fix: ensure async initial store value is noticed

fixes #12341

Author: dummdidumm

.changeset/nasty-carrots-develop.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure async initial store value is noticed

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

@@ -97,11 +97,7 @@ export function set(source, value) {
 		// reactions as we only allocate and assign the reactions after the signal
 		// has fully executed. So in the case of ensuring it registers the reaction
 		// properly for itself, we need to ensure the current effect actually gets
-		// scheduled. i.e:
-		//
-		// $effect(() => x++)
-		//
-		// We additionally want to skip this logic when initialising store sources
+		// scheduled. i.e: `$effect(() => x++)`
 		if (
 			is_runes() &&
 			current_effect !== null &&

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

@@ -43,6 +43,11 @@ export function store_get(store, store_name, stores) {
 					set(entry.source, v);
 				}
 			});
+
+			// also set initial to false right afterwards in case the store subscription callback is async,
+			// in which case the `state_unsafe_mutation` check is not relevant anymore and we would instead
+			// not get notified of the store value change
+			initial = false;
 		}
 	}
 

packages/svelte/tests/runtime-runes/samples/store-async-first-value/_config.js

@@ -0,0 +1,10 @@
+import { test } from '../../test';
+
+export default test({
+	mode: ['client'],
+	async test({ assert, target }) {
+		assert.htmlEqual(target.innerHTML, ' / ');
+		await new Promise((r) => setTimeout(r, 110));
+		assert.htmlEqual(target.innerHTML, '42 / 42');
+	}
+});

packages/svelte/tests/runtime-runes/samples/store-async-first-value/main.svelte

@@ -0,0 +1,18 @@
+<script>
+	function store() {
+		return {
+			subscribe: (cb) => {
+				setTimeout(() => {
+					cb(42);
+				}, 100);
+
+				return () => {};
+			}
+		};
+	}
+
+	const value = store();
+	const derivedValue = $derived($value);
+</script>
+
+{$value} / {derivedValue}