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

lib/render-content/plugins/rewrite-asset-urls.js

@@ -0,0 +1,52 @@
+import fs from 'fs'
+
+import { visit } from 'unist-util-visit'
+
+// Matches any <img> tags with an href that starts with `/assets/` or '/public/'
+const matcher = (node) =>
+  node.type === 'element' &&
+  node.tagName === 'img' &&
+  node.properties &&
+  node.properties.src &&
+  (node.properties.src.startsWith('/assets/') || node.properties.src.startsWith('/public/'))
+
+// Content authors write images like `![Alt](/assets/images/foo.png`, but
+// for caching purposes we want to rewrite those so they can be cached
+// indefinitely.
+export default function rewriteImgSources() {
+  return (tree) => {
+    visit(tree, matcher, (node) => {
+      const newSrc = getNewSrc(node)
+      if (newSrc) {
+        node.properties.src = newSrc
+      }
+    })
+  }
+}
+
+function getNewSrc(node) {
+  const { src } = node.properties
+  if (!src.startsWith('/')) return
+
+  try {
+    const filePath = src.slice(1)
+    const stats = fs.statSync(filePath)
+    // The size is not perfect but it's good enough. The size is
+    // very fast to pick up without needing to do a deep hashing of the
+    // image's content. It's perfectly possible that someone edits an
+    // image and it's size doesn't change. Although very unlikely.
+    // The slightest change to the image is more likely to either increase
+    // or decrease the image size by at least 1 byte.
+    // Also, because of this limitation, we're not confident to cache the
+    // image more than say 24h. But in the unlikely event that someone does
+    // edit an image and the size doesn't change, there's always the
+    // escape hatch that you can soft-purge all manual CDN surrogate keys.
+    if (!stats.size) return
+    const hash = `${stats.size}`
+    const split = src.split('/')
+    split.splice(2, 0, `cb-${hash}`)
+    return split.join('/')
+  } catch (err) {
+    console.warn(`Error trying to get a hash for ${src}`, err)
+  }
+}