fix: improve error when starting next without building (#62404)

Addresses #57066

Currently, a user running `next start` on an un-built Next.js project
receives a confusing uncaught error before the process just ends:

```
   ▲ Next.js 14.1.0
   - Local:        http://localhost:3000

[Error: ENOENT: no such file or directory, open '/home/flotwig/src/project/.next/BUILD_ID'] {
  errno: -2,
  code: 'ENOENT',
  syscall: 'open',
  path: '/home/flotwig/src/project/.next/BUILD_ID'
}
```

In my case, I ran into this because I had not worked with Next.js for a
while, and I forgot that `next dev` was used for local development, not
`next start`. I believe many of the confused users in #57066 are making
a similar mistake and not realizing it, due to the error message.

This PR catches an `ENOENT` when reading `BUILD_ID` and suggests that
the user `next build` or `next dev` to remove this friction point:

```
   ▲ Next.js 14.1.1-canary.69
   - Local:        http://localhost:3000

Error: Could not find a production build in the '.next' directory. Try building your app with 'next build' before starting the production server. https://nextjs.org/docs/messages/production-start-no-build-id
    at setupFsCheck (/home/flotwig/src/next.js/packages/next/src/server/lib/router-utils/filesystem.ts:157:13)
    at initialize (/home/flotwig/src/next.js/packages/next/src/server/lib/router-server.ts:90:21)
    at Server.<anonymous> (/home/flotwig/src/next.js/packages/next/src/server/lib/start-server.ts:295:28)
```

A few things probably need to be resolved before this PR can merge, I
would appreciate any input from Next.js maintainers.

- [x] Is this the best place to catch this? I am not familiar with the
Next.js codebase, so I just caught this in the most obvious way.
- [x] Can the error message be improved?
- [x] Add a test for the error in `test/development`

---------

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

packages/next/src/server/lib/router-utils/filesystem.ts

@@ -150,7 +150,14 @@ export async function setupFsCheck(opts: {
 
   if (!opts.dev) {
     const buildIdPath = path.join(opts.dir, opts.config.distDir, BUILD_ID_FILE)
-    buildId = await fs.readFile(buildIdPath, 'utf8')
+    try {
+      buildId = await fs.readFile(buildIdPath, 'utf8')
+    } catch (err: any) {
+      if (err.code !== 'ENOENT') throw err
+      throw new Error(
+        `Could not find a production build in the '${opts.config.distDir}' directory. Try building your app with 'next build' before starting the production server. https://nextjs.org/docs/messages/production-start-no-build-id`
+      )
+    }
 
     try {
       for (const file of await recursiveReadDir(publicFolderPath)) {

test/development/start-no-build/start-no-build.test.ts

@@ -0,0 +1,22 @@
+import { createNext } from 'e2e-utils'
+
+describe('next start without next build', () => {
+  it('should show error when there is no production build', async () => {
+    const next = await createNext({
+      files: __dirname,
+      skipStart: true,
+      startCommand: `pnpm next start`,
+    })
+
+    await next.start()
+    await new Promise<void>((resolve, reject) => {
+      next.on('stderr', (msg) => {
+        if (msg.includes('Could not find a production build in the')) {
+          resolve()
+        }
+      })
+    })
+
+    await next.destroy()
+  })
+})