fix(i18n): fallback index when routing is prefix-always (#9032)

* fix(i18n): fallback index when routing is prefix-always

* chore: add comment as per feedback

4 files changed, 106 insertions, 69 deletions

diff --git a/packages/astro/src/core/build/buildPipeline.ts b/packages/astro/src/core/build/buildPipeline.ts
index b668c0756..83e45f808 100644
--- a/packages/astro/src/core/build/buildPipeline.ts
+++ b/packages/astro/src/core/build/buildPipeline.ts
@@ -161,7 +161,11 @@ export class BuildPipeline extends Pipeline {
 			for (const pageData of pageDataList) {
 				if (routeIsRedirect(pageData.route)) {
 					pages.set(pageData, path);
-				} else if (routeIsFallback(pageData.route) && i18nHasFallback(this.getConfig())) {
+				} else if (
+					routeIsFallback(pageData.route) &&
+					(i18nHasFallback(this.getConfig()) ||
+						(routeIsFallback(pageData.route) && pageData.route.route === '/'))
+				) {
 					pages.set(pageData, path);
 				}
 			}
diff --git a/packages/astro/src/core/routing/manifest/create.ts b/packages/astro/src/core/routing/manifest/create.ts
index 6f2409091..f5b6e0ccd 100644
--- a/packages/astro/src/core/routing/manifest/create.ts
+++ b/packages/astro/src/core/routing/manifest/create.ts
@@ -485,13 +485,13 @@ export function createRouteManifest(
 		routes.push(routeData);
 	});
 	const i18n = settings.config.experimental.i18n;
-
-	if (i18n && i18n.fallback) {
-		let fallback = Object.entries(i18n.fallback);
+	if (i18n) {
+		// In this block of code we group routes based on their locale
 
 		// A map like: locale => RouteData[]
 		const routesByLocale = new Map<string, RouteData[]>();
-		// We create a set, so we can remove the routes that have been added to the previous map
+		// This type is here only as a helper. We copy the routes and make them unique, so we don't "process" the same route twice.
+		// The assumption is that a route in the file system belongs to only one locale.
 		const setRoutes = new Set(routes);
 
 		// First loop
@@ -524,70 +524,110 @@ export function createRouteManifest(
 			setRoutes.delete(route);
 		}
 
-		if (fallback.length > 0) {
-			for (const [fallbackFromLocale, fallbackToLocale] of fallback) {
-				let fallbackToRoutes;
-				if (fallbackToLocale === i18n.defaultLocale) {
-					fallbackToRoutes = routesByLocale.get(i18n.defaultLocale);
-				} else {
-					fallbackToRoutes = routesByLocale.get(fallbackToLocale);
-				}
-				const fallbackFromRoutes = routesByLocale.get(fallbackFromLocale);
+		// Work done, now we start creating "fallback" routes based on the configuration
 
-				// Technically, we should always have a fallback to. Added this to make TS happy.
-				if (!fallbackToRoutes) {
-					continue;
+		if (i18n.routingStrategy === 'prefix-always') {
+			// we attempt to retrieve the index page of the default locale
+			const defaultLocaleRoutes = routesByLocale.get(i18n.defaultLocale);
+			if (defaultLocaleRoutes) {
+				const indexDefaultRoute = defaultLocaleRoutes.find((routeData) => {
+					// it should be safe to assume that an index page has "index" in their name
+					return routeData.component.includes('index');
+				});
+				if (indexDefaultRoute) {
+					// we found the index of the default locale, now we create a root index that will redirect to the index of the default locale
+					const pathname = '/';
+					const route = '/';
+
+					const segments = removeLeadingForwardSlash(route)
+						.split(path.posix.sep)
+						.filter(Boolean)
+						.map((s: string) => {
+							validateSegment(s);
+							return getParts(s, route);
+						});
+					routes.push({
+						...indexDefaultRoute,
+						pathname,
+						route,
+						segments,
+						pattern: getPattern(segments, config),
+						type: 'fallback',
+					});
 				}
+			}
+		}
+
+		if (i18n.fallback) {
+			let fallback = Object.entries(i18n.fallback);
+
+			if (fallback.length > 0) {
+				for (const [fallbackFromLocale, fallbackToLocale] of fallback) {
+					let fallbackToRoutes;
+					if (fallbackToLocale === i18n.defaultLocale) {
+						fallbackToRoutes = routesByLocale.get(i18n.defaultLocale);
+					} else {
+						fallbackToRoutes = routesByLocale.get(fallbackToLocale);
+					}
+					const fallbackFromRoutes = routesByLocale.get(fallbackFromLocale);
 
