Add check for writeable directory (vercel#3370)

* Add check for writeable directory

Followup of vercel/vercel#175

* Add link to docs

errors/build-dir-not-writeable.md

@@ -0,0 +1,30 @@
+# Build directory not writeable
+
+#### Why This Error Occurred
+
+The filesystem does not allow writing to the specified directory. A common cause for this error is starting a [custom server](https://github.com/zeit/next.js#custom-server-and-routing) in development mode on a production server, for example, [now.sh](https://zeit.co) which [doesn't allow you to write to the filesystem after your app is built](https://zeit.co/docs/deployment-types/node#file-system-specifications).
+
+#### Possible Ways to Fix It
+
+When using a custom server with a server file, for example called `server.js`, make sure you update the scripts key in `package.json` to:
+
+```json
+{
+  "scripts": {
+    "dev": "node server.js",
+    "build": "next build",
+    "start": "NODE_ENV=production node server.js"
+  }
+}
+```
+
+and the custom server starts Next in production mode when `NODE_ENV` is `production`
+
+```js
+const dev = process.env.NODE_ENV !== 'production'
+const app = next({ dev })
+```
+
+### Useful Links
+
+- [Custom Server documentation + examples](https://github.com/zeit/next.js#custom-server-and-routing)

readme.md

@@ -669,7 +669,19 @@ export default ({ url }) =>
   </ul>
 </details></p>
 
-Typically you start your next server with `next start`. It's possible, however, to start a server 100% programmatically in order to customize routes, use route patterns, etc
+Typically you start your next server with `next start`. It's possible, however, to start a server 100% programmatically in order to customize routes, use route patterns, etc.
+
+When using a custom server with a server file, for example called `server.js`, make sure you update the scripts key in `package.json` to:
+
+```json
+{
+  "scripts": {
+    "dev": "node server.js",
+    "build": "next build",
+    "start": "NODE_ENV=production node server.js"
+  }
+}
+```
 
 This example makes `/a` resolve to `./pages/b`, and `/b` resolve to `./pages/a`:
 

server/build/index.js

@@ -10,6 +10,14 @@ import md5File from 'md5-file/promise'
 export default async function build (dir, conf = null) {
   const buildId = uuid.v4()
   const buildDir = join(tmpdir(), uuid.v4())
+
+  try {
+    await fs.access(buildDir, fs.constants.W_OK)
+  } catch (err) {
+    console.error(`> Failed, build directory is not writeable. https://err.sh/zeit/next.js/build-dir-not-writeable`)
+    throw err
+  }
+
   const compiler = await webpack(dir, { buildId, buildDir, conf })
 
   try {