use manual surrogate key for nextjs static assets (github#25096)

* use manual surrogate key for nextjs static assets

* refactor

middleware/index.js

@@ -13,10 +13,7 @@ import cookieParser from './cookie-parser.js'
 import csrf from './csrf.js'
 import handleCsrfErrors from './handle-csrf-errors.js'
 import compression from 'compression'
-import {
-  setDefaultFastlySurrogateKey,
-  setManualFastlySurrogateKeyIfChecksummed,
-} from './set-fastly-surrogate-key.js'
+import { setDefaultFastlySurrogateKey } from './set-fastly-surrogate-key.js'
 import setFastlyCacheHeaders from './set-fastly-cache-headers.js'
 import catchBadAcceptLanguage from './catch-bad-accept-language.js'
 import reqUtils from './req-utils.js'
@@ -63,6 +60,7 @@ import renderPage from './render-page.js'
 import assetPreprocessing from './asset-preprocessing.js'
 import archivedAssetRedirects from './archived-asset-redirects.js'
 import favicon from './favicon.js'
+import setStaticAssetCaching from './static-asset-caching.js'
 
 const { DEPLOYMENT_ENV, NODE_ENV } = process.env
 const isDevelopment = NODE_ENV === 'development'
@@ -115,14 +113,16 @@ export default function (app) {
 
   app.use(favicon)
 
-  // Any `/assets/cb-*` request should get the setManualFastlySurrogateKey()
-  // middleware, but it's not possible to express such a prefix in
-  // Express middlewares. Because we don't want the manual Fastly
-  // surrogate key on *all* /assets/ requests.
-  // Note, this needs to come before `assetPreprocessing` because
+  // Any static URL that contains some sort of checksum that makes it
+  // unique gets the "manual" surrogate key. If it's checksummed,
+  // it's bound to change when it needs to change. Otherwise,
+  // we want to make sure it doesn't need to be purged just because
+  // there's a production deploy.
+  // Note, for `/assets/cb-*...` requests,
+  // this needs to come before `assetPreprocessing` because
   // the `assetPreprocessing` middleware will rewrite `req.url` if
   // it applies.
-  app.use(setManualFastlySurrogateKeyIfChecksummed)
+  app.use(setStaticAssetCaching)
 
   // Must come before any other middleware for assets
   app.use(archivedAssetRedirects)

middleware/set-fastly-surrogate-key.js

@@ -23,19 +23,7 @@ export function setFastlySurrogateKey(res, enumKey) {
   res.set(KEY, enumKey)
 }
 
-export function setManualFastlySurrogateKey(req, res, next) {
-  res.set(KEY, SURROGATE_ENUMS.MANUAL)
-  return next()
-}
-
 export function setDefaultFastlySurrogateKey(req, res, next) {
   res.set(KEY, SURROGATE_ENUMS.DEFAULT)
   return next()
 }
-
-export function setManualFastlySurrogateKeyIfChecksummed(req, res, next) {
-  if (req.path.startsWith('/assets/cb-')) {
-    return setManualFastlySurrogateKey(req, res, next)
-  }
-  return next()
-}

middleware/static-asset-caching.js

@@ -0,0 +1,16 @@
+import { setFastlySurrogateKey, SURROGATE_ENUMS } from './set-fastly-surrogate-key.js'
+
+export default function setStaticAssetCaching(req, res, next) {
+  if (isChecksummed(req.path)) {
+    setFastlySurrogateKey(res, SURROGATE_ENUMS.MANUAL)
+  }
+  return next()
+}
+
+// True if the URL is known to contain some pattern of a checksum that
+// would make it intelligently different if its content has changed.
+function isChecksummed(path) {
+  if (path.startsWith('/assets/cb-')) return true
+  if (path.startsWith('/_next/static') && /[a-f0-9]{20}/.test(path)) return true
+  return false
+}