[next-server] skip setting vary header for basic routes

Author: huozhi

Diff for: packages/next/src/server/base-server.ts

@@ -1979,7 +1979,6 @@ export default abstract class Server<
     resolvedPathname: string
   ): void {
     const baseVaryHeader = `${RSC_HEADER}, ${NEXT_ROUTER_STATE_TREE_HEADER}, ${NEXT_ROUTER_PREFETCH_HEADER}, ${NEXT_ROUTER_SEGMENT_PREFETCH_HEADER}`
-    const isRSCRequest = getRequestMeta(req, 'isRSCRequest') ?? false
 
     let addedNextUrlToVary = false
 
@@ -1988,11 +1987,9 @@ export default abstract class Server<
       // We use the Vary header to signal this behavior to the client to properly cache the response.
       res.appendHeader('vary', `${baseVaryHeader}, ${NEXT_URL}`)
       addedNextUrlToVary = true
-    } else if (isAppPath || isRSCRequest) {
-      // We don't need to include `Next-URL` in the Vary header for non-interception routes since it won't affect the response.
-      // We also set this header for pages to avoid caching issues when navigating between pages and app.
-      res.appendHeader('vary', baseVaryHeader)
     }
+    // For other cases such as App Router requests or RSC requests we don't need to set vary header since we already
+    // have the _rsc query with the unique hash value.
 
     if (!addedNextUrlToVary) {
       // Remove `Next-URL` from the request headers we determined it wasn't necessary to include in the Vary header.

Diff for: test/e2e/app-dir/app/index.test.ts

@@ -323,25 +323,19 @@ describe('app dir - basic', () => {
     expect(res.headers.get('Content-Type')).toBe('text/x-component')
   })
 
-  it('should return the `vary` header from edge runtime', async () => {
+  it('should not return the `vary` header from edge runtime', async () => {
     const res = await next.fetch('/dashboard')
     expect(res.headers.get('x-edge-runtime')).toBe('1')
-    expect(res.headers.get('vary')).toBe(
-      'RSC, Next-Router-State-Tree, Next-Router-Prefetch, Next-Router-Segment-Prefetch'
-    )
+    expect(res.headers.get('vary')).toBe(null)
   })
 
-  it('should return the `vary` header from pages for flight requests', async () => {
+  it('should not return the `vary` header from pages for flight requests', async () => {
     const res = await next.fetch('/', {
       headers: {
         ['RSC'.toString()]: '1',
       },
     })
-    expect(res.headers.get('vary')).toBe(
-      isNextDeploy
-        ? 'RSC, Next-Router-State-Tree, Next-Router-Prefetch, Next-Router-Segment-Prefetch'
-        : 'RSC, Next-Router-State-Tree, Next-Router-Prefetch, Next-Router-Segment-Prefetch, Accept-Encoding'
-    )
+    expect(res.headers.get('vary')).toBe('Accept-Encoding')
   })
 
   it('should pass props from getServerSideProps in root layout', async () => {