fast HEAD requests (#27672)

* fast HEAD requests

* update test

middleware/fast-head.js

@@ -0,0 +1,16 @@
+import { cacheControlFactory } from './cache-control.js'
+
+const cacheControl = cacheControlFactory(60 * 60 * 24)
+
+export default function fastHead(req, res, next) {
+  const { context } = req
+  const { page } = context
+  if (page) {
+    // Since the *presence* is not affected by the request, we can cache
+    // this and allow the CDN to hold on to it.
+    cacheControl(res)
+
+    return res.status(200).send('')
+  }
+  next()
+}

middleware/index.js

@@ -65,6 +65,7 @@ import archivedAssetRedirects from './archived-asset-redirects.js'
 import favicons from './favicons.js'
 import setStaticAssetCaching from './static-asset-caching.js'
 import protect from './overload-protection.js'
+import fastHead from './fast-head.js'
 
 const { DEPLOYMENT_ENV, NODE_ENV } = process.env
 const isDevelopment = NODE_ENV === 'development'
@@ -309,6 +310,10 @@ export default function (app) {
   // Check for a dropped connection before proceeding (again)
   app.use(haltOnDroppedConnection)
 
+  // Specifically deal with HEAD requests before doing the slower
+  // full page rendering.
+  app.head('/*', fastHead)
+
   // *** Preparation for render-page: contextualizers ***
   app.use(asyncMiddleware(instrument(releaseNotes, './contextualizers/release-notes')))
   app.use(instrument(graphQL, './contextualizers/graphql'))

tests/rendering/server.js

@@ -29,8 +29,13 @@ describe('server', () => {
   test('supports HEAD requests', async () => {
     const res = await head('/en')
     expect(res.statusCode).toBe(200)
-    expect(res.headers).not.toHaveProperty('content-length')
+    expect(res.headers['content-length']).toBe('0')
     expect(res.text).toBe('')
+    // Because the HEAD requests can't be different no matter what's
+    // in the request headers (Accept-Language or Cookies)
+    // it's safe to let it cache. The only key is the URL.
+    expect(res.headers['cache-control']).toContain('public')
+    expect(res.headers['cache-control']).toMatch(/max-age=\d+/)
   })
 
   test('renders the homepage', async () => {