another fork fix

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

@@ -21,7 +21,7 @@ import {
 	FORK_ONLY_BRANCH
 } from '#client/constants';
 import { async_mode_flag } from '../../flags/index.js';
-import { deferred, define_property, includes } from '../../shared/utils.js';
+import { deferred, define_property } from '../../shared/utils.js';
 import {
 	active_reaction,
 	get,
@@ -911,11 +911,10 @@ export class Batch {
 				!is_derived &&
 				(!current || current.v !== value) &&
 				((source.f & ASYNC) === 0 ||
-					!depends_on(
-						/** @type {Effect} */ (/** @type {Source} */ (source).e),
-						[...batch.current.keys()].filter((s) => !this.current.has(s)),
-						new Map()
-					))
+					// If the fork ran an async effect, its pending/resolved result belongs to the
+					// fork. Revalidate it when its inputs change, not when another batch resolves
+					// the same expression with a different view of those inputs.
+					!batch.#stale_effects?.has(/** @type {Effect} */ (/** @type {Source} */ (source).e)))
 			) {
 				batch.current.delete(source);
 				batch.queue_revalidation(source);
@@ -1387,33 +1386,6 @@ function mark_eager_effects(value, effects) {
 	}
 }
 
-/**
- * @param {Reaction} reaction
- * @param {Value[]} sources
- * @param {Map<Reaction, boolean>} checked
- */
-function depends_on(reaction, sources, checked) {
-	const depends = checked.get(reaction);
-	if (depends !== undefined) return depends;
-
-	if (reaction.deps !== null) {
-		for (const dep of reaction.deps) {
-			if (includes.call(sources, dep)) {
-				return true;
-			}
-
-			if ((dep.f & DERIVED) !== 0 && depends_on(/** @type {Derived} */ (dep), sources, checked)) {
-				checked.set(/** @type {Derived} */ (dep), true);
-				return true;
-			}
-		}
-	}
-
-	checked.set(reaction, false);
-
-	return false;
-}
-
 /**
  * @param {Effect} effect
  * @returns {void}
@@ -1623,8 +1595,8 @@ export function fork(fn) {
 
 			// Apply changes and update write versions so deriveds see the change. Everything still
 			// in `batch.current` at this point is the latest value: sources that the real world has
-			// written to in the meantime were removed from the fork via `notify_fork` (an async
-			// source only survives if its effect depends on inputs that only the fork changed).
+			// written to in the meantime were removed from the fork via `notify_fork`, while
+			// async results are kept up to date by revalidating their producers when inputs change.
 			// We use fresh versions rather than the fork-time `content.wv`, because the real world
 			// may have run reactions since then whose versions would otherwise outrank them.
 			for (var [source, content] of batch.current) {

packages/svelte/tests/runtime-runes/samples/async-fork-effect-overlap-chain/_config.js

@@ -0,0 +1,48 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target, logs }) {
+		await tick();
+		const [fork, update, pop, commit] = target.querySelectorAll('button');
+		const [sum, doubled] = target.querySelectorAll('p');
+		logs.length = 0;
+
+		fork.click();
+		await tick();
+		assert.deepEqual(logs, ['sum 1,0']);
+		logs.length = 0;
+
+		// Revalidate twice to also check that retaining async results does not
+		// prevent the fork from responding to genuine changes to its inputs.
+		for (const y of [1, 2]) {
+			update.click();
+			await tick();
+			assert.deepEqual(logs, [`sum 0,${y}`, `sum 1,${y}`]);
+			logs.length = 0;
+
+			pop.click(); // resolve the fork before the real world
+			await tick();
+			assert.deepEqual(logs, [`double ${y + 1}`]);
+			logs.length = 0;
+
+			pop.click();
+			await tick();
+			assert.deepEqual(logs, [`double ${y}`]);
+			assert.htmlEqual(sum.innerHTML, String(y));
+			assert.htmlEqual(doubled.innerHTML, String(y * 2));
+			logs.length = 0;
+		}
+
+		commit.click();
+		await tick();
+		assert.deepEqual(logs, []);
+		assert.htmlEqual(sum.innerHTML, '3');
+		assert.htmlEqual(doubled.innerHTML, '6');
+
+		pop.click(); // the superseded first fork run
+		await tick();
+		assert.htmlEqual(sum.innerHTML, '3');
+		assert.htmlEqual(doubled.innerHTML, '6');
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-fork-effect-overlap-chain/main.svelte

@@ -0,0 +1,29 @@
+<script>
+	import { fork } from 'svelte';
+
+	let x = $state(0);
+	let y = $state(0);
+	let f;
+	const deferred = [];
+
+	function delay(x, y) {
+		console.log(`sum ${x},${y}`);
+		const value = x + y;
+		return value ? new Promise((resolve) => deferred.push(() => resolve(value))) : value;
+	}
+
+	async function double(value) {
+		console.log(`double ${value}`);
+		return value * 2;
+	}
+
+	let sum = $derived(await delay(x, y));
+</script>
+
+<button onclick={() => { f = fork(() => x++); }}>fork</button>
+<button onclick={() => y++}>update</button>
+<button onclick={() => deferred.pop()?.()}>pop</button>
+<button onclick={() => f.commit()}>commit</button>
+
+<p>{sum}</p>
+<p>{await double(sum)}</p>