fix: edge-middleware i18n matching

edge-runtime/lib/next-request.ts

@@ -2,6 +2,7 @@ import type { Context } from '@netlify/edge-functions'
 
 import {
   addBasePath,
+  addLocale,
   addTrailingSlash,
   normalizeDataUrl,
   normalizeLocalePath,
@@ -73,6 +74,33 @@ const normalizeRequestURL = (
   }
 }
 
+export const localizeRequest = (
+  url: URL,
+  nextConfig?: {
+    basePath?: string
+    i18n?: I18NConfig | null
+  },
+): { localizedUrl: URL; locale?: string } => {
+  const localizedUrl = new URL(url)
+  localizedUrl.pathname = removeBasePath(localizedUrl.pathname, nextConfig?.basePath)
+
+  // Detect the locale from the URL
+  const { detectedLocale } = normalizeLocalePath(localizedUrl.pathname, nextConfig?.i18n?.locales)
+
+  // Add the locale to the URL if not already present
+  localizedUrl.pathname = addLocale(
+    localizedUrl.pathname,
+    detectedLocale ?? nextConfig?.i18n?.defaultLocale,
+  )
+
+  localizedUrl.pathname = addBasePath(localizedUrl.pathname, nextConfig?.basePath)
+
+  return {
+    localizedUrl,
+    locale: detectedLocale,
+  }
+}
+
 export const buildNextRequest = (
   request: Request,
   context: Context,

edge-runtime/lib/util.ts

@@ -29,6 +29,20 @@ export const addBasePath = (path: string, basePath?: string) => {
   return path
 }
 
+// add locale prefix if not present, allowing for locale fallbacks
+export const addLocale = (path: string, locale?: string) => {
+  if (
+    locale &&
+    path.toLowerCase() !== `/${locale.toLowerCase()}` &&
+    !path.toLowerCase().startsWith(`/${locale.toLowerCase()}/`) &&
+    !path.startsWith(`/api/`) &&
+    !path.startsWith(`/_next/`)
+  ) {
+    return `/${locale}${path}`
+  }
+  return path
+}
+
 // https://github.com/vercel/next.js/blob/canary/packages/next/src/shared/lib/i18n/normalize-locale-path.ts
 
 export interface PathLocale {

edge-runtime/middleware.ts

@@ -5,7 +5,7 @@ import nextConfig from './next.config.json' with { type: 'json' }
 
 import { InternalHeaders } from './lib/headers.ts'
 import { logger, LogLevel } from './lib/logging.ts'
-import { buildNextRequest, RequestData } from './lib/next-request.ts'
+import { buildNextRequest, localizeRequest, RequestData } from './lib/next-request.ts'
 import { buildResponse, FetchEventResult } from './lib/response.ts'
 import {
   getMiddlewareRouteMatcher,
@@ -31,25 +31,29 @@ export async function handleMiddleware(
   context: Context,
   nextHandler: NextHandler,
 ) {
-  const nextRequest = buildNextRequest(request, context, nextConfig)
   const url = new URL(request.url)
+
   const reqLogger = logger
     .withLogLevel(
       request.headers.has(InternalHeaders.NFDebugLogging) ? LogLevel.Debug : LogLevel.Log,
     )
     .withFields({ url_path: url.pathname })
     .withRequestID(request.headers.get(InternalHeaders.NFRequestID))
 
+  const { localizedUrl } = localizeRequest(url, nextConfig)
   // While we have already checked the path when mapping to the edge function,
   // Next.js supports extra rules that we need to check here too, because we
   // might be running an edge function for a path we should not. If we find
   // that's the case, short-circuit the execution.
-  if (!matchesMiddleware(url.pathname, request, searchParamsToUrlQuery(url.searchParams))) {
+  if (
+    !matchesMiddleware(localizedUrl.pathname, request, searchParamsToUrlQuery(url.searchParams))
+  ) {
     reqLogger.debug('Aborting middleware due to runtime rules')
 
     return
   }
 
+  const nextRequest = buildNextRequest(request, context, nextConfig)
   try {
     const result = await nextHandler({ request: nextRequest })
     const response = await buildResponse({

src/build/functions/edge.ts

@@ -65,10 +65,7 @@ const writeHandlerFile = async (ctx: PluginContext, { matchers, name }: NextDefi
 
   // Writing a file with the matchers that should trigger this function. We'll
   // read this file from the function at runtime.
-  await writeFile(
-    join(handlerRuntimeDirectory, 'matchers.json'),
-    JSON.stringify(augmentMatchers(matchers, ctx)),
-  )
+  await writeFile(join(handlerRuntimeDirectory, 'matchers.json'), JSON.stringify(matchers))
 
   // The config is needed by the edge function to match and normalize URLs. To
   // avoid shipping and parsing a large file at runtime, let's strip it down to

tests/fixtures/middleware-conditions/middleware.ts

@@ -19,7 +19,7 @@ export const config = {
       source: '/hello',
     },
     {
-      source: '/nl-NL/about',
+      source: '/nl/about',
       locale: false,
     },
   ],

tests/integration/edge-handler.test.ts

@@ -261,26 +261,29 @@ describe("aborts middleware execution when the matcher conditions don't match th
 
     ctx.cleanup?.push(() => origin.stop())
 
-    for (const path of ['/hello', '/en/hello', '/nl-NL/hello', '/nl-NL/about']) {
+    for (const path of ['/hello', '/en/hello', '/nl/hello', '/nl/about']) {
       const response = await invokeEdgeFunction(ctx, {
         functions: ['___netlify-edge-handler-middleware'],
         origin,
         url: path,
       })
-      expect(response.headers.has('x-hello-from-middleware-res'), `does match ${path}`).toBeTruthy()
+      expect(
+        response.headers.has('x-hello-from-middleware-res'),
+        `should match ${path}`,
+      ).toBeTruthy()
       expect(await response.text()).toBe('Hello from origin!')
       expect(response.status).toBe(200)
     }
 
-    for (const path of ['/hello/invalid', '/about', '/en/about']) {
+    for (const path of ['/invalid/hello', '/hello/invalid', '/about', '/en/about']) {
       const response = await invokeEdgeFunction(ctx, {
         functions: ['___netlify-edge-handler-middleware'],
         origin,
         url: path,
       })
       expect(
         response.headers.has('x-hello-from-middleware-res'),
-        `does not match ${path}`,
+        `should not match ${path}`,
       ).toBeFalsy()
       expect(await response.text()).toBe('Hello from origin!')
       expect(response.status).toBe(200)