Allow BUILD_ID to be set using generateBuildId (minor) (#3873)

* Allow BUILD_ID to be set in the environment

This makes multi server deploys with Capistrano possible.

* next.config.js generateBuildId support

enable customising the build id via config

* Provide default for generateBuildId

* This is not used

readme.md

@@ -1122,6 +1122,20 @@ module.exports = {
 }
 ```
 
+#### Configuring the build ID
+
+Next.js uses a constant generated at build time to identify which version of your application is being served. This can cause problems in multi-server deployments when `next build` is ran on every server. In order to keep a static build id between builds you can provide the `generateBuildId` function:
+
+```js
+// next.config.js
+module.exports = {
+  generateBuildId: async () => {
+    // For example get the latest git commit hash here
+    return 'my-build-id'
+  }
+}
+```
+
 ### Customizing webpack config
 
 <p><details>

server/build/index.js

@@ -1,10 +1,9 @@
 import { join } from 'path'
 import promisify from '../lib/promisify'
 import fs from 'fs'
-import uuid from 'uuid'
 import webpack from 'webpack'
 import getConfig from '../config'
-import {PHASE_PRODUCTION_BUILD} from '../../lib/constants'
+import { PHASE_PRODUCTION_BUILD } from '../../lib/constants'
 import getBaseWebpackConfig from './webpack'
 
 const access = promisify(fs.access)
@@ -12,7 +11,7 @@ const writeFile = promisify(fs.writeFile)
 
 export default async function build (dir, conf = null) {
   const config = getConfig(PHASE_PRODUCTION_BUILD, dir, conf)
-  const buildId = uuid.v4()
+  const buildId = await config.generateBuildId() // defaults to a uuid
 
   try {
     await access(dir, (fs.constants || fs).W_OK)

server/config.js

@@ -1,5 +1,6 @@
 // @flow
 import findUp from 'find-up'
+import uuid from 'uuid'
 
 const cache = new Map()
 
@@ -11,6 +12,7 @@ const defaultConfig = {
   assetPrefix: '',
   configOrigin: 'default',
   useFileSystemPublicRoutes: true,
+  generateBuildId: () => uuid.v4(),
   generateEtags: true,
   pageExtensions: ['jsx', 'js']
 }