resolve();
+		await Promise.resolve();
+		await Promise.resolve();
+		await Promise.resolve();
+		await Promise.resolve();
+		await Promise.resolve();
+		await Promise.resolve();
+		await Promise.resolve();
+		await tick();
+
+		assert.htmlEqual(target.innerHTML, '<p>hello</p>');
+
+		assert.deepEqual(errors, []);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-derived-invalidation-during-init/main.svelte

@@ -0,0 +1,13 @@
+<script>
+	import Child from './Child.svelte';
+
+	let { promise } = $props();
+</script>
+
+<svelte:boundary>
+	<Child {promise} />
+
+	{#snippet pending()}
+		<p>pending</p>
+	{/snippet}
+</svelte:boundary>