-				for (const fallbackToRoute of fallbackToRoutes) {
-					const hasRoute =
-						fallbackFromRoutes &&
-						// we check if the fallback from locale (the origin) has already this route
-						fallbackFromRoutes.some((route) => {
+					// Technically, we should always have a fallback to. Added this to make TS happy.
+					if (!fallbackToRoutes) {
+						continue;
+					}
+
+					for (const fallbackToRoute of fallbackToRoutes) {
+						const hasRoute =
+							fallbackFromRoutes &&
+							// we check if the fallback from locale (the origin) has already this route
+							fallbackFromRoutes.some((route) => {
+								if (fallbackToLocale === i18n.defaultLocale) {
+									return (
+										route.route.replace(`/${fallbackFromLocale}`, '') === fallbackToRoute.route
+									);
+								} else {
+									return (
+										route.route.replace(`/${fallbackToLocale}`, `/${fallbackFromLocale}`) ===
+										fallbackToRoute.route
+									);
+								}
+							});
+
+						if (!hasRoute) {
+							let pathname: string | undefined;
+							let route: string;
 							if (fallbackToLocale === i18n.defaultLocale) {
-								return route.route.replace(`/${fallbackFromLocale}`, '') === fallbackToRoute.route;
+								if (fallbackToRoute.pathname) {
+									pathname = `/${fallbackFromLocale}${fallbackToRoute.pathname}`;
+								}
+								route = `/${fallbackFromLocale}${fallbackToRoute.route}`;
 							} else {
-								return (
-									route.route.replace(`/${fallbackToLocale}`, `/${fallbackFromLocale}`) ===
-									fallbackToRoute.route
+								pathname = fallbackToRoute.pathname?.replace(
+									`/${fallbackToLocale}`,
+									`/${fallbackFromLocale}`
+								);
+								route = fallbackToRoute.route.replace(
+									`/${fallbackToLocale}`,
+									`/${fallbackFromLocale}`
 								);
 							}
-						});
-
-					if (!hasRoute) {
-						let pathname: string | undefined;
-						let route: string;
-						if (fallbackToLocale === i18n.defaultLocale) {
-							if (fallbackToRoute.pathname) {
-								pathname = `/${fallbackFromLocale}${fallbackToRoute.pathname}`;
-							}
-							route = `/${fallbackFromLocale}${fallbackToRoute.route}`;
-						} else {
-							pathname = fallbackToRoute.pathname?.replace(
-								`/${fallbackToLocale}`,
-								`/${fallbackFromLocale}`
-							);
-							route = fallbackToRoute.route.replace(
-								`/${fallbackToLocale}`,
-								`/${fallbackFromLocale}`
-							);
-						}
 
-						const segments = removeLeadingForwardSlash(route)
-							.split(path.posix.sep)
-							.filter(Boolean)
-							.map((s: string) => {
-								validateSegment(s);
-								return getParts(s, route);
+							const segments = removeLeadingForwardSlash(route)
+								.split(path.posix.sep)
+								.filter(Boolean)
+								.map((s: string) => {
+									validateSegment(s);
+									return getParts(s, route);
+								});
+							routes.push({
+								...fallbackToRoute,
+								pathname,
+								route,
+								segments,
+								pattern: getPattern(segments, config),
+								type: 'fallback',
 							});
-						routes.push({
-							...fallbackToRoute,
-							pathname,
-							route,
-							segments,
-							pattern: getPattern(segments, config),
-							type: 'fallback',
-						});
+						}
 					}
 				}
 			}
diff --git a/packages/astro/src/vite-plugin-astro-server/route.ts b/packages/astro/src/vite-plugin-astro-server/route.ts
index 2b0bcbea7..89173a1ec 100644
--- a/packages/astro/src/vite-plugin-astro-server/route.ts
+++ b/packages/astro/src/vite-plugin-astro-server/route.ts
@@ -187,7 +187,8 @@ export async function handleRoute({
 				.some((segment) => {
 					return locales.includes(segment);
 				});
-			if (!pathNameHasLocale) {
+			// Even when we have `config.base`, the pathname is still `/` because it gets stripped before
+			if (!pathNameHasLocale && pathname !== '/') {
 				return handle404Response(origin, incomingRequest, incomingResponse);
 			}
 			request = createRequest({
diff --git a/packages/astro/test/fixtures/i18n-routing-prefix-always/src/pages/index.astro b/packages/astro/test/fixtures/i18n-routing-prefix-always/src/pages/index.astro
deleted file mode 100644
index 05faf7b0b..000000000
--- a/packages/astro/test/fixtures/i18n-routing-prefix-always/src/pages/index.astro
+++ /dev/null
@@ -1,8 +0,0 @@
-<html>
-<head>
-	<title>Astro</title>
-</head>
-<body>
-	Hello
-</body>
-</html>