From f9314e48a8f681503472ae62296dd92c530d0e64 Mon Sep 17 00:00:00 2001
From: Hendrik Liebau <mail@hendrik-liebau.de>
Date: Thu, 15 May 2025 20:04:12 +0200
Subject: [PATCH] Remove prospective fallback prerenders

The prospective fallback prerenders, which ensured that a fallback shell
could be prerendered without errors, were only needed when Dynamic IO
was enabled and PPR was disabled. This scenario is no longer supported.
---
 packages/next/src/build/index.ts              | 52 ------------------
 packages/next/src/export/routes/app-page.ts   | 52 ------------------
 packages/next/src/export/worker.ts            | 21 +-------
 packages/next/src/server/config-schema.ts     |  1 -
 packages/next/src/server/config-shared.ts     | 10 ----
 .../src/server/request/fallback-params.ts     |  2 +-
 ...mic-io-errors.prospective-fallback.test.ts | 54 -------------------
 .../app/blog/[slug]/page.jsx                  |  4 --
 .../prospective-fallback/app/layout.jsx       |  7 ---
 .../prospective-fallback/next.config.js       |  6 ---
 10 files changed, 2 insertions(+), 207 deletions(-)
 delete mode 100644 test/e2e/app-dir/dynamic-io-errors/dynamic-io-errors.prospective-fallback.test.ts
 delete mode 100644 test/e2e/app-dir/dynamic-io-errors/fixtures/prospective-fallback/app/blog/[slug]/page.jsx
 delete mode 100644 test/e2e/app-dir/dynamic-io-errors/fixtures/prospective-fallback/app/layout.jsx
 delete mode 100644 test/e2e/app-dir/dynamic-io-errors/fixtures/prospective-fallback/next.config.js

diff --git a/packages/next/src/build/index.ts b/packages/next/src/build/index.ts
index 614e512d78a51..e629539aef7ad 100644
--- a/packages/next/src/build/index.ts
+++ b/packages/next/src/build/index.ts
@@ -193,7 +193,6 @@ import {
 } from '../server/lib/experimental/ppr'
 import { FallbackMode, fallbackModeToFallbackField } from '../lib/fallback'
 import { RenderingMode } from './rendering-mode'
-import { getParamKeys } from '../server/request/fallback-params'
 import { InvariantError } from '../shared/lib/invariant-error'
 import { HTML_LIMITED_BOT_UA_RE_STRING } from '../shared/lib/router/utils/is-bot'
 import type { UseCacheTrackerKey } from './webpack/plugins/telemetry-plugin/use-cache-tracker-utils'
@@ -1613,10 +1612,6 @@ export default async function build(
       const serverPropsPages = new Set<string>()
       const additionalPaths = new Map<string, PrerenderedRoute[]>()
       const staticPaths = new Map<string, PrerenderedRoute[]>()
-      const prospectiveRenders = new Map<
-        string,
-        { page: string; originalAppPath: string }
-      >()
       const appNormalizedPaths = new Map<string, string>()
       const fallbackModes = new Map<string, FallbackMode>()
       const appDefaultConfigs = new Map<string, AppSegmentConfig>()
@@ -1975,31 +1970,6 @@ export default async function build(
 
                             staticPaths.set(originalAppPath, [])
                           }
-                          // As PPR isn't enabled for this route, if dynamic IO
-                          // is enabled, and this is a dynamic route, we should
-                          // complete a prospective render for the route so that
-                          // we can use the fallback behavior. This lets us
-                          // check that dynamic pages won't error when they
-                          // enable PPR.
-                          else if (config.experimental.dynamicIO && isDynamic) {
-                            // If there's a page with a more specific render
-                            // available, then we should skip the prospective
-                            // render because it'll be done as a part of the
-                            // that render to validate the dynamic state.
-                            if (
-                              // The existence of any prerendered routes when
-                              // PPR is disabled means that the route has more
-                              // specific prerendered routes that should be
-                              // used for the diagnostic render anyways.
-                              !workerResult.prerenderedRoutes ||
-                              workerResult.prerenderedRoutes.length === 0
-                            ) {
-                              prospectiveRenders.set(originalAppPath, {
-                                page,
-                                originalAppPath,
-                              })
-                            }
-                          }
 
