fix: notify deriveds of changes to sources inside forks (#17437)

* failing tests

* update tests

* update write versions when committing forks

* remove second test, for now

* changeset

* fix: correctly update writable deriveds inside forks (#17438)

* fix: correctly update writable deriveds inside forks

* tweak

* changeset

* on second thoughts, minimise the diff, and revisit later

* missed a spot

.changeset/empty-paths-smile.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: notify deriveds of changes to sources inside forks

.changeset/seven-llamas-care.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: correctly update writable deriveds inside forks

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

@@ -25,6 +25,7 @@ import { deferred, define_property } from '../../shared/utils.js';
 import {
 	active_effect,
 	get,
+	increment_write_version,
 	is_dirty,
 	is_updating_effect,
 	set_is_updating_effect,
@@ -920,13 +921,18 @@ export function fork(fn) {
 
 	flushSync(fn);
 
-	batch_values = null;
-
 	// revert state changes
 	for (var [source, value] of batch.previous) {
 		source.v = value;
 	}
 
+	// make writable deriveds dirty, so they recalculate correctly
+	for (source of batch.current.keys()) {
+		if ((source.f & DERIVED) !== 0) {
+			set_signal_status(source, DIRTY);
+		}
+	}
+
 	return {
 		commit: async () => {
 			if (committed) {
@@ -942,9 +948,10 @@ export function fork(fn) {
 
 			batch.is_fork = false;
 
-			// apply changes
+			// apply changes and update write versions so deriveds see the change
 			for (var [source, value] of batch.current) {
 				source.v = value;
+				source.wv = increment_write_version();
 			}
 
 			// trigger any `$state.eager(...)` expressions with the new state.

packages/svelte/tests/runtime-runes/samples/async-fork-derived-writable/_config.js

@@ -0,0 +1,14 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target, logs }) {
+		const [btn] = target.querySelectorAll('button');
+
+		btn.click();
+		await tick();
+		// d should be 10 (real-world: s=1, d=1*10) before commit, not 20 (fork: s=2, d=2*10)
+		// After commit, d should be 99 (the written value)
+		assert.deepEqual(logs, [10, 99]);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-fork-derived-writable/main.svelte

@@ -0,0 +1,22 @@
+<script>
+	import { fork } from 'svelte';
+
+	let s = $state(1);
+	let d = $derived(s * 10);
+</script>
+
+<button onclick={async () => {
+	const f = fork(() => {
+		// First modify s, then write to d
+		// If d is evaluated in fork context, it would see s=2 and compute d=20
+		// But it should evaluate in real-world context to get d=10
+		s = 2;
+		d = 99;
+	});
+
+	// Should be 10 (real-world value: s=1, so d=1*10=10), not 20 (fork value)
+	console.log(d);
+	await f.commit();
+	// Should be 99 (the value we wrote)
+	console.log(d);
+}}>++</button>

packages/svelte/tests/runtime-runes/samples/async-fork-derived/_config.js

@@ -0,0 +1,16 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target, logs }) {
+		const [increment] = target.querySelectorAll('button');
+
+		increment.click();
+		await tick();
+		assert.deepEqual(logs, [1, 2]);
+
+		increment.click();
+		await tick();
+		assert.deepEqual(logs, [1, 2, 2, 3]);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-fork-derived/main.svelte

@@ -0,0 +1,16 @@
+<script>
+	import { fork } from 'svelte';
+
+	let s = $state(1);
+	let d = $derived(s);
+</script>
+
+<button onclick={async () => {
+	const f = fork(() => {
+		s = s + 1;
+	});
+
+	console.log(d);
+	await f.commit();
+	console.log(d);
+}}>++</button>