fix: handle <svelte:head> rendered asynchronously

.changeset/khaki-emus-rest.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: handle `<svelte:head>` rendered asynchronously

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

@@ -2,6 +2,8 @@
 /** @import { AST } from '#compiler' */
 /** @import { ComponentContext } from '../types' */
 import * as b from '#compiler/builders';
+import { hash } from '../../../../../utils.js';
+import { filename } from '../../../../state.js';
 
 /**
  * @param {AST.SvelteHead} node
@@ -13,6 +15,7 @@ export function SvelteHead(node, context) {
 		b.stmt(
 			b.call(
 				'$.head',
+				b.literal(hash(filename)),
 				b.arrow([b.id('$$anchor')], /** @type {BlockStatement} */ (context.visit(node.fragment)))
 			)
 		)

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

@@ -2,6 +2,8 @@
 /** @import { AST } from '#compiler' */
 /** @import { ComponentContext } from '../types.js' */
 import * as b from '#compiler/builders';
+import { hash } from '../../../../../utils.js';
+import { filename } from '../../../../state.js';
 
 /**
  * @param {AST.SvelteHead} node
@@ -11,6 +13,13 @@ export function SvelteHead(node, context) {
 	const block = /** @type {BlockStatement} */ (context.visit(node.fragment));
 
 	context.state.template.push(
-		b.stmt(b.call('$.head', b.id('$$renderer'), b.arrow([b.id('$$renderer')], block)))
+		b.stmt(
+			b.call(
+				'$.head',
+				b.literal(hash(filename)),
+				b.id('$$renderer'),
+				b.arrow([b.id('$$renderer')], block)
+			)
+		)
 	);
 }

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

@@ -3,22 +3,13 @@ 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';
-
-/**
- * @type {Node | undefined}
- */
-let head_anchor;
-
-export function reset_head_anchor() {
-	head_anchor = undefined;
-}
 
 /**
+ * @param {string} hash
  * @param {(anchor: Node) => void} render_fn
  * @returns {void}
  */
-export function head(render_fn) {
+export function head(hash, render_fn) {
 	// The head function may be called after the first hydration pass and ssr comment nodes may still be present,
 	// therefore we need to skip that when we detect that we're not in hydration mode.
 	let previous_hydrate_node = null;
@@ -30,15 +21,13 @@ export function head(render_fn) {
 	if (hydrating) {
 		previous_hydrate_node = hydrate_node;
 
-		// There might be multiple head blocks in our app, so we need to account for each one needing independent hydration.
-		if (head_anchor === undefined) {
-			head_anchor = /** @type {TemplateNode} */ (get_first_child(document.head));
-		}
+		var head_anchor = /** @type {TemplateNode} */ (get_first_child(document.head));
 
+		// There might be multiple head blocks in our app, and they could have been
+		// rendered in an arbitrary order — find one corresponding to this component
 		while (
 			head_anchor !== null &&
-			(head_anchor.nodeType !== COMMENT_NODE ||
-				/** @type {Comment} */ (head_anchor).data !== HYDRATION_START)
+			(head_anchor.nodeType !== COMMENT_NODE || /** @type {Comment} */ (head_anchor).data !== hash)
 		) {
 			head_anchor = /** @type {TemplateNode} */ (get_next_sibling(head_anchor));
 		}
@@ -48,7 +37,10 @@ export function head(render_fn) {
 		if (head_anchor === null) {
 			set_hydrating(false);
 		} else {
-			head_anchor = set_hydrate_node(/** @type {TemplateNode} */ (get_next_sibling(head_anchor)));
+			var start = /** @type {TemplateNode} */ (get_next_sibling(head_anchor));
+			head_anchor.remove(); // in case this component is repeated
+
+			set_hydrate_node(start);
 		}
 	}
 
@@ -61,7 +53,6 @@ export function head(render_fn) {
 	} finally {
 		if (was_hydrating) {
 			set_hydrating(true);
-			head_anchor = hydrate_node; // so that next head block starts from the correct node
 			set_hydrate_node(/** @type {TemplateNode} */ (previous_hydrate_node));
 		}
 	}