                           if (workerResult.prerenderedRoutes) {
                             staticPaths.set(
@@ -2718,28 +2688,6 @@ export default async function build(
                 })
               })
 
-              // If the app does have dynamic IO enabled but does not have PPR
-              // enabled, then we need to perform a prospective render for all
-              // the dynamic pages to ensure that they won't error during
-              // rendering (due to a missing prelude).
-              for (const {
-                page,
-                originalAppPath,
-              } of prospectiveRenders.values()) {
-                defaultMap[page] = {
-                  page: originalAppPath,
-                  _ssgPath: page,
-                  _fallbackRouteParams: getParamKeys(page),
-                  // Prospective renders are only enabled for app pages.
-                  _isAppDir: true,
-                  // Prospective renders are only enabled when PPR is disabled.
-                  _isRoutePPREnabled: false,
-                  _isProspectiveRender: true,
-                  // Dynamic IO does not currently support `dynamic === 'error'`.
-                  _isDynamicError: false,
-                }
-              }
-
               if (i18n) {
                 for (const page of [
                   ...staticPages,
diff --git a/packages/next/src/export/routes/app-page.ts b/packages/next/src/export/routes/app-page.ts
index 7d896b57b5029..bfbaaa1a2b7b0 100644
--- a/packages/next/src/export/routes/app-page.ts
+++ b/packages/next/src/export/routes/app-page.ts
@@ -30,58 +30,6 @@ import type { RequestLifecycleOpts } from '../../server/base-server'
 import type { AppSharedContext } from '../../server/app-render/app-render'
 import type { MultiFileWriter } from '../../lib/multi-file-writer'
 
-export async function prospectiveRenderAppPage(
-  req: MockedRequest,
-  res: MockedResponse,
-  page: string,
-  pathname: string,
-  query: NextParsedUrlQuery,
-  fallbackRouteParams: FallbackRouteParams | null,
-  partialRenderOpts: Omit<RenderOpts, keyof RequestLifecycleOpts>,
-  sharedContext: AppSharedContext
-): Promise<undefined> {
-  const afterRunner = new AfterRunner()
-
-  // If the page is `/_not-found`, then we should update the page to be `/404`.
-  // UNDERSCORE_NOT_FOUND_ROUTE value used here, however we don't want to import it here as it causes constants to be inlined which we don't want here.
-  if (page === '/_not-found/page') {
-    pathname = '/404'
-  }
-
-  try {
-    await lazyRenderAppPage(
-      new NodeNextRequest(req),
-      new NodeNextResponse(res),
-      pathname,
-      query,
-      fallbackRouteParams,
-      {
-        ...partialRenderOpts,
-        waitUntil: afterRunner.context.waitUntil,
-        onClose: afterRunner.context.onClose,
-        onAfterTaskError: afterRunner.context.onTaskError,
-      },
-      undefined,
-      false,
-      sharedContext
-    )
-
-    // TODO(after): if we abort a prerender because of an error in an after-callback
-    // we should probably communicate that better (and not log the error twice)
-    await afterRunner.executeAfter()
-  } catch (err) {
-    if (!isDynamicUsageError(err)) {
-      throw err
-    }
-
-    // We should fail rendering if a client side rendering bailout
-    // occurred at the page level.
-    if (isBailoutToCSRError(err)) {
-      throw err
-    }
-  }
-}
-
 /**
  * Renders & exports a page associated with the /app directory
  */
diff --git a/packages/next/src/export/worker.ts b/packages/next/src/export/worker.ts
index 7ef928b90104e..2aa6a4e48459f 100644
--- a/packages/next/src/export/worker.ts
+++ b/packages/next/src/export/worker.ts
@@ -26,7 +26,7 @@ import { createRequestResponseMocks } from '../server/lib/mock-request'
 import { isAppRouteRoute } from '../lib/is-app-route-route'
 import { hasNextSupport } from '../server/ci-info'
 import { exportAppRoute } from './routes/app-route'
-import { exportAppPage, prospectiveRenderAppPage } from './routes/app-page'
+import { exportAppPage } from './routes/app-page'
 import { exportPagesPage } from './routes/pages'
 import { getParams } from './helpers/get-params'
 import { createIncrementalCache } from './helpers/create-incremental-cache'
@@ -104,10 +104,6 @@ async function exportPageImpl(
     // the renderOpts.
     _isRoutePPREnabled: isRoutePPREnabled,
 
