Use os.tmpDir for file system cache

packages/core/lib/FileSystemCache.js

@@ -17,20 +17,34 @@
 const fs = require('fs').promises;
 const {existsSync, mkdirSync} = require('fs');
 const crypto = require('crypto');
+const os = require('os');
 const log = require('./Log');
 const LRUCache = require('lru-cache');
 
 const path = require('path');
 
 const DEFAULT_OPTS = {
-  baseDir: path.join(__dirname, '.cache'),
   log,
   maxItems: 50,
 };
 
 class FileSystemCache {
-  static create(opts = {}) {
-    return new FileSystemCache(Object.assign(DEFAULT_OPTS, opts));
+  static create(customOpts = {}) {
+    const opts = Object.assign(DEFAULT_OPTS, customOpts);
+    // Try to create a tmp directory...
+    try {
+      if (!opts.baseDir) {
+        opts.baseDir = path.join(os.tmpdir(), 'ampproject-toolbox-optimizer');
+      }
+      if (!existsSync(opts.baseDir)) {
+        mkdirSync(opts.baseDir);
+      }
+    } catch (e) {
+      log.debug('No filesystem access, falling back to in-memory cache only', e);
+      // ... if this fails, we have no write access to the fs, use a simple memory cache instead
+      return new LRUCache(opts.maxItems);
+    }
+    return new FileSystemCache(opts);
   }
 
   constructor(opts) {
@@ -57,9 +71,6 @@ class FileSystemCache {
   async set(key, value) {
     try {
       this.cache.set(key, value);
-      if (!existsSync(this.opts.baseDir)) {
-        mkdirSync(this.opts.baseDir);
-      }
       const cacheFile = this.createCacheFileName(key);
       return fs.writeFile(cacheFile, JSON.stringify(value, null, ''), 'utf-8');
     } catch (e) {

packages/core/spec/lib/FileSystemCacheSpec.js

@@ -13,7 +13,6 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-
 const FileSystemCache = require('../../lib/FileSystemCache.js');
 
 let cache;
@@ -22,7 +21,6 @@ beforeEach(async () => {
   cache = FileSystemCache.create();
   await cache.clear();
 });
-//afterEach(async () => await cache.clear());
 
 test('returns null by default', async () => {
   const result = await cache.get('key');
@@ -39,3 +37,12 @@ test('returns cached value', async () => {
   const result = await cache.get('key');
   expect(result).toBe('value');
 });
+
+test('falls back to in memory cache', async () => {
+  cache = FileSystemCache.create({
+    baseDir: {}, // invalid pathname to trigger exception
+  });
+  await cache.set('key', 'value');
+  const result = await cache.get('key');
+  expect(result).toBe('value');
+});