cache asset images more aggressively (#23553)

* cache asset images more aggressively

* more careful about which gets the manual surrogate key

* fix rendered-content-link-checker script too

* feedbacked

middleware/asset-preprocessing.js

@@ -0,0 +1,26 @@
+// This middleware rewrites the URL of requests that contain the
+// portion of `/cb-\d+/`.
+// "cb" stands for "cache bust".
+// There's a Markdown plugin that rewrites all <img src> values
+// from `<img src="/assets/foo/bar.png">` to
+// `<img src="/assets/cb-123467/foo/bar.png">` for example.
+// We're doing this so that we can set a much more aggressive
+// Cache-Control for assets and a CDN surrogate-key that doesn't
+// soft-purge on every deployment.
+
+const regex = /\/cb-\d+\//
+
+export default function assetPreprocessing(req, res, next) {
+  if (req.path.startsWith('/assets/')) {
+    // We're only confident enough to set the *manual* surrogate key if the
+    // asset contains the cache-busting piece.
+    if (regex.test(req.url)) {
+      // We remove it so that when `express.static()` runs, it can
+      // find the file on disk by its original name.
+      req.url = req.url.replace(regex, '/')
+      // The Cache-Control is managed by the configuration
+      // for express.static() later in the middleware.
+    }
+  }
+  return next()
+}