-    // If this is a prospective render, we don't actually want to persist the
-    // result, we just want to use it to error the build if there's a problem.
-    _isProspectiveRender: isProspectiveRender = false,
-
     // Configure the rendering of the page not to throw if an empty static shell
     // is generated while rendering using PPR.
     _doNotThrowOnEmptyStaticShell: doNotThrowOnEmptyStaticShell = false,
@@ -287,21 +283,6 @@ async function exportPageImpl(
       buildId: input.buildId,
     }
 
-    // If this is a prospective render, don't return any metrics or revalidate
-    // timings as we aren't persisting this render (it was only to error).
-    if (isProspectiveRender) {
-      return prospectiveRenderAppPage(
-        req,
-        res,
-        page,
-        pathname,
-        query,
-        fallbackRouteParams,
-        renderOpts,
-        sharedContext
-      )
-    }
-
     return exportAppPage(
       req,
       res,
diff --git a/packages/next/src/server/config-schema.ts b/packages/next/src/server/config-schema.ts
index 5722af8fe92e2..cf2e060444674 100644
--- a/packages/next/src/server/config-schema.ts
+++ b/packages/next/src/server/config-schema.ts
@@ -41,7 +41,6 @@ const zExportMap: zod.ZodType<ExportPathMap> = z.record(
     _isAppDir: z.boolean().optional(),
     _isDynamicError: z.boolean().optional(),
     _isRoutePPREnabled: z.boolean().optional(),
-    _isProspectiveRender: z.boolean().optional(),
     _doNotThrowOnEmptyStaticShell: z.boolean().optional(),
   })
 )
diff --git a/packages/next/src/server/config-shared.ts b/packages/next/src/server/config-shared.ts
index 0b5314a1e3461..1d1a8c3cf277d 100644
--- a/packages/next/src/server/config-shared.ts
+++ b/packages/next/src/server/config-shared.ts
@@ -743,16 +743,6 @@ export type ExportPathMap = {
      */
     _isRoutePPREnabled?: boolean
 
-    /**
-     * When true, it indicates that this page is being rendered in an attempt to
-     * discover if the page will be safe to generate with PPR. This is only
-     * enabled when the app has `experimental.dynamicIO` enabled but does not
-     * have `experimental.ppr` enabled.
-     *
-     * @internal
-     */
-    _isProspectiveRender?: boolean
-
     /**
      * When true, it indicates that the diagnostic render for this page is
      * disabled. This is only used when the app has `experimental.ppr` and
diff --git a/packages/next/src/server/request/fallback-params.ts b/packages/next/src/server/request/fallback-params.ts
index e3af245362e80..52ebe2c2f7f65 100644
--- a/packages/next/src/server/request/fallback-params.ts
+++ b/packages/next/src/server/request/fallback-params.ts
@@ -3,7 +3,7 @@ import { getRouteRegex } from '../../shared/lib/router/utils/route-regex'
 
 export type FallbackRouteParams = ReadonlyMap<string, string>
 
-export function getParamKeys(page: string) {
+function getParamKeys(page: string) {
   const pattern = getRouteRegex(page)
   const matcher = getRouteMatcher(pattern)
 
