fix: add a warning when the misuse of reset in an error:boundary causes an error to be thrown when flushing the boundary content

.changeset/new-dogs-obey.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: handle error in correct boundary after reset

.changeset/polite-toys-report.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: make `<svelte:boundary>` reset function a noop after the first call

documentation/docs/98-reference/.generated/client-errors.md

@@ -158,3 +158,23 @@ let odd = $derived(!even);
 ```
 
 If side-effects are unavoidable, use [`$effect`]($effect) instead.
+
+### svelte_boundary_reset_onerror
+
+```
+A `<svelte:boundary>` `reset` function cannot be called while an error is still being handled
+```
+
+If a [`<svelte:boundary>`](https://svelte.dev/docs/svelte/svelte-boundary) has an `onerror` function, it must not call the provided `reset` function synchronously since the boundary is still in a broken state. Typically, `reset()` is called later, once the error has been resolved.
+
+If it's possible to resolve the error inside the `onerror` callback, you must at least wait for the boundary to settle before calling `reset()`, for example using [`tick`](https://svelte.dev/docs/svelte/lifecycle-hooks#tick):
+
+```svelte
+<svelte:boundary onerror={async (error, reset) => {
+	fixTheError();
+	+++await tick();+++
+	reset();
+}}>
+
+</svelte:boundary>
+```

documentation/docs/98-reference/.generated/client-warnings.md

@@ -232,6 +232,32 @@ Reactive `$state(...)` proxies and the values they proxy have different identiti
 
 To resolve this, ensure you're comparing values where both values were created with `$state(...)`, or neither were. Note that `$state.raw(...)` will _not_ create a state proxy.
 
+### svelte_boundary_reset_noop
+
+```
+A `<svelte:boundary>` `reset` function only resets the boundary the first time it is called
+```
+
+When an error occurs while rendering the contents of a [`<svelte:boundary>`](https://svelte.dev/docs/svelte/svelte-boundary), the `onerror` handler is called with the error plus a `reset` function that attempts to re-render the contents.
+
+This `reset` function should only be called once. After that, it has no effect — in a case like this, where a reference to `reset` is stored outside the boundary, clicking the button while `<Contents />` is rendered will _not_ cause the contents to be rendered again.
+
+```svelte
+<script>
+	let reset;
+</script>
+
+<button onclick={reset}>reset</button>
+
+<svelte:boundary onerror={(e, r) => (reset = r)}>
+	<!-- contents >
+
+	{#snippet failed(e)}
+		<p>oops! {e.message}</p>
+	{/snippet}
+</svelte:boundary>
+```
+
 ### transition_slide_display
 
