Fix serverside asset modules

packages/next/build/webpack-config.ts

@@ -1000,9 +1000,7 @@ export default async function getBaseWebpackConfig(
       ],
     },
     output: {
-      // we must set publicPath to an empty value to override the default of
-      // auto which doesn't work in IE11
-      publicPath: `${config.assetPrefix || ''}/_next/`,
+      publicPath: isServer ? '' : `${config.assetPrefix || ''}/_next/`,
       path:
         isServer && isWebpack5 && !dev
           ? path.join(outputPath, 'chunks')

test/integration/server-asset-modules/my-data.json

@@ -0,0 +1,3 @@
+{
+  "message": "hello world"
+}

test/integration/server-asset-modules/next.config.js

@@ -0,0 +1 @@
+module.exports = {}

test/integration/server-asset-modules/pages/api/test.js

@@ -0,0 +1,6 @@
+import * as fs from 'fs/promises'
+export default async (req, res) => {
+  const fileUrl = new URL('../../my-data.json', import.meta.url)
+  const content = await fs.readFile(fileUrl, { encoding: 'utf-8' })
+  res.json(JSON.parse(content))
+}

test/integration/server-asset-modules/test/index.test.js

@@ -0,0 +1,80 @@
+/* eslint-env jest */
+
+import fs from 'fs-extra'
+import {
+  fetchViaHTTP,
+  findPort,
+  killApp,
+  launchApp,
+  nextBuild,
+  nextStart,
+} from 'next-test-utils'
+import { join } from 'path'
+
+jest.setTimeout(1000 * 60 * 2)
+
+let app
+let appPort
+const appDir = join(__dirname, '../')
+
+function runTests() {
+  it('should enable reading local files in api routes', async () => {
+    const res = await fetchViaHTTP(appPort, '/api/test', null, {})
+    expect(res.status).toEqual(200)
+    const content = await res.json()
+    expect(content).toHaveProperty('message', 'hello world')
+  })
+}
+
+const nextConfig = join(appDir, 'next.config.js')
+
+describe('serverside asset modules', () => {
+  describe('dev mode', () => {
+    beforeAll(async () => {
+      appPort = await findPort()
+      app = await launchApp(appDir, appPort)
+    })
+    afterAll(() => killApp(app))
+
+    runTests()
+  })
+
+  describe('production mode', () => {
+    beforeAll(async () => {
+      const curConfig = await fs.readFile(nextConfig, 'utf8')
+
+      if (curConfig.includes('target')) {
+        await fs.writeFile(nextConfig, `module.exports = {}`)
+      }
+      await nextBuild(appDir)
+
+      appPort = await findPort()
+      app = await nextStart(appDir, appPort)
+    })
+    afterAll(() => killApp(app))
+
+    runTests()
+  })
+
+  describe('serverless mode', () => {
+    let origNextConfig
+
+    beforeAll(async () => {
+      origNextConfig = await fs.readFile(nextConfig, 'utf8')
+      await fs.writeFile(
+        nextConfig,
+        `module.exports = { target: 'serverless' }`
+      )
+
+      await nextBuild(appDir)
+
+      appPort = await findPort()
+      app = await nextStart(appDir, appPort)
+    })
+    afterAll(async () => {
+      await fs.writeFile(nextConfig, origNextConfig)
+      await killApp(app)
+    })
+    runTests()
+  })
+})