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

@@ -238,3 +238,23 @@ 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

@@ -312,6 +312,32 @@ 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

@@ -184,3 +184,21 @@ 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

@@ -272,6 +272,30 @@ 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

@@ -1,7 +1,12 @@
 /** @import { Effect, Source, TemplateNode, } from '#client' */
-import { BOUNDARY_EFFECT, EFFECT_PRESERVED, EFFECT_TRANSPARENT } from '#client/constants';
+import {
+	BOUNDARY_EFFECT,
+	EFFECT_PRESERVED,
+	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,
@@ -21,6 +26,7 @@ import {
 import { get_next_sibling } from '../operations.js';
 import { queue_micro_task } from '../task.js';
 import * as e from '../../errors.js';
+import * as w from '../../warnings.js';
 import { DEV } from 'esm-env';
 import { Batch, effect_pending_updates } from '../../reactivity/batch.js';
 import { internal_set, source } from '../../reactivity/sources.js';
@@ -196,6 +202,9 @@ export class Boundary {
 
 		try {
 			return fn();
+		} catch (e) {
+			handle_error(e);
+			return null;
 		} finally {
 			set_active_effect(previous_effect);
 			set_active_reaction(previous_reaction);
@@ -257,7 +266,42 @@ export class Boundary {
 		var onerror = this.#props.onerror;
 		let failed = this.#props.failed;
 
+		if (this.#main_effect) {
+			destroy_effect(this.#main_effect);
+			this.#main_effect = null;
+		}
+
+		if (this.#pending_effect) {
+			destroy_effect(this.#pending_effect);
+			this.#pending_effect = null;
+		}
+
+		if (this.#failed_effect) {
+			destroy_effect(this.#failed_effect);
+			this.#failed_effect = null;
+		}
+
+		if (hydrating) {
+			set_hydrate_node(this.#hydrate_open);
+			next();
+			set_hydrate_node(remove_nodes());
+		}
+
+		var did_reset = false;
+		var calling_on_error = false;
+
 		const reset = () => {
+			if (did_reset) {
+				w.svelte_boundary_reset_noop();
+				return;
+			}
+
+			did_reset = true;
+
+			if (calling_on_error) {
+				e.svelte_boundary_reset_onerror();
+			}
+
 			this.#pending_count = 0;
 
 			if (this.#failed_effect !== null) {
@@ -290,32 +334,15 @@ export class Boundary {
 
 		try {
 			set_active_reaction(null);
+			calling_on_error = true;
 			onerror?.(error, reset);
+			calling_on_error = false;
+		} catch (error) {
+			invoke_error_boundary(error, this.#effect && this.#effect.parent);
 		} finally {
 			set_active_reaction(previous_reaction);
 		}
 
-		if (this.#main_effect) {
-			destroy_effect(this.#main_effect);
-			this.#main_effect = null;
-		}
-
-		if (this.#pending_effect) {
-			destroy_effect(this.#pending_effect);
-			this.#pending_effect = null;
-		}
-
-		if (this.#failed_effect) {
-			destroy_effect(this.#failed_effect);
-			this.#failed_effect = null;
-		}
-
-		if (hydrating) {
-			set_hydrate_node(this.#hydrate_open);
-			next();
-			set_hydrate_node(remove_nodes());
-		}
-
 		if (failed) {
 			queue_micro_task(() => {
 				this.#failed_effect = this.#run(() => {

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

@@ -53,7 +53,9 @@ export function invoke_error_boundary(error, effect) {
 			try {
 				/** @type {Boundary} */ (effect.b).error(error);
 				return;
-			} catch {}
+			} catch (e) {
+				error = e;
+			}
 		}
 
 		effect = effect.parent;

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

@@ -423,4 +423,20 @@ 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

@@ -224,6 +224,17 @@ 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

@@ -489,10 +489,11 @@ 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>