Add AOT gzip content-encoding support for main build files. (#565)

* Add AOT gzip content-encoding support.
Currently we only do this for
main.js and commons.js only.

* Remove unwanted await.

* Use Promise.all to gzip assets in parallel.

server/build/gzip.js

+import fs from 'fs'
+import path from 'path'
+import zlib from 'zlib'
+
+export default async function gzipAssets (dir) {
+  const nextDir = path.resolve(dir, '.next')
+
+  await Promise.all([
+    gzip(path.resolve(nextDir, 'commons.js')),
+    gzip(path.resolve(nextDir, 'main.js'))
+  ])
+}
+
+export function gzip (filePath) {
+  const input = fs.createReadStream(filePath)
+  const output = fs.createWriteStream(`${filePath}.gz`)
+
+  return new Promise((resolve, reject) => {
+    const stream = input.pipe(zlib.createGzip()).pipe(output)
+    stream.on('error', reject)
+    stream.on('finish', resolve)
+  })
+}

server/build/index.js

 import webpack from './webpack'
 import clean from './clean'
+import gzipAssets from './gzip'
 
 export default async function build (dir) {
   const [compiler] = await Promise.all([
@@ -8,6 +9,7 @@ export default async function build (dir) {
   ])
 
   await runCompiler(compiler)
+  await gzipAssets(dir)
 }
 
 function runCompiler (compiler) {

server/index.js

 import { resolve, join } from 'path'
 import { parse } from 'url'
 import http from 'http'
+import fs from 'mz/fs'
 import send from 'send'
+import accepts from 'accepts'
 import {
   renderToHTML,
   renderErrorToHTML,
@@ -61,12 +63,12 @@ export default class Server {
 
     this.router.get('/_next/main.js', async (req, res, params) => {
       const p = join(this.dir, '.next/main.js')
-      await this.serveStatic(req, res, p)
+      await this.serveStaticWithGzip(req, res, p)
     })
 
     this.router.get('/_next/commons.js', async (req, res, params) => {
       const p = join(this.dir, '.next/commons.js')
-      await this.serveStatic(req, res, p)
+      await this.serveStaticWithGzip(req, res, p)
     })
 
     this.router.get('/_next/pages/:path*', async (req, res, params) => {
@@ -212,6 +214,22 @@ export default class Server {
     return renderErrorJSON(err, res, this.renderOpts)
   }
 
+  async serveStaticWithGzip (req, res, path) {
+    const encoding = accepts(req).encodings(['gzip'])
+    if (encoding !== 'gzip') {
+      return this.serveStatic(req, res, path)
+    }
+
+    const gzipPath = `${path}.gz`
+    const exists = await fs.exists(gzipPath)
+    if (!exists) {
+      return this.serveStatic(req, res, path)
+    }
+
+    res.setHeader('Content-Encoding', 'gzip')
+    return this.serveStatic(req, res, gzipPath)
+  }
+
   serveStatic (req, res, path) {
     return new Promise((resolve, reject) => {
       send(req, path)