more fork fixes

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

@@ -270,7 +270,8 @@ export class Batch {
 
 	/**
 	 * Async and block effects that ran or were proven clean inside this fork.
-	 * When the fork commits, only these effects can retain their speculative results
+	 * The version is that of the latest real-world execution when the effect was
+	 * validated. Fork executions do not advance it, so forks remain independent
 	 * @type {Map<Effect, number> | null}
 	 */
 	fork_effects = null;
@@ -531,15 +532,17 @@ export class Batch {
 		return false;
 	}
 
-	/** @param {Effect} effect */
+	/**
-	record_effect(effect) {
+	 * @param {Effect} effect
+	 * @param {boolean} [ran]
+	 */
+	record_effect(effect, ran = false) {
 		if ((effect.f & (ASYNC | BLOCK_EFFECT)) === 0) return;
 
-		var version = effect_version++;
-		effect_versions.set(effect, version);
-
 		if (this.is_fork) {
-			(this.fork_effects ??= new Map()).set(effect, version);
+			(this.fork_effects ??= new Map()).set(effect, effect_versions.get(effect) ?? 0);
+		} else if (ran) {
+			effect_versions.set(effect, effect_version++);
 		}
 	}
 
@@ -547,7 +550,7 @@ export class Batch {
 		if (this.fork_effects === null) return;
 
 		for (const [effect, version] of this.fork_effects) {
-			if ((effect.f & DESTROYED) !== 0 || version !== effect_versions.get(effect)) {
+			if ((effect.f & DESTROYED) !== 0 || version !== (effect_versions.get(effect) ?? 0)) {
 				this.#dirty_effects?.delete(effect);
 				this.#maybe_dirty_effects?.delete(effect);
 			}
@@ -556,7 +559,7 @@ export class Batch {
 		for (const [effect, version] of this.fork_effects) {
 			if ((effect.f & DESTROYED) !== 0) continue;
 
-			if (version !== effect_versions.get(effect)) {
+			if (version !== (effect_versions.get(effect) ?? 0)) {
 				var owner = effect.batch;
 				while (owner !== null && owner.merged_into !== null) owner = owner.merged_into;
 
@@ -1079,6 +1082,8 @@ export class Batch {
 	}
 
 	discard() {
+		this.fork_effects = null;
+
 		if (this.#discard_callbacks !== null) {
 			for (const fn of this.#discard_callbacks) fn(this);
 			this.#discard_callbacks = null;
@@ -1097,6 +1102,44 @@ export class Batch {
 	}
 
 	#commit() {
+		/** @type {Map<Batch, Effect[]> | null} */
+		var forks = null;
+
+		// A real-world run supersedes the same effect's speculative validation.
+		// Once that run commits, revalidate each affected fork against the latest
+		// real state, while keeping the fork's own writes isolated.
+		for (var fork = first_batch; fork !== null; fork = fork.#next) {
+			if (!fork.is_fork || fork.fork_effects === null) continue;
+
+			for (const [effect, version] of fork.fork_effects) {
+				if (
+					version === (effect_versions.get(effect) ?? 0) ||
+					(effect.f & (DESTROYED | INERT)) !== 0
+				) {
+					continue;
+				}
+
+				var owner = effect.batch && effect.batch.#resolved();
+				if (owner !== this) continue;
+
+				set_signal_status(effect, DIRTY);
+				fork.schedule(effect);
+				var effects = (forks ??= new Map()).get(fork);
+				if (effects === undefined) forks.set(fork, [effect]);
+				else effects.push(effect);
+			}
+		}
+
+		if (forks !== null) {
+			for (const [fork, effects] of forks) {
+				queue_micro_task(() => {
+					if (!fork.linked) return;
+					for (const effect of effects) set_signal_status(effect, DIRTY);
+					fork.flush();
+				});
+			}
+		}
+
 		if (this.stale_readers === null) return;
 
