build: caching i18n extractor (#6619)

* fix: do not attempt to cache i18n:extract

* fix: i18n extraction

* docs: improve comments

.github/actions/setup/action.yml

@@ -53,6 +53,14 @@ runs:
       shell: bash
 
     # Messages are extracted from source.
+    # A record of source file content hashes and catalogs is maintained in node_modules/.cache/lingui.
+    # Messages are always extracted, but extraction may short-circuit from the custom extractor's cache.
+    - uses: actions/cache@v3
+      id: i18n-extract-cache
+      with:
+        path: node_modules/.cache
+        key: ${{ runner.os }}-i18n-extract-${{ github.run_id }}
+        restore-keys: ${{ runner.os }}-i18n-extract-
     - run: yarn i18n:extract
       shell: bash
 

lingui.config.ts

+/* eslint-env node */
+import { default as babelExtractor } from '@lingui/cli/api/extractors/babel'
+import { createHash } from 'crypto'
+import { mkdirSync, readFileSync, writeFileSync } from 'fs'
+import { existsSync } from 'fs'
+import * as path from 'path'
+
+/** A custom caching extractor built on top of babelExtractor. */
+const cachingExtractor: typeof babelExtractor = {
+  /** Delegates to babelExtractor.match. */
+  match(filename: string) {
+    return babelExtractor.match(filename)
+  },
+  /**
+   * Checks a cache before extraction, only delegating to babelExtractor.extract if the file has changed.
+   *
+   * The lingui extractor works by extracting JSON (the catalog) from `filename` to `buildDir/filename.json`.
+   * Caching works by man-in-the-middling this:
+   * - File freshness is computed as a hash of `filename` contents.
+   * - Before extracting, we check the cache to see if we already have a fresh catalog for the file.
+   *   If we do, we copy it to `localeDir/filename.json`. Copying is significantly faster than extracting.
+   * - After extracting, we copy the catalog to the cache.
+   */
+  extract(filename: string, localeDir: string, ...options: unknown[]) {
+    // This runs from node_modules/@lingui/conf, so we need to back out to the root.
+    const root = __dirname.split('/node_modules')[0]
+
+    // This logic mimics catalogFilename in @lingui/babel-plugin-extract-messages.
+    const buildDir = path.join(localeDir, '_build')
+    const localePath = path.join(buildDir, filename + '.json')
+
+    const filePath = path.join(root, filename)
+    const fileHash = createHash('sha256').update(readFileSync(filePath)).digest('hex')
+
+    const cacheRoot = path.join(root, 'node_modules/.cache/lingui')
+    const cachePath = path.join(cacheRoot, filename + '.json')
+
+    // If we have a matching cached copy of the catalog, we can copy it to localePath and return early.
+    if (existsSync(cachePath)) {
+      const { hash, catalog } = JSON.parse(readFileSync(cachePath, 'utf8'))
+      if (hash === fileHash) {
+        if (catalog) {
+          mkdirSync(path.dirname(localePath), { recursive: true })
+          writeFileSync(localePath, JSON.stringify(catalog, null, 2))
+        }
+        return
+      }
+    }
+
+    babelExtractor.extract(filename, localeDir, ...options)
+
+    // Cache the extracted catalog.
+    mkdirSync(path.dirname(cachePath), { recursive: true })
+    if (existsSync(localePath)) {
+      const catalog = JSON.parse(readFileSync(localePath, 'utf8'))
+      writeFileSync(cachePath, JSON.stringify({ hash: fileHash, catalog }))
+    } else {
+      writeFileSync(cachePath, JSON.stringify({ hash: fileHash }))
+    }
+  },
+}
+
 const linguiConfig = {
   catalogs: [
     {
@@ -60,6 +122,7 @@ const linguiConfig = {
   runtimeConfigModule: ['@lingui/core', 'i18n'],
   sourceLocale: 'en-US',
   pseudoLocale: 'pseudo',
+  extractors: [cachingExtractor],
 }
 
 export default linguiConfig