fix: handle absolute rewrite URLs

demos/middleware/pages/index.js

@@ -16,7 +16,9 @@ export default function Home() {
           Welcome to <a href="https://nextjs.org">Next.js!</a>
         </h1>
 
-        <p><Link href="/shows/rewriteme">Rewrite me</Link></p>
+        <p><Link href="/shows/rewriteme">Rewrite URL</Link></p>
+        <p><Link href="/shows/rewrite-absolute">Rewrite to absolute URL</Link></p>
+        <p><Link href="/shows/rewrite-external">Rewrite to external URL</Link></p>
       </main>
     </div>
   )

demos/middleware/pages/shows/rewrite-absolute/_middleware.ts

@@ -0,0 +1,8 @@
+import { NextResponse } from 'next/server'
+import { NextFetchEvent, NextRequest } from 'next/server'
+
+export function middleware(req: NextRequest, ev: NextFetchEvent) {
+  const res = NextResponse.rewrite(new URL('/shows/100', req.url))
+  res.headers.set('x-modified-in-rewrite', 'true')
+  return res
+}

demos/middleware/pages/shows/rewrite-absolute/index.js

@@ -0,0 +1,9 @@
+const Show = () => {
+  return (
+    <div>
+      <p>This should have been rewritten</p>
+    </div>
+  )
+}
+
+export default Show

demos/middleware/pages/shows/rewrite-external/_middleware.ts

@@ -0,0 +1,8 @@
+import { NextResponse } from 'next/server'
+import { NextFetchEvent, NextRequest } from 'next/server'
+
+export function middleware(req: NextRequest, ev: NextFetchEvent) {
+  const res = NextResponse.rewrite('http://example.com/')
+  res.headers.set('x-modified-in-rewrite', 'true')
+  return res
+}

demos/middleware/pages/shows/rewrite-external/index.js

@@ -0,0 +1,9 @@
+const Show = () => {
+  return (
+    <div>
+      <p>This should have been rewritten</p>
+    </div>
+  )
+}
+
+export default Show

plugin/src/templates/edge/utils.ts

@@ -5,6 +5,19 @@ export interface FetchEventResult {
   waitUntil: Promise<any>
 }
 
+/**
+ * This is how Next handles rewritten URLs.
+ */
+ export function relativizeURL(url: string | string, base: string | URL) {
+  const baseURL = typeof base === 'string' ? new URL(base) : base
+  const relative = new URL(url, base)
+  const origin = `${baseURL.protocol}//${baseURL.host}`
+  return `${relative.protocol}//${relative.host}` === origin
+    ? relative.toString().replace(origin, '')
+    : relative.toString()
+}
+
+
 export const addMiddlewareHeaders = async (
   originResponse: Promise<Response> | Response,
   middlewareResponse: Response,
@@ -14,7 +27,8 @@ export const addMiddlewareHeaders = async (
     return originResponse
   }
   // We need to await the response to get the origin headers, then we can add the ones from middleware.
-  const response = await originResponse
+  const res = await originResponse
+  const response = new Response(res.body, res)
   middlewareResponse.headers.forEach((value, key) => {
     response.headers.set(key, value)
   })
@@ -33,6 +47,14 @@ export const buildResponse = async ({
   request.headers.set('x-nf-next-middleware', 'skip')
   const rewrite = res.headers.get('x-middleware-rewrite')
   if (rewrite) {
+    const rewriteUrl = new URL(rewrite, request.url)
+    const baseUrl = new URL(request.url)
+    if(rewriteUrl.hostname !== baseUrl.hostname) {
+      // Netlify Edge Functions don't support proxying to external domains, but Next middleware does
+      const proxied = fetch(new Request(rewriteUrl.toString(), request))
+      return addMiddlewareHeaders(proxied, res)
+    }
+    res.headers.set('x-middleware-rewrite', relativizeURL(rewrite, request.url))
     return addMiddlewareHeaders(context.rewrite(rewrite), res)
   }
   if (res.headers.get('x-middleware-next') === '1') {