code: func (client *clientImpl) getInstanceCache(ctx context.Context) *internal.InstanceCache {
  	client.instanceCacheInit.Do(func() {
  		if client.CacheDir == "" {
  			return
  		}
  		path := filepath.Join(client.CacheDir, "instances")
  		client.instanceCache = internal.NewInstanceCache(fs.NewFileSystem(path, ""))
  		logging.Infof(ctx, "cipd: using instance cache at %q", path)
  	})
  	return client.instanceCache
  }
docs: // getInstanceCache lazy-initializes instanceCache and returns it.
  //
  // May return nil if instance cache is disabled.