fix: prevent infinite loop when HMRing a component with an `await` (#17380)

* fix: prevent infinite loop when HMRing a component with an `await`

* update test

* fix

* reintroduce old logic to fix ever-growing stack of block effects

* update snapshot

* tweak names for clarity, simplify `update` slightly

---------

Co-authored-by: Simon Holthausen <simon.holthausen@vercel.com>

.changeset/giant-gifts-mate.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: prevent infinite loop when HMRing a component with an `await`

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

@@ -519,14 +519,9 @@ export function client_component(analysis, options) {
 
 	if (options.hmr) {
 		const id = b.id(analysis.name);
-		const HMR = b.id('$.HMR');
-
-		const existing = b.member(id, HMR, true);
-		const incoming = b.member(b.id('module.default'), HMR, true);
 
 		const accept_fn_body = [
-			b.stmt(b.assignment('=', b.member(incoming, 'source'), b.member(existing, 'source'))),
-			b.stmt(b.call('$.set', b.member(existing, 'source'), b.member(incoming, 'original')))
+			b.stmt(b.call(b.member(b.member(id, b.id('$.HMR'), true), 'update'), b.id('module.default')))
 		];
 
 		if (analysis.css.hash) {
@@ -535,8 +530,7 @@ export function client_component(analysis, options) {
 		}
 
 		const hmr = b.block([
-			b.stmt(b.assignment('=', id, b.call('$.hmr', id, b.thunk(b.member(existing, 'source'))))),
-
+			b.stmt(b.assignment('=', id, b.call('$.hmr', id))),
 			b.stmt(b.call('import.meta.hot.accept', b.arrow([b.id('module')], b.block(accept_fn_body))))
 		]);
 

packages/svelte/src/internal/client/dev/hmr.js

@@ -1,23 +1,25 @@
-/** @import { Source, Effect, TemplateNode } from '#client' */
+/** @import { Effect, TemplateNode } from '#client' */
 import { FILENAME, HMR } from '../../../constants.js';
 import { EFFECT_TRANSPARENT } from '#client/constants';
 import { hydrate_node, hydrating } from '../dom/hydration.js';
 import { block, branch, destroy_effect } from '../reactivity/effects.js';
-import { source } from '../reactivity/sources.js';
+import { set, source } from '../reactivity/sources.js';
 import { set_should_intro } from '../render.js';
 import { get } from '../runtime.js';
 
 /**
  * @template {(anchor: Comment, props: any) => any} Component
- * @param {Component} original
- * @param {() => Source<Component>} get_source
+ * @param {Component} fn
  */
-export function hmr(original, get_source) {
+export function hmr(fn) {
+	const current = source(fn);
+
 	/**
 	 * @param {TemplateNode} anchor
 	 * @param {any} props
 	 */
 	function wrapper(anchor, props) {
+		let component = {};
 		let instance = {};
 
 		/** @type {Effect} */
@@ -26,8 +28,9 @@ export function hmr(original, get_source) {
 		let ran = false;
 
 		block(() => {
-			const source = get_source();
-			const component = get(source);
+			if (component === (component = get(current))) {
+				return;
+			}
 
 			if (effect) {
 				// @ts-ignore
@@ -62,16 +65,24 @@ export function hmr(original, get_source) {
 	}
 
 	// @ts-expect-error
-	wrapper[FILENAME] = original[FILENAME];
+	wrapper[FILENAME] = fn[FILENAME];
 
 	// @ts-ignore
 	wrapper[HMR] = {
-		// When we accept an update, we set the original source to the new component
-		original,
-		// The `get_source` parameter reads `wrapper[HMR].source`, but in the `accept`
-		// function we always replace it with `previous[HMR].source`, which in practice
-		// means we only ever update the original
-		source: source(original)
+		fn,
+		current,
+		update: (/** @type {any} */ incoming) => {
+			// This logic ensures that the first version of the component is the one
+			// whose update function and therefore block effect is preserved across updates.
+			// If we don't do this dance and instead just use `incoming` as the new component
+			// and then update, we'll create an ever-growing stack of block effects.
+
+			// Trigger the original block effect
+			set(wrapper[HMR].current, incoming[HMR].fn);
+
+			// Replace the incoming source with the original one
+			incoming[HMR].current = wrapper[HMR].current;
+		}
 	};
 
 	return wrapper;

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

@@ -11,11 +11,10 @@ function Hmr($$anchor) {
 }
 
 if (import.meta.hot) {
-	Hmr = $.hmr(Hmr, () => Hmr[$.HMR].source);
+	Hmr = $.hmr(Hmr);
 
 	import.meta.hot.accept((module) => {
-		module.default[$.HMR].source = Hmr[$.HMR].source;
-		$.set(Hmr[$.HMR].source, module.default[$.HMR].original);
+		Hmr[$.HMR].update(module.default);
 	});
 }
 

playgrounds/sandbox/run.js

@@ -95,6 +95,7 @@ for (const generate of /** @type {const} */ (['client', 'server'])) {
 		if (generate === 'server' || FROM_HTML) {
 			from_html = compile(source, {
 				dev: DEV,
+				hmr: DEV,
 				filename: input,
 				generate,
 				runes: argv.values.runes,

playgrounds/sandbox/svelte.config.js

@@ -1,6 +1,6 @@
 export default {
 	compilerOptions: {
-		hmr: false,
+		hmr: true,
 
 		experimental: {
 			async: true