fix: better error message with an invalid assetPrefix (#49403)

fixes #47641

This adds a friendly error for loading a font error caused by the
invalid `assetPrefix` setting.

Current:
<img width="747" alt="image"
src="https://user-images.githubusercontent.com/250407/236685607-c03b6160-9e8c-4c88-80e1-714f6a140588.png">


This PR:
<img width="890" alt="image"
src="https://user-images.githubusercontent.com/250407/236685499-da4c3d69-0f97-458d-8709-dcc43475e0e9.png">


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---------

Co-authored-by: JJ Kasper <jj@jjsweb.site>

packages/next/src/build/webpack/loaders/next-font-loader/index.ts

@@ -49,6 +49,15 @@ export default async function nextFontLoader(this: any) {
       postcss: getPostcss,
     } = this.getOptions()
 
+    if (assetPrefix && !/^\/|https?:\/\//.test(assetPrefix)) {
+      const err = new Error(
+        'assetPrefix must start with a leading slash or be an absolute URL(http:// or https://)'
+      )
+      err.name = 'NextFontError'
+      callback(err)
+      return
+    }
+
     /**
      * Emit font files to .next/static/media as [hash].[ext].
      *

test/integration/font-optimization/fixtures/invalid-assertprefix/next.config.js

@@ -0,0 +1,3 @@
+module.exports = {
+  assetPrefix: '.',
+}

test/integration/font-optimization/fixtures/invalid-assertprefix/pages/index.js

@@ -0,0 +1,10 @@
+import React from 'react'
+import { Inter } from 'next/font/google'
+
+const inter = Inter({ subsets: ['latin'] })
+
+const Page = () => {
+  return <div className={inter.className}>Hello</div>
+}
+
+export default Page

test/integration/font-optimization/test/index.test.js

@@ -313,4 +313,15 @@ describe('Font Optimization', () => {
       )
     })
   })
+  describe('invalid configuration', () => {
+    it('should show a proper error if assetPrefix starts with .', async () => {
+      const appDir = join(fixturesDir, 'invalid-assertprefix')
+      const { stderr } = await nextBuild(appDir, undefined, {
+        stderr: true,
+      })
+      expect(stderr).toContain(
+        'assetPrefix must start with a leading slash or be an absolute URL(http:// or https://)'
+      )
+    })
+  })
 })