Method ReadAt

cpp/src/arrow/io/concurrency.h:210–213

  // to use the exclusive_guard.

  Result<int64_t> ReadAt(int64_t position, int64_t nbytes, void* out) final {
    auto guard = lock_.shared_guard();
    return derived()->DoReadAt(position, nbytes, out);
  }

  Result<std::shared_ptr<Buffer>> ReadAt(int64_t position, int64_t nbytes) final {
    auto guard = lock_.shared_guard();