fix: improve hydration of altered html

.changeset/angry-pigs-float.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: improve hydration of altered html

packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/fragment.js

@@ -6,6 +6,7 @@ import { is_event_attribute, is_text_attribute } from '../../../../../utils/ast.
 import * as b from '#compiler/builders';
 import { is_custom_element_node } from '../../../../nodes.js';
 import { build_template_chunk } from './utils.js';
+import { ELEMENT_NODE, TEXT_NODE } from '#client/constants';
 
 /**
  * Processes an array of template nodes, joining sibling text/expression nodes
@@ -25,15 +26,19 @@ export function process_children(nodes, initial, is_element, context) {
 	/** @type {Sequence} */
 	let sequence = [];
 
-	/** @param {boolean} is_text */
-	function get_node(is_text) {
+	/**
+	 * @param {boolean} is_text
+	 * @param {number} node_type
+	 **/
+	function get_node(is_text, node_type) {
 		if (skipped === 0) {
 			return prev(is_text);
 		}
 
 		return b.call(
 			'$.sibling',
 			prev(false),
+			b.literal(node_type),
 			(is_text || skipped !== 1) && b.literal(skipped),
 			is_text && b.true
 		);
@@ -44,7 +49,10 @@ export function process_children(nodes, initial, is_element, context) {
 	 * @param {string} name
 	 */
 	function flush_node(is_text, name) {
-		const expression = get_node(is_text);
+		const expression = get_node(
+			is_text,
+			name === 'text' ? TEXT_NODE : name === 'node' ? 0 : ELEMENT_NODE
+		);
 		let id = expression;
 
 		if (id.type !== 'Identifier') {

packages/svelte/src/internal/client/dom/blocks/svelte-head.js

@@ -3,7 +3,8 @@ import { hydrate_node, hydrating, set_hydrate_node, set_hydrating } from '../hyd
 import { create_text, get_first_child, get_next_sibling } from '../operations.js';
 import { block } from '../../reactivity/effects.js';
 import { COMMENT_NODE, HEAD_EFFECT } from '#client/constants';
-import { HYDRATION_START } from '../../../../constants.js';
+import { HYDRATION_END, HYDRATION_ERROR, HYDRATION_START } from '../../../../constants.js';
+import { hydration_mismatch } from '../../warnings.js';
 
 /**
  * @type {Node | undefined}
@@ -58,6 +59,36 @@ export function head(render_fn) {
 
 	try {
 		block(() => render_fn(anchor), HEAD_EFFECT);
+		check_end();
+	} catch (error) {
+		// re-mount only this svelte:head
+		if (was_hydrating && head_anchor && error === HYDRATION_ERROR) {
+			// Here head_anchor is the node next after HYDRATION_START
+			/** @type {Node | null} */
+			let prev = head_anchor.previousSibling;
+			/** @type {Node | null} */
+			let next = head_anchor;
+			// remove nodes that failed to hydrate
+			while (
+				prev !== null &&
+				(prev.nodeType !== COMMENT_NODE || /** @type {Comment} */ (prev).data !== HYDRATION_END)
+			) {
+				document.head.removeChild(prev);
+				prev = next;
+				next = get_next_sibling(/** @type {Node} */ (next));
+			}
+			if (prev?.parentNode) document.head.removeChild(prev);
+			if (next !== null) {
+				// allow the next head block try to hydrate
+				head_anchor = set_hydrate_node(/** @type {TemplateNode} */ (next));
+			}
+
+			set_hydrating(false);
+			anchor = document.head.appendChild(create_text());
+			block(() => render_fn(anchor), HEAD_EFFECT);
+		} else {
+			throw error;
+		}
 	} finally {
 		if (was_hydrating) {
 			set_hydrating(true);
@@ -66,3 +97,11 @@ export function head(render_fn) {
 		}
 	}
 }
+
+// treeshaking of hydrate node fails when this is directly in the try-catch
+function check_end() {
+	if (hydrating && /** @type {Comment|null} */ (hydrate_node)?.data !== HYDRATION_END) {
+		hydration_mismatch();
+		throw HYDRATION_ERROR;
+	}
+}

packages/svelte/src/internal/client/dom/operations.js

@@ -3,7 +3,9 @@ import { hydrate_node, hydrating, set_hydrate_node } from './hydration.js';
 import { DEV } from 'esm-env';
 import { init_array_prototype_warnings } from '../dev/equality.js';
 import { get_descriptor, is_extensible } from '../../shared/utils.js';
-import { TEXT_NODE } from '#client/constants';
+import { COMMENT_NODE, TEXT_NODE } from '#client/constants';
+import { HYDRATION_END, HYDRATION_ERROR } from '../../../constants.js';
+import { hydration_mismatch } from '../warnings.js';
 
