From 3ad05194446866d3a769d4b095ce931974b5839a Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Wed, 18 Jun 2025 09:58:11 -0400
Subject: [PATCH] correctly handle errors inside boundary during reset

---
 .../internal/client/dom/blocks/boundary.js    | 36 ++++++++++---------
 .../_config.js                                | 28 +++++++++++++++
 .../main.svelte                               | 20 +++++++++++
 3 files changed, 68 insertions(+), 16 deletions(-)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/error-boundary-reset-with-error/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/error-boundary-reset-with-error/main.svelte

diff --git a/packages/svelte/src/internal/client/dom/blocks/boundary.js b/packages/svelte/src/internal/client/dom/blocks/boundary.js
index 2edc251de9..cda73f4af2 100644
--- a/packages/svelte/src/internal/client/dom/blocks/boundary.js
+++ b/packages/svelte/src/internal/client/dom/blocks/boundary.js
@@ -2,7 +2,7 @@
 
 import { BOUNDARY_EFFECT, EFFECT_TRANSPARENT } from '#client/constants';
 import { component_context, set_component_context } from '../../context.js';
-import { invoke_error_boundary } from '../../error-handling.js';
+import { handle_error, invoke_error_boundary } from '../../error-handling.js';
 import { block, branch, destroy_effect, pause_effect } from '../../reactivity/effects.js';
 import {
 	active_effect,
@@ -36,6 +36,8 @@ function with_boundary(boundary, fn) {
 
 	try {
 		fn();
+	} catch (e) {
+		handle_error(e);
 	} finally {
 		set_active_effect(previous_effect);
 		set_active_reaction(previous_reaction);
@@ -74,7 +76,16 @@ export function boundary(node, props, boundary_fn) {
 				throw error;
 			}
 
+			if (boundary_effect) {
+				destroy_effect(boundary_effect);
+			} else if (hydrating) {
+				set_hydrate_node(hydrate_open);
+				next();
+				set_hydrate_node(remove_nodes());
+			}
+
 			var did_reset = false;
+			var calling_on_error = false;
 
 			var reset = () => {
 				if (did_reset) {
@@ -84,17 +95,16 @@ export function boundary(node, props, boundary_fn) {
 
 				did_reset = true;
 
+				if (calling_on_error) {
+					w.reset_misuse();
+					throw error;
+				}
+
 				pause_effect(boundary_effect);
 
 				with_boundary(boundary, () => {
 					is_creating_fallback = false;
-					try {
-						boundary_effect = branch(() => boundary_fn(anchor));
-					} catch (error) {
-						// If the new subtree immediately throws during mount, warn the dev.
-						w.reset_misuse();
-						throw error;
-					}
+					boundary_effect = branch(() => boundary_fn(anchor));
 				});
 			};
 
@@ -102,19 +112,13 @@ export function boundary(node, props, boundary_fn) {
 
 			try {
 				set_active_reaction(null);
+				calling_on_error = true;
 				onerror?.(error, reset);
+				calling_on_error = false;
 			} finally {
 				set_active_reaction(previous_reaction);
 			}
 
-			if (boundary_effect) {
-				destroy_effect(boundary_effect);
-			} else if (hydrating) {
-				set_hydrate_node(hydrate_open);
-				next();
-				set_hydrate_node(remove_nodes());
-			}
-
 			if (failed) {
 				// Render the `failed` snippet in a microtask
 				queue_micro_task(() => {
diff --git a/packages/svelte/tests/runtime-runes/samples/error-boundary-reset-with-error/_config.js b/packages/svelte/tests/runtime-runes/samples/error-boundary-reset-with-error/_config.js
new file mode 100644
index 0000000000..3d63345bbf
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/error-boundary-reset-with-error/_config.js
@@ -0,0 +1,28 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	test({ assert, target, warnings }) {
+		const [toggle] = target.querySelectorAll('button');
+
+		flushSync(() => toggle.click());
+		assert.htmlEqual(
+			target.innerHTML,
+			// TODO the synthetic stack shouldn't be part of the message here
+			`<button>toggle</button><p>yikes! in {expression} in undefined</p><button>reset</button>`
+		);
+
+		const [, reset] = target.querySelectorAll('button');
+		flushSync(() => reset.click());
+		assert.htmlEqual(
+			target.innerHTML,
+			`<button>toggle</button><p>yikes! in {expression} in undefined</p><button>reset</button>`
+		);
+
+		flushSync(() => toggle.click());
+
+		const [, reset2] = target.querySelectorAll('button');
+		flushSync(() => reset2.click());
+		assert.htmlEqual(target.innerHTML, `<button>toggle</button><p>hello!</p>`);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/error-boundary-reset-with-error/main.svelte b/packages/svelte/tests/runtime-runes/samples/error-boundary-reset-with-error/main.svelte
new file mode 100644
index 0000000000..91479a631a
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/error-boundary-reset-with-error/main.svelte
@@ -0,0 +1,20 @@
+<script>
+	let must_throw = $state(false);
+
+	function throw_error() {
+		throw new Error('yikes!');
+	}
+</script>
+
+<button onclick={() => must_throw = !must_throw}>toggle</button>
+
+<svelte:boundary>
+	<p>{must_throw ? throw_error() : 'hello!'}</p>
+
+	{#snippet failed(error, reset)}
+		<p>{error.message}</p>
+		<button onclick={reset}>reset</button>
+	{/snippet}
+</svelte:boundary>
+
+