diff --git a/test/e2e/app-dir/dynamic-io-errors/dynamic-io-errors.prospective-fallback.test.ts b/test/e2e/app-dir/dynamic-io-errors/dynamic-io-errors.prospective-fallback.test.ts
deleted file mode 100644
index d9c59ec790450..0000000000000
--- a/test/e2e/app-dir/dynamic-io-errors/dynamic-io-errors.prospective-fallback.test.ts
+++ /dev/null
@@ -1,54 +0,0 @@
-import { nextTestSetup } from 'e2e-utils'
-
-describe(`Dynamic IO Prospective Fallback`, () => {
-  const { next, isNextDev, skipped } = nextTestSetup({
-    files: __dirname + '/fixtures/prospective-fallback',
-    skipStart: true,
-    skipDeployment: true,
-  })
-
-  if (skipped) {
-    return
-  }
-
-  if (isNextDev) {
-    it('should not error when visiting the page', async () => {
-      // Start the server, we expect this to succeed.
-      await next.start()
-
-      const res = await next.fetch('/blog/123')
-      expect(res.status).toBe(200)
-    })
-  } else {
-    it('should error on the build due to a missing suspense boundary', async () => {
-      try {
-        await next.start()
-      } catch {
-        // we expect the build to fail
-      }
-
-      // TODO: Assert on component stack
-      expect(next.cliOutput).toContain(
-        'Route "/blog/[slug]": A component accessed data, headers, params, searchParams, or a short-lived cache without a Suspense boundary nor a "use cache" above it.'
-      )
-    })
-
-    it('should not error when we add the missing suspense boundary', async () => {
-      await next.patchFile(
-        'app/blog/[slug]/loading.jsx',
-        `
-        export default function Loading() {
-          return <div>Loading...</div>
-        }
-      `
-      )
-
-      // We expect this to succeed.
-      await next.start()
-
-      expect(next.cliOutput).not.toContain(
-        'Route "/blog/[slug]": A component accessed data, headers, params, searchParams, or a short-lived cache without a Suspense boundary nor a "use cache" above it.'
-      )
-    })
-  }
-})
diff --git a/test/e2e/app-dir/dynamic-io-errors/fixtures/prospective-fallback/app/blog/[slug]/page.jsx b/test/e2e/app-dir/dynamic-io-errors/fixtures/prospective-fallback/app/blog/[slug]/page.jsx
deleted file mode 100644
index cc65c1e63b57b..0000000000000
--- a/test/e2e/app-dir/dynamic-io-errors/fixtures/prospective-fallback/app/blog/[slug]/page.jsx
+++ /dev/null
@@ -1,4 +0,0 @@
-export default async function BlogPage({ params }) {
-  const { slug } = await params
-  return <div>Blog: {slug}</div>
-}
diff --git a/test/e2e/app-dir/dynamic-io-errors/fixtures/prospective-fallback/app/layout.jsx b/test/e2e/app-dir/dynamic-io-errors/fixtures/prospective-fallback/app/layout.jsx
deleted file mode 100644
index 803f17d863c8a..0000000000000
--- a/test/e2e/app-dir/dynamic-io-errors/fixtures/prospective-fallback/app/layout.jsx
+++ /dev/null
@@ -1,7 +0,0 @@
-export default function RootLayout({ children }) {
-  return (
-    <html>
-      <body>{children}</body>
-    </html>
-  )
-}
diff --git a/test/e2e/app-dir/dynamic-io-errors/fixtures/prospective-fallback/next.config.js b/test/e2e/app-dir/dynamic-io-errors/fixtures/prospective-fallback/next.config.js
deleted file mode 100644
index 195ad29be3c54..0000000000000
--- a/test/e2e/app-dir/dynamic-io-errors/fixtures/prospective-fallback/next.config.js
+++ /dev/null
@@ -1,6 +0,0 @@
-module.exports = {
-  experimental: {
-    dynamicIO: true,
-    serverSourceMaps: true,
-  },
-}