Wrong index path revalidation timer (#20818)

Hello friends

Ran into this bug on our production site, prerenderManifest stores revalidation info for the index as `"/": { .. }`, but the code tries to access this information as `"/index"`.

This leads to our index page always having s-max-age: 1

packages/next/next-server/server/incremental-cache.ts

@@ -156,7 +156,9 @@ export class IncrementalCache {
     ) {
       data.isStale = true
     }
-    const manifestEntry = this.prerenderManifest.routes[pathname]
+
+    const manifestPath = toRoute(pathname)
+    const manifestEntry = this.prerenderManifest.routes[manifestPath]
 
     if (data && manifestEntry) {
       data.curRevalidate = manifestEntry.initialRevalidateSeconds

test/integration/prerender/pages/index.js

@@ -5,7 +5,7 @@ export async function getStaticProps() {
   return {
     props: { world: 'world', time: new Date().getTime() },
     // bad-prop
-    revalidate: 1,
+    revalidate: 2,
   }
 }
 

test/integration/prerender/test/index.test.js

@@ -58,7 +58,7 @@ const startServer = async (optEnv = {}) => {
 const expectedManifestRoutes = () => ({
   '/': {
     dataRoute: `/_next/data/${buildId}/index.json`,
-    initialRevalidateSeconds: 1,
+    initialRevalidateSeconds: 2,
     srcRoute: null,
   },
   '/blog/[post3]': {
@@ -521,6 +521,15 @@ const runTests = (dev = false, isEmulatedServerless = false) => {
     expect(data.pageProps.post).toBe('post-3')
   })
 
+  if (!dev) {
+    it('should use correct caching headers for a revalidate page', async () => {
+      const initialRes = await fetchViaHTTP(appPort, '/')
+      expect(initialRes.headers.get('cache-control')).toBe(
+        's-maxage=2, stale-while-revalidate'
+      )
+    })
+  }
+
   it('should navigate to a normal page and back', async () => {
     const browser = await webdriver(appPort, '/')
     let text = await browser.elementByCss('p').text()