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

@@ -12,20 +12,13 @@ import { HYDRATION_END, HYDRATION_ERROR, HYDRATION_START } from '../../constants
 import { active_effect } from './runtime.js';
 import { push, pop, component_context } from './context.js';
 import { component_root } from './reactivity/effects.js';
-import {
-	hydrate_next,
-	hydrate_node,
-	hydrating,
-	set_hydrate_node,
-	set_hydrating
-} from './dom/hydration.js';
+import { hydrate_node, hydrating, set_hydrate_node, set_hydrating } from './dom/hydration.js';
 import { array_from } from '../shared/utils.js';
 import {
 	all_registered_events,
 	handle_event_propagation,
 	root_event_handles
 } from './dom/elements/events.js';
-import { reset_head_anchor } from './dom/blocks/svelte-head.js';
 import * as w from './warnings.js';
 import * as e from './errors.js';
 import { assign_nodes } from './dom/template.js';
@@ -152,7 +145,6 @@ export function hydrate(component, options) {
 	} finally {
 		set_hydrating(was_hydrating);
 		set_hydrate_node(previous_hydrate_node);
-		reset_head_anchor();
 	}
 }
 

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

@@ -64,15 +64,16 @@ export function render(component, options = {}) {
 }
 
 /**
+ * @param {string} hash
  * @param {Renderer} renderer
  * @param {(renderer: Renderer) => Promise<void> | void} fn
  * @returns {void}
  */
-export function head(renderer, fn) {
+export function head(hash, renderer, fn) {
 	renderer.head((renderer) => {
-		renderer.push(BLOCK_OPEN);
+		renderer.push(`<!--${hash}-->`);
 		renderer.child(fn);
-		renderer.push(BLOCK_CLOSE);
+		renderer.push(EMPTY_COMMENT);
 	});
 }
 

packages/svelte/tests/hydration/test.ts

@@ -132,7 +132,11 @@
 		flushSync();
 
 		const normalize = (string: string) =>
-			string.trim().replaceAll('\r\n', '\n').replaceAll('/>', '>');
+			string
+				.trim()
+				.replaceAll('\r\n', '\n')
+				.replaceAll('/>', '>')
+				.replace(/<!--.+?-->/g, '');
 
 		const expected = read(`${cwd}/_expected.html`) ?? rendered.html;
 		assert.equal(normalize(target.innerHTML), normalize(expected));

packages/svelte/tests/runtime-runes/samples/async-head/A.svelte

@@ -0,0 +1,7 @@
+<script>
+	let { name, content } = $props();
+</script>
+
+<svelte:head>
+	<meta name={name} content={content} />
+</svelte:head>

packages/svelte/tests/runtime-runes/samples/async-head/B.svelte

@@ -0,0 +1,8 @@
+<script>
+	let { name, content } = $props();
+</script>
+
+<svelte:head>
+	<meta name="{name}-1" content="{content}-1" />
+	<meta name="{name}-2" content="{content}-2" />
+</svelte:head>

packages/svelte/tests/runtime-runes/samples/async-head/_config.js

@@ -0,0 +1,23 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, window }) {
+		await tick();
+
+		const head = window.document.head;
+
+		// we don't care about the order, but we want to ensure that the
+		// elements didn't clobber each other
+		for (let n of ['1', '2', '3']) {
+			const a = head.querySelector(`meta[name="a-${n}"]`);
+			assert.equal(a?.getAttribute('content'), n);
+
+			const b1 = head.querySelector(`meta[name="b-${n}-1"]`);
+			assert.equal(b1?.getAttribute('content'), `${n}-1`);
+
+			const b2 = head.querySelector(`meta[name="b-${n}-2"]`);
+			assert.equal(b2?.getAttribute('content'), `${n}-2`);
+		}
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-head/main.svelte

@@ -0,0 +1,11 @@
+<script lang="ts">
+	import A from './A.svelte';
+	import B from './B.svelte';
+</script>
+
+<A name="a-1" content={await 1} />
+<A name="a-2" content={await 2} />
+<B name="b-1" content={1} />
+<A name="a-3" content={await 3} />
+<B name="b-2" content={2} />
+<B name="b-3" content={3} />