[WIP] Add caching plugin

lib/metalsmith-cache/cache.test.js

@@ -0,0 +1,31 @@
+const fs = require('fs')
+
+const Metalsmith = require('metalsmith')
+
+const plugin = require('./index.js')
+
+describe('metalsmith-cache plugin', () => {
+  const cacheDir = 'lib/metalsmith-cache/fixtures/.cache'
+  let callCount = 0
+  const dummyPlugin = (files, metalsmith, done) => {
+    callCount++
+    done()
+  }
+
+  it('runs plugin on first build and creates a cache marker', () => {
+    callCount = 0
+    fs.rmSync(cacheDir, { recursive: true, force: true })
+
+    Metalsmith('lib/metalsmith-cache/fixtures')
+      .use(plugin('initial-test', '**/*.md', dummyPlugin, { cacheDir }))
+      .build((err) => {
+        if (err) {
+          throw err
+        }
+        expect(callCount).toBe(1)
+
+        const cached = fs.readdirSync(cacheDir)
+        expect(cached).toHaveLength(1)
+      })
+  })
+})

lib/metalsmith-cache/fixtures/.gitignore

@@ -0,0 +1,2 @@
+build
+.cache

lib/metalsmith-cache/fixtures/src/index.md

@@ -0,0 +1 @@
+## Test file

lib/metalsmith-cache/index.js

@@ -0,0 +1,49 @@
+const { createHash } = require('crypto')
+const { existsSync, rmSync, mkdirSync, writeFileSync } = require('fs')
+const { join } = require('path')
+
+const { root } = require('../../config/paths.js')
+
+/**
+ *
+ * @param {string} name - The name for the cached process
+ * @param {string | string[]} pattern - Minimatch pattern
+ * @param {Function} plugin - A Metalsmith plugin to run with caching
+ * @param {object} [options] - Plugin options
+ * @param {boolean} [options.isProduction] - Whether the build is in production mode
+ * @param {string} [options.cacheDir] - Directory to store cache files, defaults to '.cache'
+ * @returns {Function} - The caching plugin function
+ */
+module.exports = function metalsmithCache(name, pattern, plugin, options = {}) {
+  const cacheDir = options.cacheDir || '.cache'
+
+  return function (files, metalsmith, done) {
+    // never cache in production
+    if (options.isProduction) {
+      return plugin(files, metalsmith, done)
+    }
+
+    const cachePath = join(root, cacheDir, name)
+
+    const hash = createHash('md5')
+      .update(
+        metalsmith
+          .match(pattern)
+          .sort()
+          .map((filename) => filename + files[filename].contents)
+          .join('')
+      )
+      .digest('hex')
+
+    const marker = join(cachePath, `metalsmith-cache-${name}-${hash}`)
+
+    if (!existsSync(marker)) {
+      rmSync(cachePath, { recursive: true, force: true })
+      mkdirSync(cachePath, { recursive: true })
+      writeFileSync(marker, '')
+
+      return plugin(files, metalsmith, done)
+    }
+    return done()
+  }
+}

lib/metalsmith.js

@@ -10,6 +10,7 @@ const sass = require('@metalsmith/sass') // convert Sass files to CSS using Dart
 const { glob } = require('glob') // Match files using glob patterns
 const Metalsmith = require('metalsmith') // static site generator
 const canonical = require('metalsmith-canonical') // add a canonical url property to pages
+const { default: tracer } = require('metalsmith-tracer') // debug logging
 
 // Helpers and config
 const { paths, navigation: menuItems } = require('../config')
@@ -26,14 +27,15 @@ const rebrandedNunjucksOptions = {
 // Local metalsmith plugins
 const { hashAssets } = require('./fingerprints') // rename files with hash fingerprints
 const generateSitemap = require('./generate-sitemap.js') // generate sitemap
+const cache = require('./metalsmith-cache') // cache plugin to skip expensive tasks
 const lunr = require('./metalsmith-lunr-index') // generate search index
 const renderMarkdown = require('./metalsmith-render-markdown')
 const titleChecker = require('./metalsmith-title-checker.js')
 const navigation = require('./navigation.js') // navigation plugin
 const rollup = require('./rollup') // used to build GOV.UK Frontend JavaScript
 
 // Static site generator
-const metalsmith = Metalsmith(resolve(__dirname, '../'))
+const metalsmith = tracer(Metalsmith(resolve(__dirname, '../')))
 
 // Flag production mode (to skip plugins in development)
 const isProduction = process.env.NODE_ENV !== 'development'
@@ -183,22 +185,28 @@ module.exports = metalsmith
     done()
   })
 
-  // render legacy templating syntax in source files
   .use(
-    inPlace({
-      pattern: ['**/*.{md,njk}', '!**/branded.njk'],
-      transform: 'jstransformer-nunjucks',
-      engineOptions: nunjucksOptions
-    })
+    cache(
+      'rebrand',
+      ['**/branded.njk'],
+      inPlace({
+        pattern: '**/branded.njk',
+        transform: 'jstransformer-nunjucks',
+        engineOptions: rebrandedNunjucksOptions
+      })
+    )
   )
 
-  // render rebranded templating syntax in source files
   .use(
-    inPlace({
-      pattern: '**/branded.njk',
-      transform: 'jstransformer-nunjucks',
-      engineOptions: rebrandedNunjucksOptions
-    })
+    cache(
+      'nunjucks',
+      ['**/*.{md,njk}', '!**/branded.njk'],
+      inPlace({
+        pattern: ['**/*.{md,njk}', '!**/branded.njk'],
+        transform: 'jstransformer-nunjucks',
+        engineOptions: nunjucksOptions
+      })
+    )
   )
 
   // render markdown in source files and extract page headings
@@ -241,24 +249,32 @@ module.exports = metalsmith
 
   // apply layouts to source files
   .use(
-    layouts({
-      default: 'layout.njk',
-      directory: join(paths.views, 'layouts'),
-      pattern: ['**/*.html', '!**/branded/**/*.html'],
-      engineOptions: nunjucksOptions,
-      transform: 'nunjucks'
-    })
+    cache(
+      'layouts',
+      ['**/*.html', '!**/branded/**/*.html'],
+      layouts({
+        default: 'layout.njk',
+        directory: join(paths.views, 'layouts'),
+        pattern: ['**/*.html', '!**/branded/**/*.html'],
+        engineOptions: nunjucksOptions,
+        transform: 'nunjucks'
+      })
+    )
   )
 
   // apply layouts to source files
   .use(
-    layouts({
-      default: 'layout.njk',
-      directory: join(paths.views, 'layouts'),
-      pattern: '**/branded/**/*.html',
-      engineOptions: rebrandedNunjucksOptions,
-      transform: 'nunjucks'
-    })
+    cache(
+      'rebranded-layouts',
+      ['**/branded/**/*.html'],
+      layouts({
+        default: 'layout.njk',
+        directory: join(paths.views, 'layouts'),
+        pattern: '**/branded/**/*.html',
+        engineOptions: rebrandedNunjucksOptions,
+        transform: 'nunjucks'
+      })
+    )
   )
 
   // generate a sitemap.xml in public/ folder