const std::string& filename, int64_t file_signature) {
  absl::MutexLock lock(&mu_);
  auto it = file_signature_map_.find(filename);
  if (it != file_signature_map_.end()) {
    if (it->second == file_signature) {
      return true;
    }
    // Remove the file from cache if the signatures don't match.
    RemoveFile_Locked(filename);
    it->second = file_signature;
    return false;
  }