fix post request hangs when no body is consumed on middleware (#35131)

fixes #34966

Wait for reading original request body before overwriting the request properties with `originalBody?.finalize()` call since it caused the problem when middleware does not read request body

## Bug

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

packages/next/server/body-streams.ts

@@ -58,14 +58,20 @@ export function clonableBodyForRequest<T extends IncomingMessage>(
 ) {
   let bufferedBodyStream: BodyStream | null = null
 
+  const endPromise = new Promise((resolve, reject) => {
+    incomingMessage.on('end', resolve)
+    incomingMessage.on('error', reject)
+  })
+
   return {
     /**
      * Replaces the original request body if necessary.
      * This is done because once we read the body from the original request,
      * we can't read it again.
      */
-    finalize(): void {
+    async finalize(): Promise<void> {
       if (bufferedBodyStream) {
+        await endPromise
         replaceRequestBody(
           incomingMessage,
           bodyStreamToNodeStream(bufferedBodyStream)

packages/next/server/next-server.ts

@@ -1314,7 +1314,7 @@ export default class NextNodeServer extends BaseServer {
       }
     }
 
-    originalBody?.finalize()
+    await originalBody?.finalize()
 
     return result
   }

test/production/reading-request-body-in-middleware/index.test.ts

@@ -16,7 +16,11 @@ describe('reading request body in middleware', () => {
               return new Response('No body', { status: 400 });
             }
 
-            const json = await request.json();
+            let json;
+
+            if (!request.nextUrl.searchParams.has("no_reading")) {
+              json = await request.json();
+            }
 
             if (request.nextUrl.searchParams.has("next")) {
               const res = NextResponse.next();
@@ -141,4 +145,30 @@ describe('reading request body in middleware', () => {
     })
     expect(response.headers.get('x-from-root-middleware')).toEqual('1')
   })
+
+  it('passes the body to the api endpoint when no body is consumed on middleware', async () => {
+    const response = await fetchViaHTTP(
+      next.url,
+      '/api/hi',
+      {
+        next: '1',
+        no_reading: '1',
+      },
+      {
+        method: 'POST',
+        headers: {
+          'content-type': 'application/json',
+        },
+        body: JSON.stringify({
+          foo: 'bar',
+        }),
+      }
+    )
+    expect(response.status).toEqual(200)
+    expect(await response.json()).toEqual({
+      foo: 'bar',
+      api: true,
+    })
+    expect(response.headers.get('x-from-root-middleware')).toEqual('1')
+  })
 })