From 3e92edfbf4d8a8b6a3e667604cc15bcf0eb102fd Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 13 Oct 2026 12:07:20 -0400
Subject: [PATCH] restore transferring discard callbacks in #merge

If the surviving batch is discarded (e.g. after an unhandled error), the merged batch's discard
callbacks must run too, or each blocks keep its pending state and leak items.
---
 .../src/internal/client/reactivity/batch.js   |  4 ++
 .../Item.svelte                               |  9 ++++
 .../_config.js                                | 47 +++++++++++++++++++
 .../main.svelte                               | 39 +++++++++++++++
 4 files changed, 99 insertions(+)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-merge-discard-after-error/Item.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-merge-discard-after-error/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-merge-discard-after-error/main.svelte

diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index f4c70c822f..2569acb897 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -662,6 +662,10 @@ export class Batch {
 			this.oncommit(() => c(batch));
 		}
 
+		for (const c of batch.#discard_callbacks) {
+			this.ondiscard(() => c(batch));
+		}
+
 		for (const [s, v] of batch.#skipped_branches) {
 			this.#skipped_branches.set(s, v);
 			this.unskipped_branches.delete(s);
diff --git a/packages/svelte/tests/runtime-runes/samples/async-merge-discard-after-error/Item.svelte b/packages/svelte/tests/runtime-runes/samples/async-merge-discard-after-error/Item.svelte
new file mode 100644
index 0000000000..7e30dd0125
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-merge-discard-after-error/Item.svelte
@@ -0,0 +1,9 @@
+<script>
+	let { item } = $props();
+	$effect(() => {
+		console.log('mount ' + item);
+		return () => console.log('destroy ' + item);
+	});
+</script>
+
+<span>{item}</span>
diff --git a/packages/svelte/tests/runtime-runes/samples/async-merge-discard-after-error/_config.js b/packages/svelte/tests/runtime-runes/samples/async-merge-discard-after-error/_config.js
new file mode 100644
index 0000000000..9bd1abce5c
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-merge-discard-after-error/_config.js
@@ -0,0 +1,47 @@
+import { flushSync, tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	mode: ['client'],
+	async test({ assert, target, logs }) {
+		const [a, push, clear, shift] = target.querySelectorAll('button');
+
+		shift.click();
+		await tick();
+		assert.deepEqual(logs, ['mount 1', 'mount 2', 'mount 3']);
+
+		// batch X: a=1, pending
+		a.click();
+		await tick();
+
+		// batch Y: list changes; the each block reads `a`, so Y merges into X.
+		// The each block registers commit/discard callbacks with Y
+		push.click();
+		await tick();
+
+		// X resolves and throws while processing (not handled by any boundary), so it is discarded
+		// rather than committed. Y's discard callbacks must still run
+		let error = null;
+		shift.click();
+		try {
+			for (let i = 0; i < 10; i++) {
+				await Promise.resolve();
+				flushSync();
+			}
+		} catch (e) {
+			error = e;
+		}
+		assert.match(/** @type {Error} */ (error)?.message, /^boom/);
+
+		logs.length = 0;
+		clear.click();
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`<button>a</button><button>push</button><button>clear</button><button>shift</button><p>a: 0</p>`
+		);
+		// the removed items must be destroyed, not kept around for the discarded batch Y
+		assert.deepEqual(logs, ['destroy 1', 'destroy 2', 'destroy 3']);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-merge-discard-after-error/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-merge-discard-after-error/main.svelte
new file mode 100644
index 0000000000..ea1bfd8594
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-merge-discard-after-error/main.svelte
@@ -0,0 +1,39 @@
+<script>
+	import Item from './Item.svelte';
+
+	let a = $state(0);
+	let list = $state([1, 2, 3]);
+
+	let queue = [];
+	function push(v) {
+		const d = Promise.withResolvers();
+		queue.push(() => d.resolve(v));
+		return d.promise;
+	}
+
+	function check(v) {
+		if (v === 1) throw new Error('boom');
+		return true;
+	}
+</script>
+
+<button onclick={() => a++}>a</button>
+<button onclick={() => list.push(4)}>push</button>
+<button onclick={() => (list = [])}>clear</button>
+<button onclick={() => queue.shift()?.()}>shift</button>
+
+<!-- no failed snippet/onerror: errors are not handled by the boundary -->
+<svelte:boundary>
+	{#if check(await push(a))}
+		<p>a: {a}</p>
+	{/if}
+
+	<!-- reads `a`, so batches changing `list` depend on (and merge into) a pending batch changing `a` -->
+	{#each list.slice(0, a + 10) as item (item)}
+		<Item {item} />
+	{/each}
+
+	{#snippet pending()}
+		<p>loading...</p>
+	{/snippet}
+</svelte:boundary>