Fix external rewrite with body (#49487)

This ensures we use the proper body stream when rewriting to an external
resource as currently it tries to consume an already read body stream.

Fixes: #48040

packages/next/src/server/next-server.ts

@@ -860,7 +860,12 @@ export default class NextNodeServer extends BaseServer {
             }
           })
         })
-        proxy.web(req.originalRequest, res.originalResponse)
+        proxy.web(req.originalRequest, res.originalResponse, {
+          buffer: getRequestMeta(
+            req,
+            '__NEXT_CLONABLE_BODY'
+          )?.cloneBodyStream(),
+        })
       }
     })
 

test/e2e/middleware-rewrites/app/middleware.js

@@ -19,6 +19,12 @@ export async function middleware(request) {
     })
   }
 
+  if (url.pathname.includes('/middleware-external-rewrite-body')) {
+    return NextResponse.rewrite(
+      'https://next-data-api-endpoint.vercel.app/api/echo-body'
+    )
+  }
+
   if (url.pathname.includes('/rewrite-to-static')) {
     request.nextUrl.pathname = '/static-ssg/post-1'
     return NextResponse.rewrite(request.nextUrl)

test/e2e/middleware-rewrites/app/next.config.js

@@ -24,6 +24,11 @@ module.exports = {
           source: '/config-rewrite-to-dynamic-static/:rewriteSlug',
           destination: '/ssg',
         },
+        {
+          source: '/external-rewrite-body',
+          destination:
+            'https://next-data-api-endpoint.vercel.app/api/echo-body',
+        },
       ],
       fallback: [],
     }

test/e2e/middleware-rewrites/test/index.test.ts

@@ -23,6 +23,28 @@ describe('Middleware Rewrite', () => {
   })
 
   function tests() {
+    it('should handle next.config.js rewrite with body correctly', async () => {
+      const body = JSON.stringify({ hello: 'world' })
+      const res = await next.fetch('/external-rewrite-body', {
+        redirect: 'manual',
+        method: 'POST',
+        body,
+      })
+      expect(res.status).toBe(200)
+      expect(await res.text()).toEqual(body)
+    })
+
+    it('should handle middleware rewrite with body correctly', async () => {
+      const body = JSON.stringify({ hello: 'world' })
+      const res = await next.fetch('/middleware-external-rewrite-body', {
+        redirect: 'manual',
+        method: 'POST',
+        body,
+      })
+      expect(res.status).toBe(200)
+      expect(await res.text()).toEqual(body)
+    })
+
     it('should handle static dynamic rewrite from middleware correctly', async () => {
       const browser = await webdriver(next.url, '/rewrite-to-static')