absl::MutexLock lock(&mu_);
  auto entry = block_map_.find(key);
  if (entry != block_map_.end()) {
    if (BlockNotStale(entry->second)) {
      return entry->second;
    } else {
      // Remove the stale block and continue.
      RemoveFile_Locked(key.first);
    }
  }

  // Insert a new empty block, setting the bookkeeping to sentinel values
  // in order to update them as appropriate.