fix: never set derived.v inside fork (#18037)

This started out as me implementing
https://github.com/sveltejs/svelte/pull/17998/changes#r3018047965, but
then I realised that I'd also fixed the bug that #17998 addresses. So I
guess it's an alternative to that PR

---------

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

.changeset/poor-tips-send.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: never set derived.v inside fork

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

@@ -419,18 +419,22 @@ export class Batch {
 	 * Associate a change to a given source with the current
 	 * batch, noting its previous and current values
 	 * @param {Value} source
-	 * @param {any} old_value
+	 * @param {any} value
 	 * @param {boolean} [is_derived]
 	 */
-	capture(source, old_value, is_derived = false) {
-		if (old_value !== UNINITIALIZED && !this.previous.has(source)) {
-			this.previous.set(source, old_value);
+	capture(source, value, is_derived = false) {
+		if (source.v !== UNINITIALIZED && !this.previous.has(source)) {
+			this.previous.set(source, source.v);
 		}
 
 		// Don't save errors in `batch_values`, or they won't be thrown in `runtime.js#get`
 		if ((source.f & ERROR_VALUE) === 0) {
-			this.current.set(source, [source.v, is_derived]);
-			batch_values?.set(source, source.v);
+			this.current.set(source, [value, is_derived]);
+			batch_values?.set(source, value);
+		}
+
+		if (!this.is_fork) {
+			source.v = value;
 		}
 	}
 
@@ -1162,11 +1166,6 @@ export function fork(fn) {
 
 	flushSync(fn);
 
-	// revert state changes
-	for (var [source, value] of batch.previous) {
-		source.v = value;
-	}
-
 	return {
 		commit: async () => {
 			if (committed) {

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

@@ -384,7 +384,6 @@ export function execute_derived(derived) {
  * @returns {void}
  */
 export function update_derived(derived) {
-	var old_value = derived.v;
 	var value = execute_derived(derived);
 
 	if (!derived.equals(value)) {
@@ -395,8 +394,11 @@ export function update_derived(derived) {
 		// otherwise, the next time we get here after a 'real world' state
 		// change, `derived.equals` may incorrectly return `true`
 		if (!current_batch?.is_fork || derived.deps === null) {
-			derived.v = value;
-			current_batch?.capture(derived, old_value, true);
+			if (current_batch !== null) {
+				current_batch.capture(derived, value, true);
+			} else {
+				derived.v = value;
+			}
 
 			// deriveds without dependencies should never be recomputed
 			if (derived.deps === null) {

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

@@ -180,18 +180,10 @@ export function set(source, value, should_proxy = false) {
  */
 export function internal_set(source, value, updated_during_traversal = null) {
 	if (!source.equals(value)) {
-		var old_value = source.v;
-
-		if (is_destroying_effect) {
-			old_values.set(source, value);
-		} else {
-			old_values.set(source, old_value);
-		}
-
-		source.v = value;
+		old_values.set(source, is_destroying_effect ? value : source.v);
 
 		var batch = Batch.ensure();
-		batch.capture(source, old_value);
+		batch.capture(source, value);
 
 		if (DEV) {
 			if (tracing_mode_flag || active_effect !== null) {

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

@@ -0,0 +1,22 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+		const [x, y, resolve, commit] = target.querySelectorAll('button');
+		const [p] = target.querySelectorAll('p');
+
+		y.click();
+		await tick();
+		resolve.click();
+		await tick();
+		x.click();
+		await tick();
+		assert.htmlEqual(p.innerHTML, '1 0');
+
+		await tick();
+		commit.click();
+		assert.htmlEqual(p.innerHTML, '1 1');
+	}
+});

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

@@ -0,0 +1,21 @@
+<script>
+	import { fork } from 'svelte';
+
+	let x = $state(0);
+	let y = $state(0);
+	let f;
+
+	const deferred = [];
+
+	function delay(value) {
+		if (!value) return value;
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+</script>
+
+<p>{x} {await delay(y)}</p>
+
+<button onclick={() => x += 1}>x</button>
+<button onclick={() => f = fork(() => y += 1)}>y (fork)</button>
+<button onclick={() => deferred.shift()?.()}>resolve</button>
+<button onclick={() => f.commit()}>commit</button>