code: func (cf *CachingFetcher) SetCacheHitHook(fn func(br blob.Ref, rc io.ReadCloser) (io.ReadCloser, error)) {
  	cf.cacheHitHook = fn
  }
docs: // SetCacheHitHook sets a function that will modify the return values from Fetch
  // in the case of a cache hit.
  // Its purpose is to add potential side-effects from calling the Fetcher that would
  // have happened if we had had a cache miss. It is the responsibility of fn to
  // return a ReadCloser equivalent to the state that rc was given in.