Migrate locale redirect handling to router-server (#62606)

This moves the locale redirect handling out of `base-server` as it
shouldn't be handled here and should be at the routing level. This
avoids the duplicate handling with middleware that causes the incorrect
detection/infinite looping. Test case from separate PR was carried over
to prevent regression.

Fixes: #55648
Closes: #62435
Closes: NEXT-2627
Closes: NEXT-2628

---------

Co-authored-by: Nourman Hajar <nourmanhajar@gmail.com>
Co-authored-by: samcx <sam@vercel.com>

packages/next/src/server/base-server.ts

@@ -52,7 +52,6 @@ import {
   PAGES_MANIFEST,
   STATIC_STATUS_PAGES,
 } from '../shared/lib/constants'
-import { RedirectStatusCode } from '../client/components/redirect-status-code'
 import { isDynamicRoute } from '../shared/lib/router/utils'
 import { checkIsOnDemandRevalidate } from './api-utils'
 import { setConfig } from '../shared/lib/runtime-config.external'
@@ -1198,36 +1197,6 @@ export default abstract class Server<ServerOptions extends Options = Options> {
         }
       }
 
-      if (
-        // Edge runtime always has minimal mode enabled.
-        process.env.NEXT_RUNTIME !== 'edge' &&
-        !this.minimalMode &&
-        defaultLocale
-      ) {
-        const { getLocaleRedirect } =
-          require('../shared/lib/i18n/get-locale-redirect') as typeof import('../shared/lib/i18n/get-locale-redirect')
-        const redirect = getLocaleRedirect({
-          defaultLocale,
-          domainLocale,
-          headers: req.headers,
-          nextConfig: this.nextConfig,
-          pathLocale: pathnameInfo.locale,
-          urlParsed: {
-            ...url,
-            pathname: pathnameInfo.locale
-              ? `/${pathnameInfo.locale}${url.pathname}`
-              : url.pathname,
-          },
-        })
-
-        if (redirect) {
-          return res
-            .redirect(redirect, RedirectStatusCode.TemporaryRedirect)
-            .body(redirect)
-            .send()
-        }
-      }
-
       addRequestMeta(req, 'isLocaleDomain', Boolean(domainLocale))
 
       if (pathnameInfo.locale) {

packages/next/src/server/lib/router-server.ts

@@ -25,6 +25,7 @@ import setupCompression from 'next/dist/compiled/compression'
 import { NoFallbackError } from '../base-server'
 import { signalFromNodeResponse } from '../web/spec-extension/adapters/next-request'
 import { isPostpone } from './router-utils/is-postpone'
+import { parseUrl as parseUrlUtil } from '../../shared/lib/router/utils/parse-url'
 
 import {
   PHASE_PRODUCTION_SERVER,
@@ -34,6 +35,9 @@ import { RedirectStatusCode } from '../../client/components/redirect-status-code
 import { DevBundlerService } from './dev-bundler-service'
 import { type Span, trace } from '../../trace'
 import { ensureLeadingSlash } from '../../shared/lib/page-path/ensure-leading-slash'
+import { getNextPathnameInfo } from '../../shared/lib/router/utils/get-next-pathname-info'
+import { getHostname } from '../../shared/lib/get-hostname'
+import { detectDomainLocale } from '../../shared/lib/i18n/detect-domain-locale'
 
 const debug = setupDebug('next:router-server:main')
 const isNextFont = (pathname: string | null) =>
@@ -140,6 +144,57 @@ export async function initialize(opts: {
     require('./render-server') as typeof import('./render-server')
 
   const requestHandlerImpl: WorkerRequestHandler = async (req, res) => {
+    if (
+      !opts.minimalMode &&
+      config.i18n &&
+      config.i18n.localeDetection !== false
+    ) {
+      const urlParts = (req.url || '').split('?', 1)
+      let urlNoQuery = urlParts[0] || ''
+
+      if (config.basePath) {
+        urlNoQuery = removePathPrefix(urlNoQuery, config.basePath)
+      }
+
+      const pathnameInfo = getNextPathnameInfo(urlNoQuery, {
+        nextConfig: config,
+      })
+
+      const domainLocale = detectDomainLocale(
+        config.i18n.domains,
+        getHostname({ hostname: urlNoQuery }, req.headers)
+      )
+
+      const defaultLocale =
+        domainLocale?.defaultLocale || config.i18n.defaultLocale
+
+      const { getLocaleRedirect } =
+        require('../../shared/lib/i18n/get-locale-redirect') as typeof import('../../shared/lib/i18n/get-locale-redirect')
+
+      const parsedUrl = parseUrlUtil((req.url || '')?.replace(/^\/+/, '/'))
+
+      const redirect = getLocaleRedirect({
+        defaultLocale,
+        domainLocale,
+        headers: req.headers,
+        nextConfig: config,
+        pathLocale: pathnameInfo.locale,
+        urlParsed: {
+          ...parsedUrl,
+          pathname: pathnameInfo.locale
+            ? `/${pathnameInfo.locale}${urlNoQuery}`
+            : urlNoQuery,
+        },
+      })
+
+      if (redirect) {
+        res.setHeader('Location', redirect)
+        res.statusCode = RedirectStatusCode.TemporaryRedirect
+        res.end(redirect)
+        return
+      }
+    }
+
     if (compress) {
       // @ts-expect-error not express req/res
       compress(req, res, () => {})

