Provide default fallback _document and _app for for concurrent mode

packages/next/build/webpack-config.ts

@@ -361,11 +361,16 @@ export default async function getBaseWebpackConfig(
   const hasReactRoot: boolean =
     config.experimental.reactRoot || hasReact18 || isReactExperimental
 
-  if (config.experimental.reactRoot && !(hasReact18 || isReactExperimental)) {
+  // Only inform during one of the builds
+  if (
+    !isServer &&
+    config.experimental.reactRoot &&
+    !(hasReact18 || isReactExperimental)
+  ) {
     // It's fine to only mention React 18 here as we don't recommend people to try experimental.
     Log.warn('You have to use React 18 to use `experimental.reactRoot`.')
   }
-  if (config.experimental.concurrentFeatures && !hasReactRoot) {
+  if (!isServer && config.experimental.concurrentFeatures && !hasReactRoot) {
     throw new Error(
       '`experimental.concurrentFeatures` requires `experimental.reactRoot` to be enabled along with React 18.'
     )

packages/next/build/webpack/loaders/next-middleware-ssr-loader/index.ts

@@ -1,7 +1,33 @@
 import loaderUtils from 'next/dist/compiled/loader-utils'
 import { getStringifiedAbsolutePath } from './utils'
 
-export default function middlewareRSCLoader(this: any) {
+const fallbackDocumentPage = `
+import { Html, Head, Main, NextScript } from 'next/document'
+
+function Document() {
+  return (
+    createElement(Html, null, 
+      createElement(Head),
+      createElement('body', null,
+        createElement(Main),
+        createElement(NextScript),
+      )
+    )
+  )
+}
+`
+
+function hasModule(path: string) {
+  let has
+  try {
+    has = !!require.resolve(path)
+  } catch (_) {
+    has = false
+  }
+  return has
+}
+
+export default async function middlewareRSCLoader(this: any) {
   const {
     absolutePagePath,
     basePath,
@@ -22,6 +48,21 @@ export default function middlewareRSCLoader(this: any) {
     './pages/_app'
   )
 
+  const hasProvidedAppPage = hasModule(JSON.parse(stringifiedAbsoluteAppPath))
+  const hasProvidedDocumentPage = hasModule(
+    JSON.parse(stringifiedAbsoluteDocumentPath)
+  )
+
+  let appDefinition = `const App = require(${
+    hasProvidedAppPage
+      ? stringifiedAbsoluteAppPath
+      : JSON.stringify('next/dist/pages/_app')
+  }).default`
+
+  let documentDefinition = hasProvidedDocumentPage
+    ? `const Document = require(${stringifiedAbsoluteDocumentPath}).default`
+    : fallbackDocumentPage
+
   const transformed = `
         import { adapter } from 'next/dist/server/web/adapter'
 
@@ -38,30 +79,35 @@ export default function middlewareRSCLoader(this: any) {
             : ''
         }
 
-        var {
+        ${documentDefinition}
+        ${appDefinition}
+
+        const {
           default: Page,
           config,
           getStaticProps,
           getServerSideProps,
           getStaticPaths
         } = require(${stringifiedAbsolutePagePath})
-        var Document = require(${stringifiedAbsoluteDocumentPath}).default
-        var App = require(${stringifiedAbsoluteAppPath}).default
 
         const buildManifest = self.__BUILD_MANIFEST
         const reactLoadableManifest = self.__REACT_LOADABLE_MANIFEST
         const rscManifest = self._middleware_rsc_manifest
 
         if (typeof Page !== 'function') {
-          throw new Error('Your page must export a \`default\` component');
+          throw new Error('Your page must export a \`default\` component')
+        }
+
+        function renderError(err, status) {
+          return new Response(err.toString(), {status})
         }
 
-        function wrapReadable (readable) {
-          var encoder = new TextEncoder()
-          var transformStream = new TransformStream()
-          var writer = transformStream.writable.getWriter()
-          var reader = readable.getReader()
-          var process = () => {
+        function wrapReadable(readable) {
+          const encoder = new TextEncoder()
+          const transformStream = new TransformStream()
+          const writer = transformStream.writable.getWriter()
+          const reader = readable.getReader()
+          const process = () => {
             reader.read().then(({ done, value }) => {
               if (!done) {
                 writer.write(typeof value === 'string' ? encoder.encode(value) : value)
@@ -82,7 +128,7 @@ export default function middlewareRSCLoader(this: any) {
 
         let responseCache
         const FlightWrapper = props => {
-          var response = responseCache
+          let response = responseCache
           if (!response) {
             responseCache = response = createFromReadableStream(renderFlight(props))
           }
@@ -103,6 +149,11 @@ export default function middlewareRSCLoader(this: any) {
           const url = request.nextUrl
           const query = Object.fromEntries(url.searchParams)
 
