[next]: ensure defaultLocale redirect doesn't conflict with user redi…

…rects (#12916)

When `localeDetection` is turned off, usually the user intends to handle
redirects themselves, or doesn't want them at all. However because of
the rewrites that we insert for `defaultLocale` handling, it can clobber
the user-provided redirects. eg the rewrite to `/${defaultLocale}` would
then flow through to the user's redirect handling to result in
`/${defaultLocale}/${redirectLocale}` which wouldn't have been the
intention.

When `localeDetection` is turned off, we shouldn't only attempt the
defaultLocale rewrite handling when we've already processed all other
routes (ie in the `handle: miss` case)

Closes NEXT-3975

---------

Co-authored-by: JJ Kasper <[email protected]>

.changeset/honest-seas-pull.md

@@ -0,0 +1,5 @@
+---
+'@vercel/next': patch
+---
+
+ensure defaultLocale rewrite doesn't conflict with user-defined redirects

packages/next/src/server-build.ts

@@ -1941,39 +1941,37 @@ export async function serverBuild({
                     },
                     continue: true,
                   },
+                  {
+                    src: `^${path.posix.join('/', entryDirectory)}$`,
+                    dest: `${path.posix.join(
+                      '/',
+                      entryDirectory,
+                      i18n.defaultLocale
+                    )}`,
+                    continue: true,
+                  },
+                  // Auto-prefix non-locale path with default locale
+                  // note for prerendered pages this will cause
+                  // x-now-route-matches to contain the path minus the locale
+                  // e.g. for /de/posts/[slug] x-now-route-matches would have
+                  // 1=posts%2Fpost-1
+                  {
+                    src: `^${path.posix.join(
+                      '/',
+                      entryDirectory,
+                      '/'
+                    )}(?!(?:_next/.*|${i18n.locales
+                      .map(locale => escapeStringRegexp(locale))
+                      .join('|')})(?:/.*|$))(.*)$`,
+                    dest: `${path.posix.join(
+                      '/',
+                      entryDirectory,
+                      i18n.defaultLocale
+                    )}/$1`,
+                    continue: true,
+                  },
                 ]
               : []),
-
-            {
-              src: `^${path.posix.join('/', entryDirectory)}$`,
-              dest: `${path.posix.join(
-                '/',
-                entryDirectory,
-                i18n.defaultLocale
-              )}`,
-              continue: true,
-            },
-
-            // Auto-prefix non-locale path with default locale
-            // note for prerendered pages this will cause
-            // x-now-route-matches to contain the path minus the locale
-            // e.g. for /de/posts/[slug] x-now-route-matches would have
-            // 1=posts%2Fpost-1
-            {
-              src: `^${path.posix.join(
-                '/',
-                entryDirectory,
-                '/'
-              )}(?!(?:_next/.*|${i18n.locales
-                .map(locale => escapeStringRegexp(locale))
-                .join('|')})(?:/.*|$))(.*)$`,
-              dest: `${path.posix.join(
-                '/',
-                entryDirectory,
-                i18n.defaultLocale
-              )}/$1`,
-              continue: true,
-            },
           ]
         : []),
 
@@ -2256,6 +2254,39 @@ export async function serverBuild({
       // to allow checking non-prefixed lambda outputs
       ...(i18n
         ? [
+            ...(i18n.localeDetection === false
+              ? [
+                  {
+                    src: `^${path.posix.join('/', entryDirectory)}$`,
+                    dest: `${path.posix.join(
+                      '/',
+                      entryDirectory,
+                      i18n.defaultLocale
+                    )}`,
+                    check: true,
+                  },
+                  // Auto-prefix non-locale path with default locale
+                  // note for prerendered pages this will cause
+                  // x-now-route-matches to contain the path minus the locale
+                  // e.g. for /de/posts/[slug] x-now-route-matches would have
+                  // 1=posts%2Fpost-1
+                  {
+                    src: `^${path.posix.join(
+                      '/',
+                      entryDirectory,
+                      '/'
+                    )}(?!(?:_next/.*|${i18n.locales
+                      .map(locale => escapeStringRegexp(locale))
+                      .join('|')})(?:/.*|$))(.*)$`,
+                    dest: `${path.posix.join(
+                      '/',
+                      entryDirectory,
+                      i18n.defaultLocale
+                    )}/$1`,
+                    check: true,
+                  },
+                ]
+              : []),
             {
               src: path.posix.join(
                 '/',