 // export these for reference in the compiled code, making global name deduplication unnecessary
 /** @type {Window} */
@@ -158,26 +160,40 @@ export function first_child(fragment, is_text) {
 /**
  * Don't mark this as side-effect-free, hydration needs to walk all nodes
  * @param {TemplateNode} node
+ * @param {number} node_type
  * @param {number} count
- * @param {boolean} is_text
+ * @param {boolean} add_text
  * @returns {Node | null}
  */
-export function sibling(node, count = 1, is_text = false) {
-	let next_sibling = hydrating ? hydrate_node : node;
+export function sibling(node, node_type, count = 1, add_text = false) {
+	var next_sibling = hydrating ? hydrate_node : node;
 	var last_sibling;
 
 	while (count--) {
 		last_sibling = next_sibling;
 		next_sibling = /** @type {TemplateNode} */ (get_next_sibling(next_sibling));
+		if (
+			(next_sibling === null && !add_text) ||
+			(next_sibling?.nodeType === COMMENT_NODE &&
+				/** @type {Comment} */ (next_sibling).data === HYDRATION_END)
+		) {
+			hydration_mismatch();
+			throw HYDRATION_ERROR;
+		}
 	}
 
 	if (!hydrating) {
 		return next_sibling;
 	}
 
+	if (hydrating && node_type !== 0 && !add_text && next_sibling?.nodeType !== node_type) {
+		hydration_mismatch();
+		throw HYDRATION_ERROR;
+	}
+
 	// if a sibling {expression} is empty during SSR, there might be no
 	// text node to hydrate — we must therefore create one
-	if (is_text && next_sibling?.nodeType !== TEXT_NODE) {
+	if (add_text && next_sibling?.nodeType !== TEXT_NODE) {
 		var text = create_text();
 		// If the next sibling is `null` and we're handling text then it's because
 		// the SSR content was empty for the text, so we need to generate a new text
@@ -192,7 +208,7 @@ export function sibling(node, count = 1, is_text = false) {
 	}
 
 	set_hydrate_node(next_sibling);
-	return /** @type {TemplateNode} */ (next_sibling);
+	return next_sibling;
 }
 
 /**

packages/svelte/tests/hydration/samples/head-corrupted-2/_config.js

@@ -0,0 +1,18 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	expect_hydration_error: true,
+	test(assert, target, snapshot, component, window) {
+		assert.equal(window.document.querySelectorAll('meta').length, 5);
+
+		const [button] = target.getElementsByTagName('button');
+		button.click();
+		flushSync();
+
+		/** @type {NodeList} */
+		const metas = window.document.querySelectorAll('meta[name=count]');
+		assert.equal(metas.length, 4);
+		metas.forEach((meta) => assert.equal(/** @type {HTMLMetaElement} */ (meta).content, '2'));
+	}
+});

packages/svelte/tests/hydration/samples/head-corrupted-2/_expected_head.html

@@ -0,0 +1,2 @@
+<!--[--><meta name="count" content="1"><!----><!--]--><!--[--><meta name="count" content="1"><!----><!--]-->
+<!----><meta name="count" content="1"> <meta name="will-be-missing"> <meta name="count" content="1">

packages/svelte/tests/hydration/samples/head-corrupted-2/_override_head.html

@@ -0,0 +1,5 @@
+<!--[--><meta name="count" content="1" /><!----><!--]--><!--[--><meta name="count" content="1" />
+<meta name="count" content="1" /><!----><!--]--><!--[--><meta
+	name="count"
+	content="1"
+/><!----><!--]-->

packages/svelte/tests/hydration/samples/head-corrupted-2/head.svelte

@@ -0,0 +1,6 @@
+<script>
+	let { children } = $props();
+</script>
+<svelte:head>
+	{@render children()}
+</svelte:head>

packages/svelte/tests/hydration/samples/head-corrupted-2/main.svelte

@@ -0,0 +1,19 @@
+<script>
+	import Head from './head.svelte';
+
+	let count = $state(1);
+</script>
+
+<Head>
+	<meta name="count" content={count}>
+</Head>
+<Head>
+	<meta name="count" content={count}>
+	<meta name="will-be-missing">
+	<meta name="count" content={count}>
+</Head>
+<Head>
+	<meta name="count" content={count}>
+</Head>
+
+<button onclick={() => count++}>inc</button>

packages/svelte/tests/hydration/samples/head-corrupted-3/_config.js

@@ -0,0 +1,8 @@
+import { test } from '../../test';
+
+export default test({
+	expect_hydration_error: true,
+	test(assert, target, snapshot, component, window) {
+		assert.equal(window.document.querySelectorAll('meta').length, 2);
+	}
+});

packages/svelte/tests/hydration/samples/head-corrupted-3/_expected_head.html

@@ -0,0 +1 @@
+<meta name="description" content="some description"> <meta name="keywords" content="some keywords">

packages/svelte/tests/hydration/samples/head-corrupted-3/_override_head.html

@@ -0,0 +1,2 @@
+<!--[--><meta name="description" content="some description" />
+<meta name="foreign" content="alien" /> <meta name="keywords" content="some keywords" /><!--]-->