+          if (Document.getInitialProps) {
+            const err = new Error('Document.getInitialProps is not supported with server components, please remove it from pages/_document')
+            return renderError(err, 500)
+          }
+
           // Preflight request
           if (request.method === 'HEAD') {
             return new Response('OK.', {

test/integration/react-rsc-basic/test/index.test.js

@@ -5,6 +5,8 @@ import { join } from 'path'
 import fs from 'fs-extra'
 
 import {
+  File,
+  fetchViaHTTP,
   findPort,
   killApp,
   launchApp,
@@ -18,6 +20,38 @@ import css from './css'
 const nodeArgs = ['-r', join(__dirname, '../../react-18/test/require-hook.js')]
 const appDir = join(__dirname, '../app')
 const distDir = join(__dirname, '../app/.next')
+const documentPage = new File(join(appDir, 'pages/_document.js'))
+const appPage = new File(join(appDir, 'pages/_app.js'))
+
+const documentWithGip = `
+import { Html, Head, Main, NextScript } from 'next/document'
+
+export default function Document() {
+  return (
+    <Html>
+      <Head />
+      <body>
+        <Main />
+        <NextScript />
+      </body>
+    </Html>
+  )
+}
+
+Document.getInitialProps = (ctx) => {
+  return ctx.defaultGetInitialProps(ctx)
+}
+`
+
+const appWithGlobalCss = `
+import '../styles.css'
+
+function App({ Component, pageProps }) {
+  return <Component {...pageProps} />
+}
+
+export default App
+`
 
 async function nextBuild(dir) {
   return await _nextBuild(dir, [], {
@@ -99,7 +133,7 @@ describe('RSC prod', () => {
       expect(content.clientInfo).toContainEqual(item)
     }
   })
-  runTests(context)
+  runBasicTests(context)
 })
 
 describe('RSC dev', () => {
@@ -112,39 +146,38 @@ describe('RSC dev', () => {
   afterAll(async () => {
     await killApp(context.server)
   })
-  runTests(context)
+  runBasicTests(context)
 })
 
-describe('CSS prod', () => {
-  const context = { appDir }
-
-  beforeAll(async () => {
-    context.appPort = await findPort()
-    await nextBuild(context.appDir)
-    context.server = await nextStart(context.appDir, context.appPort)
-  })
-  afterAll(async () => {
-    await killApp(context.server)
-  })
+const cssSuite = {
+  runTests: css,
+  before: () => appPage.write(appWithGlobalCss),
+  after: () => appPage.delete(),
+}
 
-  css(context)
-})
+runSuite('CSS', 'dev', cssSuite)
+runSuite('CSS', 'prod', cssSuite)
 
-describe('CSS dev', () => {
-  const context = { appDir }
+const documentSuite = {
+  runTests: (context) => {
+    it('should error when custom _document has getInitialProps method', async () => {
+      const res = await fetchViaHTTP(context.appPort, '/')
+      const html = await res.text()
 
-  beforeAll(async () => {
-    context.appPort = await findPort()
-    context.server = await nextDev(context.appDir, context.appPort)
-  })
-  afterAll(async () => {
-    await killApp(context.server)
-  })
+      expect(res.status).toBe(500)
+      expect(html).toContain(
+        'Document.getInitialProps is not supported with server components, please remove it from pages/_document'
+      )
+    })
+  },
+  before: () => documentPage.write(documentWithGip),
+  after: () => documentPage.delete(),
+}
 
-  css(context)
-})
+runSuite('document', 'dev', documentSuite)
+runSuite('document', 'prod', documentSuite)
 
-async function runTests(context) {
+async function runBasicTests(context) {
   it('should render the correct html', async () => {
     const homeHTML = await renderViaHTTP(context.appPort, '/')
 
@@ -181,3 +214,29 @@ async function runTests(context) {
     expect(imageTag.attr('src')).toContain('data:image')
   })
 }
+
+function runSuite(suiteName, env, { runTests, before, after }) {
+  const context = { appDir }
+  describe(`${suiteName} ${env}`, () => {
+    if (env === 'prod') {
+      beforeAll(async () => {
+        before?.()
+        context.appPort = await findPort()
+        context.server = await nextDev(context.appDir, context.appPort)
+      })
+    }
+    if (env === 'dev') {
+      beforeAll(async () => {
+        before?.()
+        context.appPort = await findPort()
+        context.server = await nextDev(context.appDir, context.appPort)
+      })
+    }
+    afterAll(async () => {
+      after?.()
+      await killApp(context.server)
+    })
+
+    runTests(context)
+  })
+}