 		var readers = this.stale_readers;
@@ -1226,7 +1269,9 @@ export class Batch {
 		// changes, we should only see values that have been committed. Overlapping
 		// batches are merged unless one is waiting behind a sealed predecessor
 		for (let other = first_batch; other !== null; other = other.#next) {
-			if (other === batch || other.is_fork) continue;
+			// Forks are based on the latest real world, not an independently
+			// held-back snapshot of it. Other forks remain separate worlds.
+			if (other === batch || other.is_fork || batch.is_fork) continue;
 
 			for (const [source, previous] of other.previous) {
 				if (!values.has(source)) {
@@ -1255,12 +1300,10 @@ export class Batch {
 	schedule(effect) {
 		last_scheduled_effect = effect;
 
-		if ((effect.f & (ASYNC | BLOCK_EFFECT)) !== 0) {
+		if (this.is_fork && (effect.f & (ASYNC | BLOCK_EFFECT)) !== 0) {
-			effect_versions.set(effect, effect_version++);
+			this.fork_effects?.delete(effect);
 		}
 
-		if (this.is_fork) this.fork_effects?.delete(effect);
-
 		if (this.claim(effect)) {
 			this.#dirty_effects ??= new Set();
 			this.#maybe_dirty_effects ??= new Set();
@@ -1596,7 +1639,8 @@ function mark_committed_reactions(value, batch, marked, status) {
 			while (owner !== null && owner.merged_into !== null) owner = owner.merged_into;
 
 			var fork_version = batch.fork_effects?.get(effect);
-			var validated = fork_version !== undefined && fork_version === effect_versions.get(effect);
+			var validated =
+				fork_version !== undefined && fork_version === (effect_versions.get(effect) ?? 0);
 			var stale =
 				batch.stale_readers?.has(effect) === true || owner?.stale_readers?.has(effect) === true;
 
@@ -1734,9 +1778,9 @@ export function fork(fn) {
 
 			// Marking can entangle the fork with newer work and replace values in
 			// `current`, so only write through once the final world is known.
-			for (var [source, value] of batch.current) {
+			for (var [signal, value] of batch.current) {
-				source.v = value;
+				signal.v = value;
-				source.wv = increment_write_version();
+				signal.wv = increment_write_version();
 			}
 
 			// Effects retained from the speculative world now belong to the real

packages/svelte/src/internal/client/runtime.js

@@ -487,7 +487,7 @@ export function update_effect(effect) {
 		var teardown = update_reaction(effect);
 		effect.teardown = typeof teardown === 'function' ? teardown : null;
 		effect.wv = write_version;
-		batch?.record_effect(effect);
+		batch?.record_effect(effect, true);
 
 		// In DEV, increment versions of any sources that were written to during the effect,
 		// so that they are correctly marked as dirty when the effect re-runs

packages/svelte/tests/runtime-runes/samples/async-fork-effect-overlap-1/_config.js

@@ -0,0 +1,35 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target, logs }) {
+		await tick();
+		const [x, y, shift, pop, commit] = target.querySelectorAll('button');
+		const [p] = target.querySelectorAll('p');
+		logs.length = 0;
+
+		x.click();
+		await tick();
+		assert.deepEqual(logs, ['called with 1,0']);
+		logs.length = 0;
+
+		y.click();
+		await tick();
+		assert.deepEqual(logs, ['called with 0,1']);
+		assert.htmlEqual(p.innerHTML, '0');
+		logs.length = 0;
+
+		pop.click();
+		await tick();
+		assert.deepEqual(logs, ['called with 1,1']);
+		assert.htmlEqual(p.innerHTML, '1');
+		logs.length = 0;
+
+		commit.click();
+		await tick();
+		pop.click();
+		await tick();
+		assert.deepEqual(logs, []);
+		assert.htmlEqual(p.innerHTML, '2');
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-fork-effect-overlap-1/main.svelte

