split cache into subdirs

src/FSCache.js

@@ -20,14 +20,14 @@ class FSCache {
     );
   }
 
-  async ensureDirExists() {
-    await fs.mkdirp(this.dir);
+  async ensureDirExists(dir = this.dir) {
+    await fs.mkdirp(dir);
     this.dirExists = true;
   }
 
   getCacheFile(filename) {
     let hash = md5(this.optionsHash + filename);
-    return path.join(this.dir, hash + '.json');
+    return path.join(this.dir, hash.substring(0, 2), hash + '.json');
   }
 
   async writeDepMtimes(data) {
@@ -42,9 +42,10 @@ class FSCache {
 
   async write(filename, data) {
     try {
-      await this.ensureDirExists();
+      let cacheFile = this.getCacheFile(filename);
+      await this.ensureDirExists(path.dirname(cacheFile));
       await this.writeDepMtimes(data);
-      await fs.writeFile(this.getCacheFile(filename), JSON.stringify(data));
+      await fs.writeFile(cacheFile, JSON.stringify(data));
       this.invalidated.delete(filename);
     } catch (err) {
       logger.error('Error writing to cache', err);
@@ -83,7 +84,7 @@ class FSCache {
 
       let json = await fs.readFile(cacheFile);
       let data = JSON.parse(json);
-      if (!await this.checkDepMtimes(data)) {
+      if (!(await this.checkDepMtimes(data))) {
         return null;
       }