fix: handle hydration mismatches in await blocks

.changeset/wild-carrots-eat.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: handle hydration mismatches in await blocks

packages/svelte/src/compiler/phases/3-transform/server/visitors/AwaitBlock.js

@@ -2,32 +2,28 @@
 /** @import { AST } from '#compiler' */
 /** @import { ComponentContext } from '../types.js' */
 import * as b from '../../../../utils/builders.js';
-import { empty_comment } from './shared/utils.js';
+import { block_close } from './shared/utils.js';
 
 /**
  * @param {AST.AwaitBlock} node
  * @param {ComponentContext} context
  */
 export function AwaitBlock(node, context) {
 	context.state.template.push(
-		empty_comment,
 		b.stmt(
 			b.call(
 				'$.await',
+				b.id('$$payload'),
 				/** @type {Expression} */ (context.visit(node.expression)),
 				b.thunk(
 					node.pending ? /** @type {BlockStatement} */ (context.visit(node.pending)) : b.block([])
 				),
 				b.arrow(
 					node.value ? [/** @type {Pattern} */ (context.visit(node.value))] : [],
 					node.then ? /** @type {BlockStatement} */ (context.visit(node.then)) : b.block([])
-				),
-				b.arrow(
-					node.error ? [/** @type {Pattern} */ (context.visit(node.error))] : [],
-					node.catch ? /** @type {BlockStatement} */ (context.visit(node.catch)) : b.block([])
 				)
 			)
 		),
-		empty_comment
+		block_close
 	);
 }

packages/svelte/src/constants.js

@@ -22,6 +22,7 @@ export const HYDRATION_START = '[';
 /** used to indicate that an `{:else}...` block was rendered */
 export const HYDRATION_START_ELSE = '[!';
 export const HYDRATION_END = ']';
+export const HYDRATION_AWAIT_THEN = '!';
 export const HYDRATION_ERROR = {};
 
 export const ELEMENT_IS_NAMESPACED = 1;

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

@@ -4,9 +4,16 @@ import { is_promise } from '../../../shared/utils.js';
 import { block, branch, pause_effect, resume_effect } from '../../reactivity/effects.js';
 import { internal_set, mutable_source, source } from '../../reactivity/sources.js';
 import { flushSync, set_active_effect, set_active_reaction } from '../../runtime.js';
-import { hydrate_next, hydrate_node, hydrating } from '../hydration.js';
+import {
+	hydrate_next,
+	hydrate_node,
+	hydrating,
+	remove_nodes,
+	set_hydrate_node,
+	set_hydrating
+} from '../hydration.js';
 import { queue_micro_task } from '../task.js';
-import { UNINITIALIZED } from '../../../../constants.js';
+import { HYDRATION_START_ELSE, UNINITIALIZED } from '../../../../constants.js';
 import {
 	component_context,
 	is_runes,
@@ -113,6 +120,19 @@ export function await_block(node, get_input, pending_fn, then_fn, catch_fn) {
 	var effect = block(() => {
 		if (input === (input = get_input())) return;
 
+		/** Whether or not there was a hydration mismatch. Needs to be a `let` or else it isn't treeshaken out */
+		// @ts-ignore coercing `anchor` to a `Comment` causes TypeScript and Prettier to fight
+		let mismatch = hydrating && is_promise(input) === (anchor.data === HYDRATION_START_ELSE);
+
+		if (mismatch) {
+			// Hydration mismatch: remove everything inside the anchor and start fresh
+			anchor = remove_nodes();
+
+			set_hydrate_node(anchor);
+			set_hydrating(false);
+			mismatch = true;
+		}
+
 		if (is_promise(input)) {
 			var promise = input;
 
@@ -155,6 +175,11 @@ export function await_block(node, get_input, pending_fn, then_fn, catch_fn) {
 			update(THEN, false);
 		}
 
+		if (mismatch) {
+			// continue in hydration mode
+			set_hydrating(true);
+		}
+
 		// Set the input to something else, in order to disable the promise callbacks
 		return () => (input = UNINITIALIZED);
 	});

packages/svelte/src/internal/server/index.js

@@ -13,7 +13,7 @@ import {
 import { escape_html } from '../../escaping.js';
 import { DEV } from 'esm-env';
 import { current_component, pop, push } from './context.js';
-import { EMPTY_COMMENT, BLOCK_CLOSE, BLOCK_OPEN } from './hydration.js';
+import { EMPTY_COMMENT, BLOCK_CLOSE, BLOCK_OPEN, BLOCK_OPEN_ELSE } from './hydration.js';
 import { validate_store } from '../shared/validate.js';
 import { is_boolean_attribute, is_raw_text_element, is_void } from '../../utils.js';
 import { reset_elements } from './dev.js';
@@ -474,18 +474,21 @@ export function bind_props(props_parent, props_now) {
 
 /**
  * @template V
+ * @param {Payload} payload
  * @param {Promise<V>} promise
  * @param {null | (() => void)} pending_fn
  * @param {(value: V) => void} then_fn
  * @returns {void}
  */
-function await_block(promise, pending_fn, then_fn) {
+function await_block(payload, promise, pending_fn, then_fn) {
 	if (is_promise(promise)) {
+		payload.out += BLOCK_OPEN;
 		promise.then(null, noop);
 		if (pending_fn !== null) {
 			pending_fn();
 		}
 	} else if (then_fn !== null) {
+		payload.out += BLOCK_OPEN_ELSE;
 		then_fn(promise);
 	}
 }

packages/svelte/tests/runtime-runes/samples/await-hydrate-maybe-promise/_config.js

@@ -0,0 +1,23 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	ssrHtml: '<button>fulfil</button><p>42</p><hr><p>loading...</p>',
+	html: '<button>fulfil</button><p>loading...</p><hr><p>42</p>',
+
+	props: {
+		browser: true
+	},
+
+	server_props: {
+		browser: false
+	},
+
+	async test({ assert, target }) {
+		const button = target.querySelector('button');
+
+		flushSync(() => button?.click());
+		await Promise.resolve();
+		assert.htmlEqual(target.innerHTML, '<button>fulfil</button><p>42</p><hr><p>42</p>');
+	}
+});

packages/svelte/tests/runtime-runes/samples/await-hydrate-maybe-promise/main.svelte

@@ -0,0 +1,25 @@
+<script>
+	let { browser } = $props();
+
+	let fulfil;
+	let promise = new Promise((f) => (fulfil = f));
+
+	let a = browser ? promise : 42;
+	let b = browser ? 42 : promise;
+</script>
+
+<button onclick={() => fulfil(42)}>fulfil</button>
+
+{#await a}
+	{#if true}<p>loading...</p>{/if}
+{:then a}
+	<p>{a}</p>
+{/await}
+
+<hr>
+
+{#await b}
+	{#if true}<p>loading...</p>{/if}
+{:then b}
+	<p>{b}</p>
+{/await}

packages/svelte/tests/snapshot/samples/await-block-scope/_expected/server/index.svelte.js

@@ -8,7 +8,7 @@ export default function Await_block_scope($$payload) {
 		counter.count += 1;
 	}
 
-	$$payload.out += `<button>clicks: ${$.escape(counter.count)}</button> <!---->`;
-	$.await(promise, () => {}, (counter) => {}, () => {});
-	$$payload.out += `<!----> ${$.escape(counter.count)}`;
+	$$payload.out += `<button>clicks: ${$.escape(counter.count)}</button> `;
+	$.await($$payload, promise, () => {}, (counter) => {});
+	$$payload.out += `<!--]--> ${$.escape(counter.count)}`;
 }