cache asset images more aggressively (github#23553)

* cache asset images more aggressively

* more careful about which gets the manual surrogate key

* fix rendered-content-link-checker script too

* feedbacked

Author: peterbe

lib/render-content/create-processor.js

@@ -18,6 +18,7 @@ import HighlightjsGraphql from 'highlightjs-graphql'
 import remarkCodeExtra from 'remark-code-extra'
 import codeHeader from './plugins/code-header.js'
 import rewriteLocalLinks from './plugins/rewrite-local-links.js'
+import rewriteImgSources from './plugins/rewrite-asset-urls.js'
 import useEnglishHeadings from './plugins/use-english-headings.js'
 import rewriteLegacyAssetPaths from './plugins/rewrite-legacy-asset-paths.js'
 import wrapInElement from './plugins/wrap-in-element.js'
@@ -45,6 +46,7 @@ export default function createProcessor(context) {
     .use(raw)
     .use(rewriteLegacyAssetPaths, context)
     .use(wrapInElement, { selector: 'ol > li img', wrapper: 'span.procedural-image-wrapper' })
+    .use(rewriteImgSources)
     .use(rewriteLocalLinks, {
       languageCode: context.currentLanguage,
       version: context.currentVersion,

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)
+  }
+}

lib/rewrite-local-links.js

@@ -26,8 +26,8 @@ function getNewHref(link, languageCode, version) {
   const href = link.attr('href')
 
   // Exceptions to link rewriting
-  if (href.startsWith('/assets')) return
-  if (href.startsWith('/public')) return
+  if (href.startsWith('/assets/')) return
+  if (href.startsWith('/public/')) return
   if (Object.keys(externalRedirects).includes(href)) return
 
   let newHref = href

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()
+}

middleware/index.js

@@ -14,7 +14,10 @@ import csrf from './csrf.js'
 import handleCsrfErrors from './handle-csrf-errors.js'
 import compression from 'compression'
 import disableCachingOnSafari from './disable-caching-on-safari.js'
-import setDefaultFastlySurrogateKey from './set-fastly-surrogate-key.js'
+import {
+  setDefaultFastlySurrogateKey,
+  setManualFastlySurrogateKey,
+} 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'
@@ -57,6 +60,7 @@ import featuredLinks from './featured-links.js'
 import learningTrack from './learning-track.js'
 import next from './next.js'
 import renderPage from './render-page.js'
+import assetPreprocessing from './asset-preprocessing.js'
 
 const { NODE_ENV } = process.env
 const isDevelopment = NODE_ENV === 'development'
@@ -95,17 +99,25 @@ export default function (app) {
       instrument(archivedEnterpriseVersionsAssets, './archived-enterprise-versions-assets')
     )
   )
+  // This must come before the express.static('assets') middleware.
+  app.use(assetPreprocessing)
+  // By specifying '/assets/cb-' and not just '/assets/' we
+  // avoid possibly legacy enterprise assets URLs and asset image URLs
+  // that don't have the cache-busting piece in it.
+  app.use('/assets/cb-', setManualFastlySurrogateKey)
   app.use(
-    '/assets',
+    '/assets/',
     express.static('assets', {
       index: false,
       etag: false,
       lastModified: false,
-      maxAge: '1 day', // Relatively short in case we update images
+      // Can be aggressive because images inside the content get unique
+      // URLs with a cache busting prefix.
+      maxAge: '7 days',
     })
   )
   app.use(
-    '/public',
+    '/public/',
     express.static('data/graphql', {
       index: false,
       etag: false,

middleware/set-fastly-surrogate-key.js

@@ -23,7 +23,12 @@ export function setFastlySurrogateKey(res, enumKey) {
   res.set(KEY, enumKey)
 }
 
-export default function setDefaultFastlySurrogateKey(req, res, next) {
+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()
 }

script/rendered-content-link-checker.mjs

@@ -240,7 +240,13 @@ async function processPermalink(permalink, page, pageMap, redirects, opts) {
 
   if (checkImages) {
     $('img[src]').each((i, img) => {
-      const { src } = img.attribs
+      let { src } = img.attribs
+
+      // Images get a cache-busting prefix injected in the image
+      // E.g. <img src="/assets/cb-123456/foo/bar.png">
+      // We need to remove that otherwise we can't look up the image
+      // on disk.
+      src = src.replace(/\/cb-\d+\//, '/')
 
       if (globalImageSrcCheckCache.has(src)) {
         globalCacheHitCount++

tests/rendering/server.js

@@ -375,6 +375,7 @@ describe('server', () => {
   })
 
