fix: increment signal versions when discarding forks (#17577)

* fix: always update an UNINITIALIZED derived on read

Co-authored-by: David Roizenman <hmnd@users.noreply.github.com>

* obsolete comments

* rename test

* add tests, fix

* revert

* Update .changeset/vast-hornets-draw.md

---------

Co-authored-by: David Roizenman <hmnd@users.noreply.github.com>

Author: Rich-Harris

.changeset/vast-hornets-draw.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: increment signal versions when discarding forks

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

@@ -972,6 +972,13 @@ export function fork(fn) {
 			await settled;
 		},
 		discard: () => {
+			// cause any MAYBE_DIRTY deriveds to update
+			// if they depend on things thath changed
+			// inside the discarded fork
+			for (var source of batch.current.keys()) {
+				source.wv = increment_write_version();
+			}
+
 			if (!committed && batches.has(batch)) {
 				batches.delete(batch);
 				batch.discard();

packages/svelte/tests/runtime-runes/samples/fork-derived-uncached-1/_config.js

@@ -0,0 +1,40 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	skip_no_async: true,
+	async test({ assert, target }) {
+		const [fork, toggle, increment] = target.querySelectorAll('button');
+
+		// derived is first evaluated in block effect, then discarded
+		flushSync(() => fork.click());
+
+		// should not throw "Cannot convert a Symbol value to a string" due to cached UNINITIALIZED from first fork
+		flushSync(() => fork.click());
+
+		// should not reflect the temporary change to `clicks` inside the fork
+		flushSync(() => toggle.click());
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>fork</button>
+				<button>toggle</button>
+				<button>clicks: 0</button>
+				<p>0</p>
+			`
+		);
+
+		flushSync(() => increment.click());
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>fork</button>
+				<button>toggle</button>
+				<button>clicks: 1</button>
+				<p>2</p>
+			`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/fork-derived-uncached-1/main.svelte

@@ -0,0 +1,23 @@
+<script>
+	import { fork } from "svelte";
+
+	let clicks = $state(0);
+	let show = $state(false);
+	const derived = $derived(clicks * 2);
+</script>
+
+<button onclick={() => {
+	fork(() => {
+		clicks += 1;
+		show = true;
+	}).discard();
+}}>fork</button>
+
+<button onclick={() => show = !show}>toggle</button>
+
+<button onclick={() => clicks++}>clicks: {clicks}</button>
+
+{#if show}
+	<p>{derived}</p>
+{/if}
+

packages/svelte/tests/runtime-runes/samples/fork-derived-uncached-2/_config.js

@@ -0,0 +1,38 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	skip_no_async: true,
+	async test({ assert, target }) {
+		const [fork, toggle, increment] = target.querySelectorAll('button');
+
+		// derived is first evaluated in block effect, then discarded
+		flushSync(() => fork.click());
+
+		// should not reflect the temporary change to `clicks` inside the fork
+		// or throw "Cannot convert a Symbol value to a string" due to cached UNINITIALIZED
+		flushSync(() => toggle.click());
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>fork</button>
+				<button>toggle</button>
+				<button>clicks: 0</button>
+				<p>0</p>
+			`
+		);
+
+		flushSync(() => increment.click());
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>fork</button>
+				<button>toggle</button>
+				<button>clicks: 1</button>
+				<p>2</p>
+			`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/fork-derived-uncached-2/main.svelte

@@ -0,0 +1,23 @@
+<script>
+	import { fork } from "svelte";
+
+	let clicks = $state(0);
+	let show = $state(false);
+	const derived = $derived(clicks * 2);
+</script>
+
+<button onclick={() => {
+	fork(() => {
+		clicks += 1;
+		show = true;
+	}).discard();
+}}>fork</button>
+
+<button onclick={() => show = !show}>toggle</button>
+
+<button onclick={() => clicks++}>clicks: {clicks}</button>
+
+{#if show}
+	<p>{derived}</p>
+{/if}
+

packages/svelte/tests/runtime-runes/samples/fork-derived-uncached-3/_config.js

@@ -0,0 +1,32 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	skip_no_async: true,
+	async test({ assert, target }) {
+		const [fork, toggle, increment] = target.querySelectorAll('button');
+
+		// initialize derived by showing it
+		flushSync(() => toggle.click());
+		flushSync(() => toggle.click());
+
+		// increment clicks
+		flushSync(() => increment.click());
+
+		// update derived, but without writing to `derived.v`
+		flushSync(() => fork.click());
+
+		// show derived
+		flushSync(() => toggle.click());
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>fork</button>
+				<button>toggle</button>
+				<button>clicks: 1</button>
+				<p>2</p>
+			`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/fork-derived-uncached-3/main.svelte

@@ -0,0 +1,23 @@
+<script>
+	import { fork } from "svelte";
+
+	let clicks = $state(0);
+	let show = $state(false);
+	const derived = $derived(clicks * 2);
+</script>
+
+<button onclick={() => {
+	fork(() => {
+		clicks += 1;
+		show = true;
+	}).discard();
+}}>fork</button>
+
+<button onclick={() => show = !show}>toggle</button>
+
+<button onclick={() => clicks++}>clicks: {clicks}</button>
+
+{#if show}
+	<p>{derived}</p>
+{/if}
+