fix(#30300): force export 404.html (#36827)

## Bug

- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

The PR fixes #30300.

The previous integration test case only checks if `/out/404.html` exists. However, the test passes since `/out/404.html/index.html` is being exported instead.
The PR changes that by checking if a given path exists and is a file.

packages/next/export/worker.ts

@@ -221,8 +221,13 @@ export default async function exportPage({
       // extension of `.slug]`
       const pageExt = isDynamic ? '' : extname(page)
       const pathExt = isDynamic ? '' : extname(path)
+
+      // force output 404.html for backwards compat
+      if (path === '/404.html') {
+        htmlFilename = path
+      }
       // Make sure page isn't a folder with a dot in the name e.g. `v1.2`
-      if (pageExt !== pathExt && pathExt !== '') {
+      else if (pageExt !== pathExt && pathExt !== '') {
         const isBuiltinPaths = ['/500', '/404'].some(
           (p) => p === path || p === path + '.html'
         )

test/integration/export/test/index.test.js

@@ -20,7 +20,7 @@ import { promises } from 'fs'
 import dynamic from './dynamic'
 import apiRoutes from './api-routes'
 
-const { access, mkdir, writeFile } = promises
+const { access, mkdir, writeFile, stat } = promises
 const appDir = join(__dirname, '../')
 const outdir = join(appDir, 'out')
 const outNoTrailSlash = join(appDir, 'outNoTrailSlash')
@@ -29,6 +29,12 @@ context.appDir = appDir
 const devContext = {}
 const nextConfig = new File(join(appDir, 'next.config.js'))
 
+const fileExist = (path) =>
+  access(path)
+    .then(() => stat(path))
+    .then((stats) => (stats.isFile() ? true : false))
+    .catch(() => false)
+
 describe('Static Export', () => {
   it('should delete existing exported files', async () => {
     const tempfile = join(outdir, 'temp.txt')
@@ -87,60 +93,28 @@ describe('Static Export', () => {
   })
 
   it('should honor exportTrailingSlash for 404 page', async () => {
-    expect(
-      await access(join(outdir, '404/index.html'))
-        .then(() => true)
-        .catch(() => false)
-    ).toBe(true)
+    expect(await fileExist(join(outdir, '404/index.html'))).toBe(true)
 
     // we still output 404.html for backwards compat
-    expect(
-      await access(join(outdir, '404.html'))
-        .then(() => true)
-        .catch(() => false)
-    ).toBe(true)
+    expect(await fileExist(join(outdir, '404.html'))).toBe(true)
   })
 
   it('should handle trailing slash in getStaticPaths', async () => {
-    expect(
-      await access(join(outdir, 'gssp/foo/index.html'))
-        .then(() => true)
-        .catch(() => false)
-    ).toBe(true)
-
-    expect(
-      await access(join(outNoTrailSlash, 'gssp/foo.html'))
-        .then(() => true)
-        .catch(() => false)
-    ).toBe(true)
+    expect(await fileExist(join(outdir, 'gssp/foo/index.html'))).toBe(true)
+
+    expect(await fileExist(join(outNoTrailSlash, 'gssp/foo.html'))).toBe(true)
   })
 
   it('should only output 404.html without exportTrailingSlash', async () => {
-    expect(
-      await access(join(outNoTrailSlash, '404/index.html'))
-        .then(() => true)
-        .catch(() => false)
-    ).toBe(false)
-
-    expect(
-      await access(join(outNoTrailSlash, '404.html'))
-        .then(() => true)
-        .catch(() => false)
-    ).toBe(true)
+    expect(await fileExist(join(outNoTrailSlash, '404/index.html'))).toBe(false)
+
+    expect(await fileExist(join(outNoTrailSlash, '404.html'))).toBe(true)
   })
 
   it('should not duplicate /index with exportTrailingSlash', async () => {
-    expect(
-      await access(join(outdir, 'index/index.html'))
-        .then(() => true)
-        .catch(() => false)
-    ).toBe(false)
-
-    expect(
-      await access(join(outdir, 'index.html'))
-        .then(() => true)
-        .catch(() => false)
-    ).toBe(true)
+    expect(await fileExist(join(outdir, 'index/index.html'))).toBe(false)
+
+    expect(await fileExist(join(outdir, 'index.html'))).toBe(true)
   })
 
   ssr(context)