bugfix: only replace with full prefetch if existing data was partial

packages/next/src/client/components/router-reducer/prefetch-cache-utils.ts

@@ -191,16 +191,30 @@ export function getOrCreatePrefetchCacheEntry({
       kind === PrefetchKind.FULL
 
     if (switchedToFullPrefetch) {
-      return createLazyPrefetchEntry({
-        tree,
-        url,
-        buildId,
-        nextUrl,
-        prefetchCache,
-        // If we didn't get an explicit prefetch kind, we want to set a temporary kind
-        // rather than assuming the same intent as the previous entry, to be consistent with how we
-        // lazily create prefetch entries when intent is left unspecified.
-        kind: kind ?? PrefetchKind.TEMPORARY,
+      // If we switched to a full prefetch, validate that the existing cache entry contained partial data.
+      // It's possible that the cache entry was seeded with full data but has a cache type of "auto" (ie when cache entries
+      // are seeded but without a prefetch intent)
+      existingCacheEntry.data.then((prefetchResponse) => {
+        const isFullPrefetch =
+          Array.isArray(prefetchResponse.flightData) &&
+          prefetchResponse.flightData.some((flightData) => {
+            // If we started rendering from the root and we returned RSC data (seedData), we already had a full prefetch.
+            return flightData.isRootRender && flightData.seedData !== null
+          })
+
+        if (!isFullPrefetch) {
+          return createLazyPrefetchEntry({
+            tree,
+            url,
+            buildId,
+            nextUrl,
+            prefetchCache,
+            // If we didn't get an explicit prefetch kind, we want to set a temporary kind
+            // rather than assuming the same intent as the previous entry, to be consistent with how we
+            // lazily create prefetch entries when intent is left unspecified.
+            kind: kind ?? PrefetchKind.TEMPORARY,
+          })
+        }
       })
     }
 

test/e2e/app-dir/app-prefetch/app/dynamic-page/page.js

@@ -0,0 +1,21 @@
+import Link from 'next/link'
+
+export const dynamic = 'force-dynamic'
+
+export default async function Page() {
+  return (
+    <>
+      <p id="dynamic-page">Dynamic Page</p>
+      <p>
+        <Link href="/" id="to-home">
+          To home
+        </Link>
+      </p>
+      <p>
+        <Link href="/dynamic-page" prefetch>
+          To Same Page
+        </Link>
+      </p>
+    </>
+  )
+}

test/e2e/app-dir/app-prefetch/app/static-page/page.js

@@ -9,6 +9,11 @@ export default async function Page() {
           To home
         </Link>
       </p>
+      <p>
+        <Link href="/static-page" prefetch>
+          To Same Page
+        </Link>
+      </p>
     </>
   )
 }

test/e2e/app-dir/app-prefetch/prefetching.test.ts

@@ -301,6 +301,70 @@ describe('app dir - prefetching', () => {
     await browser.waitForElementByCss('#prefetch-auto-page-data')
   })
 
+  describe('prefetch cache seeding', () => {
+    it('should not re-fetch the initial static page if the same page is prefetched with prefetch={true}', async () => {
+      const rscRequests = []
+      const browser = await next.browser('/static-page', {
+        beforePageLoad(page: Page) {
+          page.on('request', async (req: Request) => {
+            const url = new URL(req.url())
+            const headers = await req.allHeaders()
+            if (headers['rsc']) {
+              rscRequests.push(url.pathname)
+            }
+          })
+        },
+      })
+
+      expect(
+        await browser.hasElementByCssSelector('[href="/static-page"]')
+      ).toBe(true)
+
+      // sanity check: we should see a prefetch request to the root page
+      await retry(async () => {
+        expect(rscRequests.filter((req) => req === '/').length).toBe(1)
+      })
+
+      // We shouldn't see any requests to the static page since the prefetch cache was seeded as part of the SSR render
+      await retry(async () => {
+        expect(rscRequests.filter((req) => req === '/static-page').length).toBe(
+          0
+        )
+      })
+    })
+
+    it('should not re-fetch the initial dynamic page if the same page is prefetched with prefetch={true}', async () => {
+      const rscRequests = []
+      const browser = await next.browser('/dynamic-page', {
+        beforePageLoad(page: Page) {
+          page.on('request', async (req: Request) => {
+            const url = new URL(req.url())
+            const headers = await req.allHeaders()
+            if (headers['rsc']) {
+              rscRequests.push(url.pathname)
+            }
+          })
+        },
+      })
+
+      expect(
+        await browser.hasElementByCssSelector('[href="/dynamic-page"]')
+      ).toBe(true)
+
+      // sanity check: we should see a prefetch request to the root page
+      await retry(async () => {
+        expect(rscRequests.filter((req) => req === '/').length).toBe(1)
+      })
+
+      // We shouldn't see any requests to the dynamic page since the prefetch cache was seeded as part of the SSR render
+      await retry(async () => {
+        expect(
+          rscRequests.filter((req) => req === '/dynamic-page').length
+        ).toBe(0)
+      })
+    })
+  })
+
   // These tests are skipped when deployed as they rely on runtime logs
   if (!isNextDeploy) {
     describe('dynamic rendering', () => {