@@ -0,0 +1,23 @@
+<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>
+
+<button onclick={() => {f = fork(() => x++)}}>x</button>
+<button onclick={() => y++}>y</button>
+<button onclick={() => deferred.shift()?.()}>shift</button>
+<button onclick={() => deferred.pop()?.()}>pop</button>
+<button onclick={() => f.commit()}>commit</button>
+
+<p>{await delay(console.log('called with ' + x + ',' + y), x + y)}</p>

packages/svelte/tests/runtime-runes/samples/async-fork-effect-overlap-2/_config.js

@@ -0,0 +1,34 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target, logs }) {
+		await tick();
+		const [x, y, shift, pop, commit] = target.querySelectorAll('button');
+		const [p] = target.querySelectorAll('p');
+		logs.length = 0;
+
+		y.click();
+		await tick();
+		assert.deepEqual(logs, ['called with 0,1']);
+		logs.length = 0;
+
+		x.click();
+		await tick();
+		assert.deepEqual(logs, ['called with 1,1']);
+		assert.htmlEqual(p.innerHTML, '0');
+		logs.length = 0;
+
+		shift.click();
+		await tick();
+		assert.deepEqual(logs, []);
+		assert.htmlEqual(p.innerHTML, '1');
+
+		commit.click();
+		await tick();
+		pop.click();
+		await tick();
+		assert.deepEqual(logs, []);
+		assert.htmlEqual(p.innerHTML, '2');
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-fork-effect-overlap-2/main.svelte

@@ -0,0 +1,23 @@
+<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>
+
+<button onclick={() => {f = fork(() => x++)}}>x</button>
+<button onclick={() => y++}>y</button>
+<button onclick={() => deferred.shift()?.()}>shift</button>
+<button onclick={() => deferred.pop()?.()}>pop</button>
+<button onclick={() => f.commit()}>commit</button>
+
+<p>{await delay(console.log('called with ' + x + ',' + y), x + y)}</p>

packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-fork-1/_config.js

@@ -27,6 +27,11 @@ export default test({
 		await tick();
 		assert.htmlEqual(p.innerHTML, 'a 1 | b 0 | c 1');
 
+		// resolve the fork revalidation triggered by the real batch settling
+		pop.click();
+		await tick();
+		assert.htmlEqual(p.innerHTML, 'a 1 | b 0 | c 1');
+
 		commit.click();
 		await tick();
 		assert.htmlEqual(p.innerHTML, 'a 1 | b 1 | c 1');

packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-fork-2/_config.js

@@ -23,6 +23,10 @@ export default test({
 		await tick();
 		assert.htmlEqual(p.innerHTML, 'a 0 | b 1 | c 0 | d 1');
 
+		pop.click();
+		await tick();
+		assert.htmlEqual(p.innerHTML, 'a 0 | b 1 | c 0 | d 1');
+
 		pop.click();
 		await tick();
 		assert.htmlEqual(p.innerHTML, 'a 1 | b 1 | c 1 | d 1');
@@ -35,6 +39,11 @@ export default test({
 		await tick();
 		assert.htmlEqual(p.innerHTML, 'a 1 | b 1 | c 1 | d 1');
 
+		// resolve the fork revalidation triggered by the real batch settling
+		pop.click();
+		await tick();
+		assert.htmlEqual(p.innerHTML, 'a 1 | b 1 | c 1 | d 1');
+
 		commit.click();
 		await tick();
 		assert.htmlEqual(p.innerHTML, 'a 1 | b 1 | c 1 | d 1');

packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-fork-3/_config.js

@@ -35,6 +35,13 @@ export default test({
 		await tick();
 		assert.htmlEqual(p.innerHTML, 'a 1 | b 1 | c 1 | d 1');
 
+		// resolve the fork revalidations triggered by the real batches settling
+		shift.click();
+		await tick();
+		shift.click();
+		await tick();
+		assert.htmlEqual(p.innerHTML, 'a 1 | b 1 | c 1 | d 1');
+
 		commit.click();
 		await tick();
 		assert.htmlEqual(p.innerHTML, 'a 1 | b 1 | c 1 | d 1');