[fix] clone the config module to avoid mutation (#80573)

Author: huozhi

packages/next/src/server/config.ts

@@ -30,6 +30,7 @@ import { normalizeZodErrors } from '../shared/lib/zod'
 import { HTML_LIMITED_BOT_UA_RE_STRING } from '../shared/lib/router/utils/is-bot'
 import { findDir } from '../lib/find-pages-dir'
 import { CanaryOnlyError, isStableBuild } from '../shared/lib/canary-only'
+import { interopDefault } from '../lib/interop-default'
 
 export { normalizeConfig } from './config-shared'
 export type { DomainLocale, NextConfig } from './config-shared'
@@ -1192,10 +1193,12 @@ export default async function loadConfig(
       throw err
     }
 
-    const userConfig = (await normalizeConfig(
+    // Clone a new userConfig each time to avoid mutating the original
+    const loadedConfig = (await normalizeConfig(
       phase,
-      userConfigModule.default || userConfigModule
+      interopDefault(userConfigModule)
     )) as NextConfig
+    const userConfig = cloneObject(loadedConfig) as NextConfig
 
     if (!process.env.NEXT_MINIMAL) {
       // We only validate the config against schema in non minimal mode
@@ -1312,7 +1315,7 @@ export default async function loadConfig(
       userConfig.htmlLimitedBots = userConfig.htmlLimitedBots.source
     }
 
-    onLoadUserConfig?.(userConfig)
+    onLoadUserConfig?.(Object.freeze(loadedConfig))
     const completeConfig = assignDefaults(
       dir,
       {
@@ -1401,3 +1404,22 @@ export function getConfiguredExperimentalFeatures(
   }
   return configuredExperimentalFeatures
 }
+
+function cloneObject(obj: any): any {
+  if (obj === null || typeof obj !== 'object') {
+    return obj
+  }
+
+  if (Array.isArray(obj)) {
+    return obj.map(cloneObject)
+  }
+  const keys = Object.keys(obj)
+  if (keys.length === 0) {
+    return obj
+  }
+
+  return keys.reduce((acc, key) => {
+    ;(acc as any)[key] = cloneObject(obj[key])
+    return acc
+  }, {})
+}

test/e2e/app-dir/metadata-streaming/metadata-streaming-customized-rule.test.ts

@@ -1,7 +1,7 @@
 import { nextTestSetup } from 'e2e-utils'
 
 describe('app-dir - metadata-streaming-customized-rule', () => {
-  const { next } = nextTestSetup({
+  const { next, isNextDev } = nextTestSetup({
     files: __dirname,
     overrideFiles: {
       'next.config.js': `
@@ -39,4 +39,12 @@ describe('app-dir - metadata-streaming-customized-rule', () => {
     expect(await $('head title').length).toBe(0)
     expect(await $('body title').length).toBe(1)
   })
+
+  if (isNextDev) {
+    it('should not have schema issue', () => {
+      expect(next.cliOutput).not.toContain(
+        'Invalid next.config.js options detected'
+      )
+    })
+  }
 })