[Segment Cache] No data on tree prefetch if no PPR

packages/next/src/server/app-render/app-render.tsx

@@ -53,6 +53,7 @@ import {
   NEXT_ROUTER_STALE_TIME_HEADER,
   NEXT_URL,
   RSC_HEADER,
+  NEXT_ROUTER_SEGMENT_PREFETCH_HEADER,
 } from '../../client/components/app-router-headers'
 import {
   createTrackedMetadataContext,
@@ -234,6 +235,7 @@ interface ParsedRequestHeaders {
    */
   readonly flightRouterState: FlightRouterState | undefined
   readonly isPrefetchRequest: boolean
+  readonly isRouteTreePrefetchRequest: boolean
   readonly isDevWarmupRequest: boolean
   readonly isHmrRefresh: boolean
   readonly isRSCRequest: boolean
@@ -267,6 +269,10 @@ function parseRequestHeaders(
       )
     : undefined
 
+  // Checks if this is a prefetch of the Route Tree by the Segment Cache
+  const isRouteTreePrefetchRequest =
+    headers[NEXT_ROUTER_SEGMENT_PREFETCH_HEADER.toLowerCase()] === '/_tree'
+
   const csp =
     headers['content-security-policy'] ||
     headers['content-security-policy-report-only']
@@ -277,6 +283,7 @@ function parseRequestHeaders(
   return {
     flightRouterState,
     isPrefetchRequest,
+    isRouteTreePrefetchRequest,
     isHmrRefresh,
     isRSCRequest,
     isDevWarmupRequest,

packages/next/src/server/app-render/walk-tree-with-flight-router-state.tsx

@@ -62,6 +62,7 @@ export async function walkTreeWithFlightRouterState({
     query,
     isPrefetch,
     getDynamicParamFromSegment,
+    parsedRequestHeaders,
   } = ctx
 
   const [segment, parallelRoutes, modules] = loaderTreeToFilter
@@ -115,9 +116,13 @@ export async function walkTreeWithFlightRouterState({
   // somewhere in the tree, we'll recursively render the component tree up until we encounter that loading component, and then stop.
   const shouldSkipComponentTree =
     !experimental.isRoutePPREnabled &&
-    isPrefetch &&
-    !Boolean(modules.loading) &&
-    !hasLoadingComponentInTree(loaderTreeToFilter)
+    // If PPR is disabled, and this is a request for the route tree, then we
+    // never render any components. Only send the router state.
+    (parsedRequestHeaders.isRouteTreePrefetchRequest ||
+      // Otherwise, check for the presence of a `loading` component.
+      (isPrefetch &&
+        !Boolean(modules.loading) &&
+        !hasLoadingComponentInTree(loaderTreeToFilter)))
 
   if (!parentRendered && renderComponentsOnThisLevel) {
     const overriddenSegment =

test/e2e/app-dir/segment-cache/incremental-opt-in/app/ppr-disabled-with-loading-boundary/loading.tsx

@@ -0,0 +1,15 @@
+import { Suspense } from 'react'
+import { connection } from 'next/server'
+
+async function Content() {
+  await connection()
+  return 'Dynamic Content'
+}
+
+export default function PPRDisabled() {
+  return (
+    <Suspense fallback="Loading...">
+      <Content />
+    </Suspense>
+  )
+}

test/e2e/app-dir/segment-cache/incremental-opt-in/app/ppr-disabled-with-loading-boundary/page.tsx

@@ -0,0 +1,6 @@
+import { connection } from 'next/server'
+
+export default async function PPRDisabledWithLoadingBoundary() {
+  await connection()
+  return 'Dynamic Content'
+}

test/e2e/app-dir/segment-cache/incremental-opt-in/segment-cache-incremental-opt-in.test.ts

@@ -40,4 +40,28 @@ describe('segment cache (incremental opt in)', () => {
     const result = extractPseudoJSONFromFlightResponse(flightText)
     expect(typeof result.b === 'string').toBe(true)
   })
+
+  // TODO: Replace with e2e test once the client part is implemented
+  it('route tree prefetch does not include any component data even if loading.tsx is defined', async () => {
+    await next.browser('/')
+    const response = await next.fetch('/ppr-disabled-with-loading-boundary', {
+      headers: {
+        RSC: '1',
+        'Next-Router-Prefetch': '1',
+        'Next-Router-Segment-Prefetch': '/_tree',
+      },
+    })
+    expect(response.status).toBe(200)
+    expect(response.headers.get('x-nextjs-postponed')).toBe(null)
+
+    // Usually when PPR is disabled, a prefetch to a route that has a
+    // loading.tsx boundary will include component data in the response, up to
+    // the first loading boundary. But since this is specifically a prefetch
+    // of the route tree, it should skip all the component data and only return
+    // the router state.
+    const flightText = await response.text()
+    // Confirm that the response does not include any component data by checking
+    // for the absence of the loading component.
+    expect(flightText).not.toContain('Loading...')
+  })
 })