Fix running server with Polyfilled fetch (vercel#31935)

This PR fixes vercel#30398

By default Next will polyfill some fetch APIs (Request, Response, Header and fetch) only if fetch is not found in the global scope in certain entry points. If we have a custom server which is adding a global fetch (and only fetch) at the very top then the rest of APIs will not be polyfilled.

This PR adds a test on the custom server where we can add a custom polyfill for fetch with an env variable. This reproduces the issue since next-server.js will be required without having a polyfill for Response which makes it fail on requiring NextResponse. Then we remove the code that checks for subrequests to happen within the **sandbox** so that we don't need to polyfill `next-server` anymore.

The we also introduce an improvement on how we handle relative requests. Since vercel#31858 introduced a `port` and `hostname` options for the server, we can always pass absolute URLs to the Middleware so we can always use the original `nextUrl` to pass it to fetch. This brings a lot of simplification for `NextURL` since we don't have to consider relative URLs no more.

## Bug

- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [x] Errors have helpful link attached, see `contributing.md`

packages/next/server/next-server.ts

@@ -95,10 +95,10 @@ import { parseNextUrl } from '../shared/lib/router/utils/parse-next-url'
 import isError from '../lib/is-error'
 import { getMiddlewareInfo } from './require'
 import { MIDDLEWARE_ROUTE } from '../lib/constants'
-import { NextResponse } from './web/spec-extension/response'
 import { run } from './web/sandbox'
 import { addRequestMeta, getRequestMeta } from './request-meta'
 import { toNodeHeaders } from './web/utils'
+import { relativizeURL } from '../shared/lib/router/utils/relativize-url'
 
 const getCustomRouteMatcher = pathMatch(true)
 
@@ -383,7 +383,13 @@ export default class Server {
       parsedUrl.query = parseQs(parsedUrl.query)
     }
 
-    addRequestMeta(req, '__NEXT_INIT_URL', req.url)
+    // When there are hostname and port we build an absolute URL
+    const initUrl =
+      this.hostname && this.port
+        ? `http://${this.hostname}:${this.port}${req.url}`
+        : req.url
+
+    addRequestMeta(req, '__NEXT_INIT_URL', initUrl)
     addRequestMeta(req, '__NEXT_INIT_QUERY', { ...parsedUrl.query })
 
     const url = parseNextUrl({
@@ -659,6 +665,14 @@ export default class Server {
   }): Promise<FetchEventResult | null> {
     this.middlewareBetaWarning()
 
+    // For middleware to "fetch" we must always provide an absolute URL
+    const url = getRequestMeta(params.request, '__NEXT_INIT_URL')!
+    if (!url.startsWith('http')) {
+      throw new Error(
+        'To use middleware you must provide a `hostname` and `port` to the Next.js Server'
+      )
+    }
+
     const page: { name?: string; params?: { [key: string]: string } } = {}
     if (await this.hasPage(params.parsedUrl.pathname)) {
       page.name = params.parsedUrl.pathname
@@ -673,8 +687,6 @@ export default class Server {
       }
     }
 
-    const subreq = params.request.headers[`x-middleware-subrequest`]
-    const subrequests = typeof subreq === 'string' ? subreq.split(':') : []
     const allHeaders = new Headers()
     let result: FetchEventResult | null = null
 
@@ -694,14 +706,6 @@ export default class Server {
           serverless: this._isLikeServerless,
         })
 
-        if (subrequests.includes(middlewareInfo.name)) {
-          result = {
-            response: NextResponse.next(),
-            waitUntil: Promise.resolve(),
-          }
-          continue
-        }
-
         result = await run({
           name: middlewareInfo.name,
           paths: middlewareInfo.paths,
@@ -713,7 +717,7 @@ export default class Server {
               i18n: this.nextConfig.i18n,
               trailingSlash: this.nextConfig.trailingSlash,
             },
-            url: getRequestMeta(params.request, '__NEXT_INIT_URL')!,
+            url: url,
             page: page,
           },
           useCache: !this.nextConfig.experimental.concurrentFeatures,
@@ -1173,9 +1177,13 @@ export default class Server {
         type: 'route',
         name: 'middleware catchall',
         fn: async (req, res, _params, parsed) => {
-          const fullUrl = getRequestMeta(req, '__NEXT_INIT_URL')
+          if (!this.middleware?.length) {
+            return { finished: false }
+          }
+
+          const initUrl = getRequestMeta(req, '__NEXT_INIT_URL')!
           const parsedUrl = parseNextUrl({
-            url: fullUrl,
+            url: initUrl,
             headers: req.headers,
             nextConfig: {
               basePath: this.nextConfig.basePath,
@@ -1214,6 +1222,18 @@ export default class Server {
             return { finished: true }
           }
 
+          if (result.response.headers.has('x-middleware-rewrite')) {
+            const value = result.response.headers.get('x-middleware-rewrite')!
+            const rel = relativizeURL(value, initUrl)
+            result.response.headers.set('x-middleware-rewrite', rel)
+          }
+
+          if (result.response.headers.has('Location')) {
+            const value = result.response.headers.get('Location')!
+            const rel = relativizeURL(value, initUrl)
+            result.response.headers.set('Location', rel)
+          }
+
           if (
             !result.response.headers.has('x-middleware-rewrite') &&
             !result.response.headers.has('x-middleware-next') &&

packages/next/server/web/adapter.ts

@@ -1,8 +1,9 @@
 import type { NextMiddleware, RequestData, FetchEventResult } from './types'
+import type { RequestInit } from './spec-extension/request'
 import { DeprecationError } from './error'
 import { fromNodeHeaders } from './utils'
 import { NextFetchEvent } from './spec-extension/fetch-event'
-import { NextRequest, RequestInit } from './spec-extension/request'
+import { NextRequest } from './spec-extension/request'
 import { NextResponse } from './spec-extension/response'
 import { waitUntilSymbol } from './spec-compliant/fetch-event'
 
@@ -11,13 +12,9 @@ export async function adapter(params: {
   page: string
   request: RequestData
 }): Promise<FetchEventResult> {
-  const url = params.request.url.startsWith('/')
-    ? `https://${params.request.headers.host}${params.request.url}`
-    : params.request.url
-
   const request = new NextRequestHint({
     page: params.page,
-    input: url,
+    input: params.request.url,
     init: {
       geo: params.request.geo,
       headers: fromNodeHeaders(params.request.headers),