packages/svelte/tests/hydration/samples/head-corrupted-3/main.svelte

@@ -0,0 +1,10 @@
+<script>
+	let content = "some keywords"
+</script>
+
+<svelte:head>
+	<meta name="description" content="some description" />
+	<meta name="keywords" {content} />
+</svelte:head>
+
+<div>Just a dummy page.</div>

packages/svelte/tests/hydration/samples/head-corrupted/_config.js

@@ -0,0 +1,8 @@
+import { test } from '../../test';
+
+export default test({
+	expect_hydration_error: true,
+	test(assert, target, snapshot, component, window) {
+		assert.equal(window.document.querySelectorAll('meta').length, 2);
+	}
+});

packages/svelte/tests/hydration/samples/head-corrupted/_expected_head.html

@@ -0,0 +1 @@
+<!--[--><meta name="description" content="some description"> <meta name="keywords" content="some keywords">

packages/svelte/tests/hydration/samples/head-corrupted/_override_head.html

@@ -0,0 +1 @@
+<!--[--><meta name="description" content="some description" /><!--]-->

packages/svelte/tests/hydration/samples/head-corrupted/main.svelte

@@ -0,0 +1,6 @@
+<svelte:head>
+	<meta name="description" content="some description" />
+	<meta name="keywords" content="some keywords" />
+</svelte:head>
+
+<div>Just a dummy page.</div>

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

@@ -19,11 +19,11 @@ export default function Await_block_scope($$anchor) {
 
 	$.reset(button);
 
-	var node = $.sibling(button, 2);
+	var node = $.sibling(button, 0, 2);
 
 	$.await(node, () => $.get(promise), null, ($$anchor, counter) => {});
 
-	var text_1 = $.sibling(node);
+	var text_1 = $.sibling(node, 3);
 
 	$.template_effect(() => {
 		$.set_text(text, `clicks: ${counter.count ?? ''}`);

packages/svelte/tests/snapshot/samples/bind-component-snippet/_expected/client/index.svelte.js

@@ -27,7 +27,7 @@ export default function Bind_component_snippet($$anchor) {
 		}
 	});
 
-	var text_1 = $.sibling(node);
+	var text_1 = $.sibling(node, 3);
 
 	$.template_effect(() => $.set_text(text_1, ` value: ${$.get(value) ?? ''}`));
 	$.append($$anchor, fragment);

packages/svelte/tests/snapshot/samples/dynamic-attributes-casing/_expected/client/main.svelte.js

@@ -12,17 +12,17 @@ export default function Main($$anchor) {
 
 	$.set_attribute(div, 'foobar', x);
 
-	var svg = $.sibling(div, 2);
+	var svg = $.sibling(div, 1, 2);
 
 	$.set_attribute(svg, 'viewBox', x);
 
-	var custom_element = $.sibling(svg, 2);
+	var custom_element = $.sibling(svg, 1, 2);
 
 	$.set_custom_element_data(custom_element, 'fooBar', x);
 
-	var div_1 = $.sibling(custom_element, 2);
-	var svg_1 = $.sibling(div_1, 2);
-	var custom_element_1 = $.sibling(svg_1, 2);
+	var div_1 = $.sibling(custom_element, 1, 2);
+	var svg_1 = $.sibling(div_1, 1, 2);
+	var custom_element_1 = $.sibling(svg_1, 1, 2);
 
 	$.template_effect(() => $.set_custom_element_data(custom_element_1, 'fooBar', y()));
 

packages/svelte/tests/snapshot/samples/nullish-coallescence-omittance/_expected/client/index.svelte.js

@@ -12,19 +12,19 @@ export default function Nullish_coallescence_omittance($$anchor) {
 
 	h1.textContent = 'Hello, world!';
 
-	var b = $.sibling(h1, 2);
+	var b = $.sibling(h1, 1, 2);
 
 	b.textContent = '123';
 
-	var button = $.sibling(b, 2);
+	var button = $.sibling(b, 1, 2);
 
 	button.__click = [on_click, count];
 
 	var text = $.child(button);
 
 	$.reset(button);
 
-	var h1_1 = $.sibling(button, 2);
+	var h1_1 = $.sibling(button, 1, 2);
 
 	h1_1.textContent = 'Hello, world';
 	$.template_effect(() => $.set_text(text, `Count is ${$.get(count) ?? ''}`));

packages/svelte/tests/snapshot/samples/purity/_expected/client/index.svelte.js

@@ -12,11 +12,11 @@ export default function Purity($$anchor) {
 		$.untrack(() => Math.max(0, Math.min(0, 100)))
 	);
 
-	var p_1 = $.sibling(p, 2);
+	var p_1 = $.sibling(p, 1, 2);
 
 	p_1.textContent = ($.untrack(() => location.href));
 
-	var node = $.sibling(p_1, 2);
+	var node = $.sibling(p_1, 0, 2);
 
 	Child(node, { prop: encodeURIComponent('hello') });
 	$.append($$anchor, fragment);