   describe('image asset paths', () => {
+    const localImageCacheBustBasePathRegex = /^\/assets\/cb-\d+\/images\//
     const localImageBasePath = '/assets/images'
     const legacyImageBasePath = '/assets/enterprise'
     const latestEnterprisePath = `/en/enterprise/${enterpriseServerReleases.latest}`
@@ -384,7 +385,10 @@ describe('server', () => {
       const $ = await getDOM(
         '/en/github/authenticating-to-github/configuring-two-factor-authentication'
       )
-      expect($('img').first().attr('src').startsWith(localImageBasePath)).toBe(true)
+      const imageSrc = $('img').first().attr('src')
+      expect(
+        localImageCacheBustBasePathRegex.test(imageSrc) || imageSrc.startsWith(localImageBasePath)
+      ).toBe(true)
     })
 
     test('github articles on GHE have images that point to local assets dir', async () => {
@@ -393,7 +397,9 @@ describe('server', () => {
       )
       const imageSrc = $('img').first().attr('src')
       expect(
-        imageSrc.startsWith(localImageBasePath) || imageSrc.startsWith(legacyImageBasePath)
+        localImageCacheBustBasePathRegex.test(imageSrc) ||
+          imageSrc.startsWith(localImageBasePath) ||
+          imageSrc.startsWith(legacyImageBasePath)
       ).toBe(true)
     })
 
@@ -403,7 +409,9 @@ describe('server', () => {
       )
       const imageSrc = $('img').first().attr('src')
       expect(
-        imageSrc.startsWith(localImageBasePath) || imageSrc.startsWith(legacyImageBasePath)
+        localImageCacheBustBasePathRegex.test(imageSrc) ||
+          imageSrc.startsWith(localImageBasePath) ||
+          imageSrc.startsWith(legacyImageBasePath)
       ).toBe(true)
     })
 
@@ -413,7 +421,9 @@ describe('server', () => {
       )
       const imageSrc = $('img').first().attr('src')
       expect(
-        imageSrc.startsWith(localImageBasePath) || imageSrc.startsWith(legacyImageBasePath)
+        localImageCacheBustBasePathRegex.test(imageSrc) ||
+          imageSrc.startsWith(localImageBasePath) ||
+          imageSrc.startsWith(legacyImageBasePath)
       ).toBe(true)
     })
 
@@ -428,14 +438,20 @@ describe('server', () => {
       const $ = await getDOM(
         '/en/enterprise-cloud@latest/billing/managing-billing-for-your-github-account/viewing-the-subscription-and-usage-for-your-enterprise-account'
       )
-      expect($('img').first().attr('src').startsWith(localImageBasePath)).toBe(true)
+      const imageSrc = $('img').first().attr('src')
+      expect(
+        localImageCacheBustBasePathRegex.test(imageSrc) || imageSrc.startsWith(localImageBasePath)
+      ).toBe(true)
     })
 
     test('admin articles on GHEC have images that point to local assets dir', async () => {
       const $ = await getDOM(
         '/en/enterprise-cloud@latest/admin/configuration/configuring-your-enterprise/verifying-or-approving-a-domain-for-your-enterprise'
       )
-      expect($('img').first().attr('src').startsWith(localImageBasePath)).toBe(true)
+      const imageSrc = $('img').first().attr('src')
+      expect(
+        localImageCacheBustBasePathRegex.test(imageSrc) || imageSrc.startsWith(localImageBasePath)
+      ).toBe(true)
     })
 
     test('github articles on GHAE have images that point to local assets dir', async () => {
@@ -444,13 +460,18 @@ describe('server', () => {
       )
       const imageSrc = $('img').first().attr('src')
       expect(
-        imageSrc.startsWith(localImageBasePath) || imageSrc.startsWith(legacyImageBasePath)
+        localImageCacheBustBasePathRegex.test(imageSrc) ||
+          imageSrc.startsWith(localImageBasePath) ||
+          imageSrc.startsWith(legacyImageBasePath)
       ).toBe(true)
     })
 
     test('admin articles on GHAE have images that point to local assets dir', async () => {
       const $ = await getDOM('/en/github-ae@latest/admin/user-management/managing-dormant-users')
-      expect($('img').first().attr('src').startsWith(localImageBasePath)).toBe(true)
+      const imageSrc = $('img').first().attr('src')
+      expect(
+        localImageCacheBustBasePathRegex.test(imageSrc) || imageSrc.startsWith(localImageBasePath)
+      ).toBe(true)
     })
   })
 
@@ -1003,6 +1024,16 @@ describe('static routes', () => {
     expect(res.headers['surrogate-key']).toBeTruthy()
   })
 
+  it('rewrites /assets requests from a cache-busting prefix', async () => {
+    // The rewrite-asset-urls.js Markdown plugin will do this to img tags.
+    const res = await get('/assets/cb-123456/images/site/be-social.gif')
+    expect(res.statusCode).toBe(200)
+    expect(res.headers['set-cookie']).toBeUndefined()
+    expect(res.headers['cache-control']).toContain('public')
+    expect(res.headers['cache-control']).toMatch(/max-age=\d+/)
+    expect(res.headers['surrogate-key']).toBeTruthy()
+  })
+
   it('serves schema files from the /data/graphql directory at /public', async () => {
     const res = await get('/public/schema.docs.graphql')
     expect(res.statusCode).toBe(200)