fix: update original source in HMR update (#12547)

* fix: update original source in HMR update

* tidy up

* comments

* oops

Author: Rich-Harris

.changeset/small-planets-destroy.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: update original source in HMR update

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

@@ -417,8 +417,19 @@ export function client_component(source, analysis, options) {
 	);
 
 	if (options.hmr) {
+		const id = b.id(analysis.name);
+		const HMR = b.id('$.HMR');
+
+		const existing = b.member(id, HMR, true);
+		const incoming = b.member(b.id('module.default'), HMR, true);
+
 		const accept_fn_body = [
-			b.stmt(b.call('$.set', b.id('s'), b.member(b.id('module.default'), b.id('$.ORIGINAL'), true)))
+			b.stmt(
+				b.assignment('=', b.member(incoming, b.id('source')), b.member(existing, b.id('source')))
+			),
+			b.stmt(
+				b.call('$.set', b.member(existing, b.id('source')), b.member(incoming, b.id('original')))
+			)
 		];
 
 		if (analysis.css.hash) {
@@ -438,26 +449,10 @@ export function client_component(source, analysis, options) {
 		}
 
 		const hmr = b.block([
-			b.const(b.id('s'), b.call('$.source', b.id(analysis.name))),
-			b.const(b.id('$$filename'), b.member(b.id(analysis.name), b.id('$.FILENAME'), true)),
-			b.const(b.id('$$original'), b.id(analysis.name)),
-			b.stmt(b.assignment('=', b.id(analysis.name), b.call('$.hmr', b.id('s')))),
 			b.stmt(
-				b.assignment(
-					'=',
-					b.member(b.id(analysis.name), b.id('$.FILENAME'), true),
-					b.id('$$filename')
-				)
-			),
-			// Assign the original component to the wrapper so we can use it on hot reload patching,
-			// else we would call the HMR function two times
-			b.stmt(
-				b.assignment(
-					'=',
-					b.member(b.id(analysis.name), b.id('$.ORIGINAL'), true),
-					b.id('$$original')
-				)
+				b.assignment('=', id, b.call('$.hmr', id, b.thunk(b.member(existing, b.id('source')))))
 			),
+
 			b.stmt(b.call('import.meta.hot.accept', b.arrow([b.id('module')], b.block(accept_fn_body))))
 		]);
 

packages/svelte/src/constants.js

@@ -32,7 +32,7 @@ export const UNINITIALIZED = Symbol();
 
 // Dev-time component properties
 export const FILENAME = Symbol('filename');
-export const ORIGINAL = Symbol('original');
+export const HMR = Symbol('hmr');
 
 /** List of elements that require raw contents and should not have SSR comments put in them */
 export const RawTextElements = ['textarea', 'script', 'style', 'title'];

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

@@ -1,19 +1,22 @@
 /** @import { Source, Effect } from '#client' */
+import { FILENAME, HMR } from '../../../constants.js';
 import { EFFECT_TRANSPARENT } from '../constants.js';
 import { block, branch, destroy_effect } from '../reactivity/effects.js';
+import { 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 {Source<Component>} source
+ * @param {Component} original
+ * @param {() => Source<Component>} get_source
  */
-export function hmr(source) {
+export function hmr(original, get_source) {
 	/**
 	 * @param {Comment} anchor
 	 * @param {any} props
 	 */
-	return function (anchor, props) {
+	function wrapper(anchor, props) {
 		let instance = {};
 
 		/** @type {Effect} */
@@ -22,6 +25,7 @@ export function hmr(source) {
 		let ran = false;
 
 		block(() => {
+			const source = get_source();
 			const component = get(source);
 
 			if (effect) {
@@ -50,5 +54,20 @@ export function hmr(source) {
 		ran = true;
 
 		return instance;
+	}
+
+	// @ts-expect-error
+	wrapper[FILENAME] = original[FILENAME];
+
+	// @ts-expect-error
+	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)
 	};
+
+	return wrapper;
 }

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

@@ -1,4 +1,4 @@
-export { FILENAME, ORIGINAL } from '../../constants.js';
+export { FILENAME, HMR } from '../../constants.js';
 export { add_locations } from './dev/elements.js';
 export { hmr } from './dev/hmr.js';
 export {

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

@@ -1,6 +1,6 @@
 /** @import { Component, Payload, RenderOutput } from '#server' */
 /** @import { Store } from '#shared' */
-export { FILENAME, ORIGINAL } from '../../constants.js';
+export { FILENAME, HMR } from '../../constants.js';
 import { is_promise, noop } from '../shared/utils.js';
 import { subscribe_to_store } from '../../store/utils.js';
 import {

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

@@ -10,16 +10,11 @@ function Hmr($$anchor) {
 }
 
 if (import.meta.hot) {
-	const s = $.source(Hmr);
-	const $$filename = Hmr[$.FILENAME];
-	const $$original = Hmr;
-
-	Hmr = $.hmr(s);
-	Hmr[$.FILENAME] = $$filename;
-	Hmr[$.ORIGINAL] = $$original;
+	Hmr = $.hmr(Hmr, () => Hmr[$.HMR].source);
 
 	import.meta.hot.accept((module) => {
-		$.set(s, module.default[$.ORIGINAL]);
+		module.default[$.HMR].source = Hmr[$.HMR].source;
+		$.set(Hmr[$.HMR].source, module.default[$.HMR].original);
 	});
 }