fixes 404 no longer returned when error with code ENOENT is thrown.

errors/getinitialprops-enoent.md

@@ -0,0 +1,16 @@
+# ENOENT from getInitialProps
+
+#### Why This Error Occurred
+
+In one of your pages you threw an error with the code `ENOENT`, this was previously an internal feature used to trigger rendering the 404 page and should not be relied on.
+
+#### Possible Ways to Fix It
+
+In `getServerSideProps` or `getInitialProps` you can set the status code with the ServerResponse (`res`) object provided e.g. `res.statusCode = 404`.
+
+In `getStaticProps` you can handle a 404 by rendering the 404 state on the current page instead of throwing and trying to render a separate page.
+
+### Useful Links
+
+- [Google 404 Error Guide](https://developers.google.com/search/docs/guides/fix-search-javascript)
+- [404 Page Documentation](https://nextjs.org/docs/advanced-features/custom-error-page#404-page)

packages/next/next-server/server/next-server.ts

@@ -1245,9 +1245,14 @@ export default class Server {
         }
       }
     } catch (err) {
-      this.logError(err)
-      res.statusCode = 500
-      return await this.renderErrorToHTML(err, req, res, pathname, query)
+      if (err && err.code === 'ENOENT') {
+        res.statusCode = 404
+        return await this.renderErrorToHTML(null, req, res, pathname, query)
+      } else {
+        this.logError(err)
+        res.statusCode = 500
+        return await this.renderErrorToHTML(err, req, res, pathname, query)
+      }
     }
 
     res.statusCode = 404

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

@@ -42,6 +42,7 @@ import { LoadComponentsReturnType, ManifestItem } from './load-components'
 import optimizeAmp from './optimize-amp'
 import { UnwrapPromise } from '../../lib/coalesced-function'
 import { GetStaticProps, GetServerSideProps } from '../../types'
+import chalk from 'next/dist/compiled/chalk'
 
 function noRouter() {
   const message =
@@ -637,7 +638,14 @@ export async function renderToHTML(
       ;(renderOpts as any).pageData = props
     }
   } catch (err) {
-    if (isDataReq || !dev || !err) throw err
+    if (process.env.NODE_ENV !== 'production' && err?.code === 'ENOENT') {
+      console.warn(
+        chalk.yellow('Warning:') +
+          ` page "${pathname}" threw an error with a code of ENOENT. This was an internal feature to render the 404 page and should not be relied on.\nSee more info here: https://err.sh/next.js/getinitialprops-enoent`
+      )
+    }
+
+    if (isDataReq || !dev || !err || err.code === 'ENOENT') throw err
     ctx.err = err
     renderOpts.err = err
     console.error(err)

test/integration/custom-error/pages/throw-404.js

@@ -0,0 +1,9 @@
+const Page = () => 'hi'
+
+Page.getInitialProps = () => {
+  const error = new Error('to 404 we go')
+  error.code = 'ENOENT'
+  throw error
+}
+
+export default Page

test/integration/custom-error/test/index.test.js

@@ -14,23 +14,36 @@ import {
 jasmine.DEFAULT_TIMEOUT_INTERVAL = 1000 * 60 * 2
 const appDir = join(__dirname, '..')
 const nextConfig = join(appDir, 'next.config.js')
+
+let stderr = ''
 let appPort
 let app
 
-const runTests = () => {
+const runTests = isDev => {
   it('renders custom _error successfully', async () => {
     const html = await renderViaHTTP(appPort, '/')
-    expect(html).toMatch(/Custom error/)
+    expect(html).toMatch(isDev ? /oof/ : /Custom error/)
+  })
+
+  it('renders 404 page when ENOENT error is thrown', async () => {
+    const html = await renderViaHTTP(appPort, '/throw-404')
+    expect(html).toContain('Custom error')
+    expect(html).not.toContain('to 404 we go')
+
+    if (isDev) {
+      expect(stderr).toContain(
+        'page "/throw-404" threw an error with a code of ENOENT. This was an internal feature to render the 404 page and should not be relied on'
+      )
+    }
   })
 }
 
 const customErrNo404Match = /You have added a custom \/_error page without a custom \/404 page/
 
 describe('Custom _error', () => {
   describe('dev mode', () => {
-    let stderr = ''
-
     beforeAll(async () => {
+      stderr = ''
       appPort = await findPort()
       app = await launchApp(appDir, appPort, {
         onStderr(msg) {
@@ -56,6 +69,8 @@ describe('Custom _error', () => {
       expect(html).toContain('An error 404 occurred on server')
       expect(stderr).toMatch(customErrNo404Match)
     })
+
+    runTests(true)
   })
 
   describe('production mode', () => {

test/integration/prerender/pages/enoent.js

@@ -0,0 +1,14 @@
+export async function getStaticProps() {
+  if (process.env.NODE_ENV === 'development') {
+    const error = new Error('oof')
+    error.code = 'ENOENT'
+    throw error
+  }
+  return {
+    props: {
+      hi: 'hi',
+    },
+  }
+}
+
+export default () => 'hi'

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

@@ -129,6 +129,11 @@ const expectedManifestRoutes = () => ({
     initialRevalidateSeconds: false,
     srcRoute: null,
   },
+  '/enoent': {
+    dataRoute: `/_next/data/${buildId}/enoent.json`,
+    initialRevalidateSeconds: false,
+    srcRoute: null,
+  },
   '/something': {
     dataRoute: `/_next/data/${buildId}/something.json`,
     initialRevalidateSeconds: false,
@@ -505,6 +510,12 @@ const runTests = (dev = false, looseMode = false) => {
     //   )
     // })
 
+    it('should handle throw ENOENT correctly', async () => {
+      const res = await fetchViaHTTP(appPort, '/enoent')
+      const html = await res.text()
+      expect(html).toContain('oof')
+    })
+
     it('should not show warning from url prop being returned', async () => {
       const urlPropPage = join(appDir, 'pages/url-prop.js')
       await fs.writeFile(
@@ -810,6 +821,12 @@ const runTests = (dev = false, looseMode = false) => {
           ),
           page: '/default-revalidate',
         },
+        {
+          dataRouteRegex: normalizeRegEx(
+            `^\\/_next\\/data\\/${escapeRegex(buildId)}\\/enoent.json$`
+          ),
+          page: '/enoent',
+        },
         {
           dataRouteRegex: normalizeRegEx(
             `^\\/_next\\/data\\/${escapeRegex(