packages/next/src/utils.ts

@@ -554,11 +554,14 @@ export function localizeDynamicRoutes(
   isCorrectLocaleAPIRoutes?: boolean,
   inversedAppPathRoutesManifest?: Record<string, string>
 ): RouteWithSrc[] {
-  return dynamicRoutes.map((route: RouteWithSrc) => {
-    // i18n is already handled for middleware
-    if (route.middleware !== undefined || route.middlewarePath !== undefined)
-      return route;
+  const finalDynamicRoutes: RouteWithSrc[] = [];
 
+  for (const route of dynamicRoutes as RouteWithSrc[]) {
+    // i18n is already handled for middleware
+    if (route.middleware !== undefined || route.middlewarePath !== undefined) {
+      finalDynamicRoutes.push(route);
+      continue;
+    }
     const { i18n } = routesManifest || {};
 
     if (i18n) {
@@ -577,6 +580,18 @@ export function localizeDynamicRoutes(
       const isLocalePrefixed =
         isFallback || isBlocking || isAutoExport || isServerMode;
 
+      // when locale detection is disabled we don't add the default locale
+      // to the path while resolving routes so we need to be able to match
+      // without it being present
+      if (isLocalePrefixed && routesManifest?.i18n?.localeDetection === false) {
+        const nonLocalePrefixedRoute = JSON.parse(JSON.stringify(route));
+        nonLocalePrefixedRoute.src = nonLocalePrefixedRoute.src.replace(
+          '^',
+          `^${dynamicPrefix || ''}[/]?`
+        );
+        finalDynamicRoutes.push(nonLocalePrefixedRoute);
+      }
+
       route.src = route.src.replace(
         '^',
         `^${dynamicPrefix ? `${dynamicPrefix}[/]?` : '[/]?'}(?${
@@ -599,8 +614,10 @@ export function localizeDynamicRoutes(
     } else {
       route.src = route.src.replace('^', `^${dynamicPrefix}`);
     }
-    return route;
-  });
+    finalDynamicRoutes.push(route);
+  }
+
+  return finalDynamicRoutes;
 }
 
 type LoaderKey = 'imgix' | 'cloudinary' | 'akamai' | 'default';

packages/next/test/fixtures/00-i18n-support-no-locale-detection/next.config.js

@@ -2,6 +2,34 @@ module.exports = {
   generateBuildId() {
     return 'testing-build-id';
   },
+  async redirects() {
+    return [
+      {
+        source: '/',
+        has: [
+          {
+            type: 'header',
+            key: 'x-redirect-me',
+          },
+        ],
+        destination: '/fr-BE',
+        permanent: false,
+        locale: false,
+      },
+      {
+        source: '/:path+',
+        has: [
+          {
+            type: 'header',
+            key: 'x-redirect-me',
+          },
+        ],
+        destination: '/fr-BE/:path+',
+        permanent: false,
+        locale: false,
+      },
+    ];
+  },
   i18n: {
     localeDetection: false,
     locales: ['nl-NL', 'nl-BE', 'nl', 'fr-BE', 'fr', 'en-US', 'en'],

packages/next/test/fixtures/00-i18n-support-no-locale-detection/vercel.json

@@ -520,6 +520,19 @@
       "path": "/_next/data/testing-build-id/fr/gsp/blocking/first.json",
       "status": 200,
       "mustContain": "\"catchall\":\"yes\""
+    },
+    {
+      "path": "/",
+      "status": 307,
+      "headers": {
+        "x-redirect-me": "1"
+      },
+      "fetchOptions": {
+        "redirect": "manual"
+      },
+      "responseHeaders": {
+        "location": "//fr-BE/"
+      }
     }
   ]
 }