test/e2e/i18n-preferred-locale-detection/app/middleware.js

@@ -0,0 +1,4 @@
+export async function middleware() {
+  const noop = () => {}
+  noop()
+}

test/e2e/i18n-preferred-locale-detection/app/next.config.js

@@ -0,0 +1,10 @@
+module.exports = {
+  i18n: {
+    locales: ['en', 'id'],
+    defaultLocale: 'en',
+  },
+  experimental: {
+    clientRouterFilter: true,
+    clientRouterFilterRedirects: true,
+  },
+}

test/e2e/i18n-preferred-locale-detection/app/pages/index.js

@@ -0,0 +1,21 @@
+import Link from 'next/link'
+
+export const getServerSideProps = async ({ locale }) => {
+  return {
+    props: {
+      locale,
+    },
+  }
+}
+
+export default function Home({ locale }) {
+  return (
+    <div>
+      <div id="index">Index</div>
+      <div id="current-locale">{locale}</div>
+      <Link href="/new" id="to-new">
+        To new
+      </Link>
+    </div>
+  )
+}

test/e2e/i18n-preferred-locale-detection/app/pages/new.js

@@ -0,0 +1,21 @@
+import Link from 'next/link'
+
+export const getServerSideProps = async ({ locale }) => {
+  return {
+    props: {
+      locale,
+    },
+  }
+}
+
+export default function New({ locale }) {
+  return (
+    <div>
+      <div id="new">New</div>
+      <div id="current-locale">{locale}</div>
+      <Link href="/" id="to-index">
+        To index (No Locale Specified)
+      </Link>
+    </div>
+  )
+}

test/e2e/i18n-preferred-locale-detection/i18n-preferred-locale-detection.test.ts

@@ -0,0 +1,63 @@
+import type { Request } from 'playwright'
+import { join } from 'path'
+import { FileRef, nextTestSetup } from 'e2e-utils'
+
+describe('i18-preferred-locale-redirect', () => {
+  const { next } = nextTestSetup({
+    files: new FileRef(join(__dirname, './app/')),
+  })
+
+  it('should request a path prefixed with my preferred detected locale when accessing index', async () => {
+    const browser = await next.browser('/new', {
+      locale: 'id',
+    })
+
+    let requestedPreferredLocalePathCount = 0
+    browser.on('request', (request: Request) => {
+      if (new URL(request.url(), 'http://n').pathname === '/id') {
+        requestedPreferredLocalePathCount++
+      }
+    })
+
+    const goToIndex = async () => {
+      await browser.get(next.url)
+    }
+
+    await expect(goToIndex()).resolves.not.toThrow(/ERR_TOO_MANY_REDIRECTS/)
+
+    await browser.waitForElementByCss('#index')
+
+    expect(await browser.elementByCss('#index').text()).toBe('Index')
+    expect(await browser.elementByCss('#current-locale').text()).toBe('id')
+
+    expect(requestedPreferredLocalePathCount).toBe(1)
+  })
+
+  it('should not request a path prefixed with my preferred detected locale when clicking link to index from a non-locale-prefixed path', async () => {
+    const browser = await next.browser('/new', {
+      locale: 'id',
+    })
+
+    await browser
+      .waitForElementByCss('#to-index')
+      .click()
+      .waitForElementByCss('#index')
+
+    expect(await browser.elementByCss('#index').text()).toBe('Index')
+    expect(await browser.elementByCss('#current-locale').text()).toBe('en')
+  })
+
+  it('should request a path prefixed with my preferred detected locale when clicking link to index from a locale-prefixed path', async () => {
+    const browser = await next.browser('/id/new', {
+      locale: 'id',
+    })
+
+    await browser
+      .waitForElementByCss('#to-index')
+      .click()
+      .waitForElementByCss('#index')
+
+    expect(await browser.elementByCss('#index').text()).toBe('Index')
+    expect(await browser.elementByCss('#current-locale').text()).toBe('id')
+  })
+})