fix(middleware): consider localhost variations (vercel#31603)

Since `localhost` is actually an alias for `127.0.0.1` that points to loopback, we should take that into consideration at `NextURL` when we handle local URLs.

The implementation is based on [is-localhost-url](https://github.com/Kikobeats/is-localhost-url); I added some tests over local URLs variations present at the library to ensure other variations are working fine.

Additionally, I refactor some things over the code to avoid doing the same twice and added some legibility that is always welcome when you are working with URLs stuff.

 closes vercel#31533

packages/next/server/web/next-url.ts

@@ -20,6 +20,9 @@ interface Options {
   trailingSlash?: boolean
 }
 
+const REGEX_LOCALHOST_HOSTNAME =
+  /(?!^https?:\/\/)(127(?:\.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)){3}|::1)/
+
 export class NextURL extends URL {
   private _basePath: string
   private _locale?: {
@@ -33,11 +36,12 @@ export class NextURL extends URL {
   private _options: Options
   private _url: URL
 
-  constructor(url: string, options: Options = {}) {
-    super(formatRelative(url))
+  constructor(input: string, options: Options = {}) {
+    const url = createWHATWGURL(input)
+    super(url)
     this._options = options
     this._basePath = ''
-    this._url = formatRelative(url)
+    this._url = url
     this.analyzeUrl()
   }
 
@@ -163,7 +167,7 @@ export class NextURL extends URL {
   }
 
   set href(url: string) {
-    this._url = formatRelative(url)
+    this._url = createWHATWGURL(url)
     this.analyzeUrl()
   }
 
@@ -228,8 +232,13 @@ export class NextURL extends URL {
   }
 }
 
-function formatRelative(url: string) {
-  return url.startsWith('/')
+function createWHATWGURL(url: string) {
+  url = url.replace(REGEX_LOCALHOST_HOSTNAME, 'localhost')
+  return isRelativeURL(url)
     ? new URL(url.replace(/^\/+/, '/'), new URL('https://localhost'))
     : new URL(url)
 }
+
+function isRelativeURL(url: string) {
+  return url.startsWith('/')
+}

test/unit/web-runtime/utils.test.ts → test/unit/web-runtime/next-url.test.ts

@@ -119,3 +119,15 @@ it('parses and formats the default locale', () => {
   expect(url.locale).toEqual('fr')
   expect(url.toString()).toEqual('/root/fr/bar')
 })
+
+it('consider 127.0.0.1 and variations as localhost', () => {
+  const httpUrl = new NextURL('http://localhost:3000/hello')
+  expect(new NextURL('http://127.0.0.1:3000/hello')).toStrictEqual(httpUrl)
+  expect(new NextURL('http://127.0.1.0:3000/hello')).toStrictEqual(httpUrl)
+  expect(new NextURL('http://::1:3000/hello')).toStrictEqual(httpUrl)
+
+  const httpsUrl = new NextURL('https://localhost:3000/hello')
+  expect(new NextURL('https://127.0.0.1:3000/hello')).toStrictEqual(httpsUrl)
+  expect(new NextURL('https://127.0.1.0:3000/hello')).toStrictEqual(httpsUrl)
+  expect(new NextURL('https://::1:3000/hello')).toStrictEqual(httpsUrl)
+})