fix: replace version in generateBundle (#12700)

In #8957 one of the desired outcomes was that the framework code would have its own chunk with a stable hash for better caching, but that didn't really happen because the client imports something that references version, through __sveltekit/environment for created_updated_store and version hash through __sveltekit/path, which meant that when the developer changes their app code and increments/changes the version it'd also change the above mentioned modules and subsequently most chunks with the framework code

In this PR the version is injected in generateBundle once the chunk hashes have already been calculated so that just changing the version doesn't affect the hashes for chunks without any actual code changes

fixes #12260

Author: gtm-nayan

.changeset/moody-zoos-compare.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: ensure a changing `version` doesn't affect the hashes for chunks without any actual code changes

packages/kit/src/constants.js

@@ -9,3 +9,15 @@ export const GENERATED_COMMENT = '// this file is generated — do not edit it\n
 export const ENDPOINT_METHODS = ['GET', 'POST', 'PUT', 'PATCH', 'DELETE', 'OPTIONS', 'HEAD'];
 
 export const PAGE_METHODS = ['GET', 'POST', 'HEAD'];
+
+/**
+ * Placeholders for the hash of the app version.
+ * Later replaced in the generateBundle hook to avoid affecting the chunk hash.
+ */
+export const APP_VERSION_HASH_PLACEHOLDER_BASE = '__SVELTEKIT_APP_VERSION_HASH__';
+
+/**
+ * Placeholder for the app version.
+ * Later replaced in the generateBundle hook to avoid affecting the chunk hash.
+ */
+export const APP_VERSION_PLACEHOLDER_BASE = '__SVELTEKIT_APP_VERSION__';

packages/kit/src/exports/vite/index.js

@@ -35,6 +35,10 @@ import {
 	sveltekit_server
 } from './module_ids.js';
 import { resolve_peer_dependency } from '../../utils/import.js';
+import {
+	APP_VERSION_PLACEHOLDER_BASE,
+	APP_VERSION_HASH_PLACEHOLDER_BASE
+} from '../../constants.js';
 
 const cwd = process.cwd();
 
@@ -184,7 +188,16 @@ async function kit({ svelte_config }) {
 	const { kit } = svelte_config;
 	const out = `${kit.outDir}/output`;
 
-	const version_hash = hash(kit.version.name);
+	const app_version = kit.version.name;
+	const version_hash = hash(app_version);
+
+	// if the app version or hash is longer than the placeholder, we need to pad it to avoid
+	// source map damage
+	const app_version_placeholder = APP_VERSION_PLACEHOLDER_BASE.padEnd(app_version.length, '_');
+	const app_version_hash_placeholder = APP_VERSION_HASH_PLACEHOLDER_BASE.padEnd(
+		version_hash.length,
+		'_'
+	);
 
 	/** @type {import('vite').ResolvedConfig} */
 	let vite_config;
@@ -384,7 +397,7 @@ async function kit({ svelte_config }) {
 			const browser = !options?.ssr;
 
 			const global = is_build
-				? `globalThis.__sveltekit_${version_hash}`
+				? `globalThis.__sveltekit_${browser ? app_version_hash_placeholder : version_hash}`
 				: 'globalThis.__sveltekit_dev';
 
 			if (options?.ssr === false && process.env.TEST !== 'true') {
@@ -470,10 +483,8 @@ async function kit({ svelte_config }) {
 				}
 
 				case sveltekit_environment: {
-					const { version } = svelte_config.kit;
-
 					return dedent`
-						export const version = ${s(version.name)};
+						export const version = ${is_build && browser ? app_version_placeholder : s(kit.version.name)};
 						export let building = false;
 						export let prerendering = false;
 
@@ -923,7 +934,50 @@ async function kit({ svelte_config }) {
 		}
 	};
 
-	return [plugin_setup, plugin_virtual_modules, plugin_guard, plugin_compile];
+	/** @type {import('vite').Plugin} */
+	const plugin_replace_version_and_hash = {
+		name: 'vite-plugin-svelte-replace-version-and-hash',
+		enforce: 'post',
+
+		generateBundle(_, bundle, __) {
+			if (vite_config.build.ssr) return;
+
+			for (const file in bundle) {
+				if (bundle[file].type !== 'chunk') continue;
+				const chunk = /** @type {import('rollup').OutputChunk} */ (bundle[file]);
+				let code = chunk.code;
+				if (
+					!(code.includes(app_version_placeholder) || code.includes(app_version_hash_placeholder))
+				)
+					continue;
+
+				// replace the version and version after the chunk hash has already been calculated
+				// to avoid affecting the chunk hash
+				const substitutions = [
+					[app_version_hash_placeholder, version_hash],
+					[app_version_placeholder, JSON.stringify(kit.version.name)]
+				];
+
+				for (const [placeholder, replacement] of substitutions) {
+					code = code.replaceAll(
+						placeholder,
+						// pad the replacement to mitigate source map changes
+						replacement.padEnd(placeholder.length, ' ')
+					);
+				}
+
+				chunk.code = code;
+			}
+		}
+	};
+
+	return [
+		plugin_setup,
+		plugin_virtual_modules,
+		plugin_guard,
+		plugin_compile,
+		plugin_replace_version_and_hash
+	];
 }
 
 /**