fix: remove traceparent from cachekey should not remove traceparent from original object (#64727)

### What?

I submitted PR #64499 , it got merged, but it contains a mistake.
I'm terribly sorry about this!

By removing the traceparent from the cachekey, we mistakenly removed the
header from the original object.
Causing the actual request to be executed without the traceparent
header.

### Why?

Creating a cachekey should not alter the original object.

### How?

Flip the arguments for Object.assign

---------

Co-authored-by: Jeffrey <[email protected]>
Co-authored-by: JJ Kasper <[email protected]>

Author: 3 people

packages/next/src/server/lib/incremental-cache/index.ts

@@ -383,7 +383,7 @@ export class IncrementalCache implements IncrementalCacheType {
     const headers =
       typeof (init.headers || {}).keys === 'function'
         ? Object.fromEntries(init.headers as Headers)
-        : Object.assign(init.headers || {}, {})
+        : Object.assign({}, init.headers)
 
     if ('traceparent' in headers) delete headers['traceparent']
 

test/e2e/app-dir/app-static/app-static.test.ts

@@ -40,6 +40,22 @@ createNextDescribe(
       }
     })
 
+    it('should still cache even though the `traceparent` header was different', async () => {
+      const res = await next.fetch('/strip-header-traceparent')
+      expect(res.status).toBe(200)
+
+      const html = await res.text()
+      const $ = cheerio.load(html)
+
+      const data1 = $('#data1').text()
+      const data2 = $('#data2').text()
+      expect(data1).toBeTruthy()
+      expect(data1).toBe(data2)
+
+      const echoedHeaders = JSON.parse($('#echoedHeaders').text())
+      expect(echoedHeaders.headers.traceparent).toEqual('C')
+    })
+
     it('should warn for too many cache tags', async () => {
       const res = await next.fetch('/too-many-cache-tags')
       expect(res.status).toBe(200)
@@ -790,6 +806,10 @@ createNextDescribe(
             "static-to-dynamic-error-forced/[id]/page_client-reference-manifest.js",
             "static-to-dynamic-error/[id]/page.js",
             "static-to-dynamic-error/[id]/page_client-reference-manifest.js",
+            "strip-header-traceparent.html",
+            "strip-header-traceparent.rsc",
+            "strip-header-traceparent/page.js",
+            "strip-header-traceparent/page_client-reference-manifest.js",
             "too-many-cache-tags/page.js",
             "too-many-cache-tags/page_client-reference-manifest.js",
             "unstable-cache/dynamic-undefined/page.js",
@@ -1512,6 +1532,22 @@ createNextDescribe(
               "initialRevalidateSeconds": false,
               "srcRoute": "/ssg-draft-mode/[[...route]]",
             },
+            "/strip-header-traceparent": {
+              "dataRoute": "/strip-header-traceparent.rsc",
+              "experimentalBypassFor": [
+                {
+                  "key": "Next-Action",
+                  "type": "header",
+                },
+                {
+                  "key": "content-type",
+                  "type": "header",
+                  "value": "multipart/form-data;.*",
+                },
+              ],
+              "initialRevalidateSeconds": 50,
+              "srcRoute": "/strip-header-traceparent",
+            },
             "/variable-config-revalidate/revalidate-3": {
               "dataRoute": "/variable-config-revalidate/revalidate-3.rsc",
               "experimentalBypassFor": [

test/e2e/app-dir/app-static/app/strip-header-traceparent/page.tsx

@@ -0,0 +1,33 @@
+export default async function Page() {
+  const data1 = await fetch(
+    'https://next-data-api-endpoint.vercel.app/api/random',
+    {
+      headers: { traceparent: 'A' },
+      next: { revalidate: 50 },
+    }
+  ).then((res) => res.text())
+
+  const data2 = await fetch(
+    'https://next-data-api-endpoint.vercel.app/api/random',
+    {
+      headers: { traceparent: 'B' },
+      next: { revalidate: 50 },
+    }
+  ).then((res) => res.text())
+
+  const echoedHeaders = await fetch(
+    'https://next-data-api-endpoint.vercel.app/api/echo-headers',
+    {
+      headers: { traceparent: 'C' },
+      next: { revalidate: 50 },
+    }
+  ).then((res) => res.text())
+
+  return (
+    <>
+      <p id="data1">{data1}</p>
+      <p id="data2">{data2}</p>
+      <p id="echoedHeaders">{echoedHeaders}</p>
+    </>
+  )
+}