 ```

packages/svelte/messages/client-errors/errors.md

@@ -114,3 +114,21 @@ let odd = $derived(!even);
 ```
 
 If side-effects are unavoidable, use [`$effect`]($effect) instead.
+
+## svelte_boundary_reset_onerror
+
+> A `<svelte:boundary>` `reset` function cannot be called while an error is still being handled
+
+If a [`<svelte:boundary>`](https://svelte.dev/docs/svelte/svelte-boundary) has an `onerror` function, it must not call the provided `reset` function synchronously since the boundary is still in a broken state. Typically, `reset()` is called later, once the error has been resolved.
+
+If it's possible to resolve the error inside the `onerror` callback, you must at least wait for the boundary to settle before calling `reset()`, for example using [`tick`](https://svelte.dev/docs/svelte/lifecycle-hooks#tick):
+
+```svelte
+<svelte:boundary onerror={async (error, reset) => {
+	fixTheError();
+	+++await tick();+++
+	reset();
+}}>
+
+</svelte:boundary>
+```

packages/svelte/messages/client-warnings/warnings.md

@@ -196,6 +196,30 @@ To silence the warning, ensure that `value`:
 
 To resolve this, ensure you're comparing values where both values were created with `$state(...)`, or neither were. Note that `$state.raw(...)` will _not_ create a state proxy.
 
+## svelte_boundary_reset_noop
+
+> A `<svelte:boundary>` `reset` function only resets the boundary the first time it is called
+
+When an error occurs while rendering the contents of a [`<svelte:boundary>`](https://svelte.dev/docs/svelte/svelte-boundary), the `onerror` handler is called with the error plus a `reset` function that attempts to re-render the contents.
+
+This `reset` function should only be called once. After that, it has no effect — in a case like this, where a reference to `reset` is stored outside the boundary, clicking the button while `<Contents />` is rendered will _not_ cause the contents to be rendered again.
+
+```svelte
+<script>
+	let reset;
+</script>
+
+<button onclick={reset}>reset</button>
+
+<svelte:boundary onerror={(e, r) => (reset = r)}>
+	<!-- contents >
+
+	{#snippet failed(e)}
+		<p>oops! {e.message}</p>
+	{/snippet}
+</svelte:boundary>
+```
+
 ## transition_slide_display
 
 > The `slide` transition does not work correctly for elements with `display: %value%`

packages/svelte/src/internal/client/dom/blocks/boundary.js

@@ -1,8 +1,8 @@
 /** @import { Effect, TemplateNode, } from '#client' */
 
-import { BOUNDARY_EFFECT, EFFECT_TRANSPARENT } from '#client/constants';
+import { BOUNDARY_EFFECT, EFFECT_RAN, 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,
@@ -19,6 +19,8 @@ import {
 	set_hydrate_node
 } from '../hydration.js';
 import { queue_micro_task } from '../task.js';
+import * as w from '../../warnings.js';
+import * as e from '../../errors.js';
 
 /**
  * @param {Effect} boundary
@@ -35,6 +37,8 @@ function with_boundary(boundary, fn) {
 
 	try {
 		fn();
+	} catch (e) {
+		handle_error(e);
 	} finally {
 		set_active_effect(previous_effect);
 		set_active_reaction(previous_reaction);
@@ -73,7 +77,29 @@ 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) {
+					w.svelte_boundary_reset_noop();
+					return;
+				}
+
+				did_reset = true;
+
+				if (calling_on_error) {
+					e.svelte_boundary_reset_onerror();
+				}
+
 				pause_effect(boundary_effect);
 
 				with_boundary(boundary, () => {
@@ -86,19 +112,19 @@ export function boundary(node, props, boundary_fn) {
 
 			try {
 				set_active_reaction(null);
+				calling_on_error = true;
 				onerror?.(error, reset);
+				calling_on_error = false;
+			} catch (error) {
+				if ((boundary.f & EFFECT_RAN) !== 0) {
+					invoke_error_boundary(error, /** @type {Effect} */ (boundary.parent));
+				} else {
+					throw error;
+				}
 			} 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(() => {

packages/svelte/src/internal/client/error-handling.js

@@ -42,7 +42,13 @@ export function invoke_error_boundary(error, effect) {
 				// @ts-expect-error
 				effect.fn(error);
 				return;
-			} catch {}
+			} catch (e) {
+				if (DEV && e instanceof Error) {
+					adjust_error(e, effect);
+				}
+
+				error = e;
+			}
 		}
 
 		effect = effect.parent;

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

@@ -319,4 +319,19 @@ export function state_unsafe_mutation() {
 	} else {
 		throw new Error(`https://svelte.dev/e/state_unsafe_mutation`);
 	}
+}
+
+/**
