fix(i18n): fallback SSR

Author: ematipico

.changeset/old-pugs-jog.md

@@ -0,0 +1,5 @@
+---
+"astro": patch
+---
+
+Fixes a case where the i18n fallback faild to correctly redirect to the index when the fallback is enabled in SSR

packages/astro/src/i18n/index.ts

@@ -343,6 +343,7 @@ export function redirectToFallback({
 	locales,
 	defaultLocale,
 	strategy,
+	base,
 }: MiddlewarePayload) {
 	return function (context: APIContext, response: Response): Response {
 		if (response.status >= 300 && fallback) {
@@ -370,7 +371,11 @@ export function redirectToFallback({
 				// If a locale falls back to the default locale, we want to **remove** the locale because
 				// the default locale doesn't have a prefix
 				if (pathFallbackLocale === defaultLocale && strategy === 'pathname-prefix-other-locales') {
-					newPathname = context.url.pathname.replace(`/${urlLocale}`, ``);
+					if (context.url.pathname.includes(`${base}`)) {
+						newPathname = context.url.pathname.replace(`/${urlLocale}`, ``);
+					} else {
+						newPathname = context.url.pathname.replace(`/${urlLocale}`, `/`);
+					}
 				} else {
 					newPathname = context.url.pathname.replace(`/${urlLocale}`, `/${pathFallbackLocale}`);
 				}

packages/astro/test/i18n-routing.test.js

@@ -1871,3 +1871,36 @@ describe('i18n routing does not break assets and endpoints', () => {
 		});
 	});
 });
+
+describe.only('SSR fallback from missing locale index to default locale index', () => {
+	/** @type {import('./test-utils').Fixture} */
+	let fixture;
+	let app;
+
+	before(async () => {
+		fixture = await loadFixture({
+			root: './fixtures/i18n-routing-prefix-other-locales/',
+			output: 'server',
+			adapter: testAdapter(),
+			i18n: {
+				defaultLocale: 'en',
+				locales: ['en', 'fr'],
+				routing: {
+					prefixDefaultLocale: false,
+				},
+				fallback: {
+					fr: 'en',
+				},
+			},
+		});
+		await fixture.build();
+		app = await fixture.loadTestAdapterApp();
+	});
+
+	it.only('should correctly redirect', async () => {
+		let request = new Request('http://example.com/fr');
+		let response = await app.render(request);
+		assert.equal(response.status, 302);
+		assert.equal(response.headers.get('location'), '/');
+	});
+});