Properly handle external middleware rewrites (#568)

.changeset/olive-needles-lay.md

@@ -0,0 +1,22 @@
+---
+'@cloudflare/next-on-pages': patch
+---
+
+fix external middleware rewrites
+
+Currently Middleware rewrites (`NextResponse.rewrite()`) assume that the rewrite destination
+is on the same host as the application, meaning that the following operations would work as intended:
+
+```ts
+NextResponse.rewrite(new URL('/rewrite-dest', request.url));
+```
+
+while something like this would not:
+
+```ts
+return NextResponse.rewrite(
+	new URL('https://my-customer-rewrite-site.come/rewrite-dest', request.url),
+);
+```
+
+Remove such assumption and allow such external rewrites to take place (as they do on Vercel)

packages/next-on-pages/templates/_worker.js/routes-matcher.ts

@@ -189,9 +189,14 @@ export class RoutesMatcher {
 
 		const rewriteKey = 'x-middleware-rewrite';
 		const rewriteHeader = resp.headers.get(rewriteKey);
+
 		if (rewriteHeader) {
 			const newUrl = new URL(rewriteHeader, this.url);
-			this.path = newUrl.pathname;
+
+			const rewriteIsExternal = this.url.hostname !== newUrl.hostname;
+
+			this.path = rewriteIsExternal ? `${newUrl}` : newUrl.pathname;
+
 			applySearchParams(this.searchParams, newUrl.searchParams);
 
 			resp.headers.delete(rewriteKey);