From 17d3d817a7e01bcaed34ce870b2c16013cbf6a05 Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Thu, 16 Jul 2026 13:25:38 +0200
Subject: [PATCH] prevent infinite loop in eager batches

---
 .../src/internal/client/reactivity/batch.js   | 27 +++++++++++++++++--
 .../async-eager-effect-loop/_config.js        | 16 +++++++++++
 .../async-eager-effect-loop/main.svelte       | 14 ++++++++++
 3 files changed, 55 insertions(+), 2 deletions(-)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-eager-effect-loop/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-eager-effect-loop/main.svelte

diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index bf2651227f..38604427f1 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -235,6 +235,14 @@ export class Batch {
 	 */
 	#maybe_dirty_effects = null;
 
+	/**
+	 * Effects run by an eager batch that must be re-established in this batch's world.
+	 * Unlike deferred effects, these are scheduled only once.
+	 * Lazily initialised for perf reasons
+	 * @type {Set<Effect> | null}
+	 */
+	#reestablish_effects = null;
+
 	/**
 	 * A map of branches that still exist, but will be destroyed when this batch
 	 * is committed — we skip over these during `process`.
@@ -512,8 +520,8 @@ export class Batch {
 				owner.linked &&
 				!owner.is_fork
 			) {
-				// TODO
-				(owner.#dirty_effects ??= new Set()).add(/** @type {Effect} */ (signal));
+				// Not putting this into #dirty_effects to prevent infinite loops
+				(owner.#reestablish_effects ??= new Set()).add(/** @type {Effect} */ (signal));
 			}
 
 			return false;
@@ -641,6 +649,9 @@ export class Batch {
 		other.#dirty_effects = null;
 		other.#maybe_dirty_effects = null;
 
+		this.#reestablish_effects = transfer_set(this.#reestablish_effects, other.#reestablish_effects);
+		other.#reestablish_effects = null;
+
 		if (other.#skipped_branches !== null) {
 			this.#skipped_branches ??= new Map();
 
@@ -782,6 +793,18 @@ export class Batch {
 			}
 		}
 
+		if (this.#reestablish_effects !== null) {
+			var reestablish_effects = this.#reestablish_effects;
+			this.#reestablish_effects = null;
+
+			for (const e of reestablish_effects) {
+				this.#dirty_effects?.delete(e);
+				this.#maybe_dirty_effects?.delete(e);
+				set_signal_status(e, DIRTY);
+				this.schedule(e);
+			}
+		}
+
 		// We always reschedule previously-deferred effects, not just when
 		// #is_deferred() is true, because traversing the tree could make
 		// an if block that contains the last blocking pending effect falsy,
diff --git a/packages/svelte/tests/runtime-runes/samples/async-eager-effect-loop/_config.js b/packages/svelte/tests/runtime-runes/samples/async-eager-effect-loop/_config.js
new file mode 100644
index 0000000000..4dee4cc511
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-eager-effect-loop/_config.js
@@ -0,0 +1,16 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+		const [increment] = target.querySelectorAll('button');
+
+		assert.htmlEqual(target.innerHTML, '<button>increment</button><p>0</p>');
+
+		increment.click();
+		await tick();
+
+		assert.htmlEqual(target.innerHTML, '<button>increment</button><p>2</p>');
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-eager-effect-loop/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-eager-effect-loop/main.svelte
new file mode 100644
index 0000000000..8ba1897206
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-eager-effect-loop/main.svelte
@@ -0,0 +1,14 @@
+<script>
+	let count = $state(0);
+	let runs = 0;
+
+	function delay(value, eager_value) {
+		if (runs++ > 10) throw new Error('infinite loop detected')
+		if (value === 0) return 0;
+		return Promise.resolve(value + eager_value);
+	}
+</script>
+
+<button onclick={() => count += 1}>increment</button>
+
+<p>{await delay(count, $state.eager(count))}</p>