extract mutation logic into a function

and remove the body modification example

Author: Schniz

packages/next/server/next-server.ts

@@ -1244,7 +1244,7 @@ export default class NextNodeServer extends BaseServer {
     const method = (params.request.method || 'GET').toUpperCase()
     let originalBody =
       method !== 'GET' && method !== 'HEAD'
-        ? requestToBodyStream(params.request.body)
+        ? teeableStream(requestToBodyStream(params.request.body))
         : undefined
 
     for (const middleware of this.middleware || []) {
@@ -1255,12 +1255,7 @@ export default class NextNodeServer extends BaseServer {
         }
 
         await this.ensureMiddleware(middleware.page, middleware.ssr)
-        let currentBody: typeof originalBody = undefined
-        if (originalBody) {
-          const [body1, body2] = originalBody.tee()
-          originalBody = body1
-          currentBody = body2
-        }
+        const currentBody = originalBody?.duplicate()
 
         const middlewareInfo = this.getMiddlewareInfo(middleware.page)
 
@@ -1290,12 +1285,7 @@ export default class NextNodeServer extends BaseServer {
         })
 
         for (let [key, value] of result.response.headers) {
-          // TODO: remove, or rethink if really needed
-          if (key === 'x-middleware-use-body' && value === '1') {
-            if (result.response.body) {
-              originalBody = result.response.body as any
-            }
-          } else if (key !== 'x-middleware-next') {
+          if (key !== 'x-middleware-next') {
             allHeaders.append(key, value)
           }
         }
@@ -1373,3 +1363,19 @@ function requestToBodyStream(
 
   return transform.readable
 }
+
+/**
+ * A simple utility to take an original stream and have
+ * an API to duplicate it without closing it or mutate any variables
+ */
+function teeableStream<T>(originalStream: ReadableStream<T>): {
+  duplicate(): ReadableStream<T>
+} {
+  return {
+    duplicate() {
+      const [stream1, stream2] = originalStream.tee()
+      originalStream = stream1
+      return stream2
+    },
+  }
+}

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

@@ -36,17 +36,6 @@ describe('reading request body in middleware', () => {
 
             if (request.nextUrl.searchParams.has("next")) {
               return NextResponse.next();
-            } else if (request.nextUrl.searchParams.has("change")) {
-              return new Response(JSON.stringify({
-                ...json,
-                changeFromRoot: true,
-              }), {
-                status: 200,
-                headers: {
-                  'x-middleware-next': '1',
-                  'x-middleware-use-body': '1',
-                }
-              });
             }
 
             return new Response(JSON.stringify({
@@ -135,26 +124,4 @@ describe('reading request body in middleware', () => {
       root: false,
     })
   })
-
-  it('modifies the body (returns changeFromRoot: true)', async () => {
-    const response = await fetchViaHTTP(
-      next.url,
-      '/nested/hello',
-      {
-        change: '1',
-      },
-      {
-        method: 'POST',
-        body: JSON.stringify({
-          foo: 'bar',
-        }),
-      }
-    )
-    expect(response.status).toEqual(200)
-    expect(await response.json()).toEqual({
-      foo: 'bar',
-      changeFromRoot: true,
-      root: false,
-    })
-  })
 })