+ * A `<svelte:boundary>` `reset` function cannot be called while an error is still being handled
+ * @returns {never}
+ */
+export function svelte_boundary_reset_onerror() {
+	if (DEV) {
+		const error = new Error(`svelte_boundary_reset_onerror\nA \`<svelte:boundary>\` \`reset\` function cannot be called while an error is still being handled\nhttps://svelte.dev/e/svelte_boundary_reset_onerror`);
+
+		error.name = 'Svelte error';
+		throw error;
+	} else {
+		throw new Error(`https://svelte.dev/e/svelte_boundary_reset_onerror`);
+	}
 }

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

@@ -181,6 +181,17 @@ export function state_proxy_equality_mismatch(operator) {
 	}
 }
 
+/**
+ * A `<svelte:boundary>` `reset` function only resets the boundary the first time it is called
+ */
+export function svelte_boundary_reset_noop() {
+	if (DEV) {
+		console.warn(`%c[svelte] svelte_boundary_reset_noop\n%cA \`<svelte:boundary>\` \`reset\` function only resets the boundary the first time it is called\nhttps://svelte.dev/e/svelte_boundary_reset_noop`, bold, normal);
+	} else {
+		console.warn(`https://svelte.dev/e/svelte_boundary_reset_noop`);
+	}
+}
+
 /**
  * The `slide` transition does not work correctly for elements with `display: %value%`
  * @param {string} value

packages/svelte/tests/runtime-legacy/shared.ts

@@ -450,10 +450,11 @@ async function run_test_variant(
 					'Expected component to unmount and leave nothing behind after it was destroyed'
 				);
 
-				// TODO: This seems useless, unhandledRejection is only triggered on the next task
-				// by which time the test has already finished and the next test resets it to null above
+				// uncaught errors like during template effects flush
 				if (unhandled_rejection) {
-					throw unhandled_rejection; // eslint-disable-line no-unsafe-finally
+					if (!config.expect_unhandled_rejections) {
+						throw unhandled_rejection; // eslint-disable-line no-unsafe-finally
+					}
 				}
 			}
 		}

packages/svelte/tests/runtime-runes/samples/error-boundary-reset-onerror/_config.js

@@ -0,0 +1,15 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	test({ assert, target }) {
+		const btn = target.querySelector('button');
+
+		btn?.click();
+
+		assert.throws(flushSync, 'svelte_boundary_reset_onerror');
+
+		// boundary content empty; only button remains
+		assert.htmlEqual(target.innerHTML, `<button>trigger throw</button>`);
+	}
+});

packages/svelte/tests/runtime-runes/samples/error-boundary-reset-onerror/main.svelte

@@ -0,0 +1,17 @@
+<script>
+	let must_throw = $state(false);
+
+	function throw_error() {
+		throw new Error("error on template render");
+	}
+</script>
+
+<svelte:boundary onerror={(_, reset) => reset()}>
+	{must_throw ? throw_error() : 'normal content'}
+
+	{#snippet failed()}
+		<div>err</div>
+	{/snippet}
+</svelte:boundary>
+
+<button onclick={() => must_throw = true}>trigger throw</button>

packages/svelte/tests/runtime-runes/samples/error-boundary-reset-premature/_config.js

@@ -0,0 +1,28 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	html: `
+		normal content
+		<button>toggle</button>
+	`,
+
+	async test({ assert, target, warnings }) {
+		const [btn] = target.querySelectorAll('button');
+
+		flushSync(() => btn.click());
+		assert.htmlEqual(target.innerHTML, `<div>err</div><button>toggle</button>`);
+		assert.deepEqual(warnings, []);
+
+		flushSync(() => btn.click());
+		assert.htmlEqual(target.innerHTML, `normal content <button>toggle</button>`);
+		assert.deepEqual(warnings, []);
+
+		flushSync(() => btn.click());
+		assert.htmlEqual(target.innerHTML, `<div>err</div><button>toggle</button>`);
+
+		assert.deepEqual(warnings, [
+			'A `<svelte:boundary>` `reset` function only resets the boundary the first time it is called'
+		]);
+	}
+});

packages/svelte/tests/runtime-runes/samples/error-boundary-reset-premature/main.svelte

@@ -0,0 +1,26 @@
+<script>
+	let must_throw = $state(false);
+	let reset = $state(null);
+
+	function throw_error() {
+		throw new Error("error on template render");
+	}
+</script>
+
+<svelte:boundary onerror={console.error}>
+	<svelte:boundary onerror={(_, fn) => (reset = fn)}>
+		{must_throw ? throw_error() : 'normal content'}
+
+		{#snippet failed()}
+			<div>err</div>
+		{/snippet}
+	</svelte:boundary>
+</svelte:boundary>
+
+<button
+	onclick={() => {
+		must_throw = !must_throw;
+		if (reset) reset();
+	}}>
+	toggle
+</button>

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>`);
+	}
+});