Leverage blocked page for _error (vercel#26748)

## Enhance

simplify detection for visiting `_error`

x-ref: vercel#26610

Author: huozhi

packages/next/server/next-server.ts

@@ -1447,7 +1447,6 @@ export default class Server {
   ): Promise<string | null> {
     const is404Page = pathname === '/404'
     const is500Page = pathname === '/500'
-    const isErrorPage = pathname === '/_error'
 
     const isLikeServerless =
       typeof components.Component === 'object' &&
@@ -1466,10 +1465,6 @@ export default class Server {
       res.statusCode = 404
     }
 
-    if (isErrorPage && res.statusCode === 200) {
-      res.statusCode = 404
-    }
-
     // ensure correct status is set when visiting a status page
     // directly e.g. /500
     if (STATIC_STATUS_PAGES.includes(pathname)) {

packages/next/shared/lib/constants.ts

@@ -17,7 +17,7 @@ export const SERVER_DIRECTORY = 'server'
 export const SERVERLESS_DIRECTORY = 'serverless'
 export const CONFIG_FILE = 'next.config.js'
 export const BUILD_ID_FILE = 'BUILD_ID'
-export const BLOCKED_PAGES = ['/_document', '/_app']
+export const BLOCKED_PAGES = ['/_document', '/_app', '/_error']
 export const CLIENT_PUBLIC_FILES_PATH = 'public'
 export const CLIENT_STATIC_FILES_PATH = 'static'
 export const CLIENT_STATIC_FILES_RUNTIME = 'runtime'

test/integration/client-navigation/test/index.test.js

@@ -29,6 +29,7 @@ describe('Client Navigation', () => {
   afterAll(() => killApp(context.server))
 
   it('should not reload when visiting /_error directly', async () => {
+    const { status } = await fetchViaHTTP(context.appPort, '/_error')
     const browser = await webdriver(context.appPort, '/_error')
 
     await browser.eval('window.hello = true')
@@ -41,6 +42,7 @@ describe('Client Navigation', () => {
     }
     const html = await browser.eval('document.documentElement.innerHTML')
 
+    expect(status).toBe(404)
     expect(html).toContain('This page could not be found')
     expect(html).toContain('404')
   })