Method IsEmptyDirectory

cpp/src/arrow/filesystem/s3fs.cc:2423–2464

If this method is called after HEAD on "bucket/key" already returned a 404, can pass the given outcome to spare a spurious HEAD call.

  // If this method is called after HEAD on "bucket/key" already returned a 404,
  // can pass the given outcome to spare a spurious HEAD call.
  Result<bool> IsEmptyDirectory(
      const std::string& bucket, const std::string& key,
      const S3Model::HeadObjectOutcome* previous_outcome = nullptr) {
    ARROW_ASSIGN_OR_RAISE(auto client_lock, holder_->Lock());

    if (previous_outcome) {
      // Fetch the backend from the previous error
      if (GetOrSetBackend(previous_outcome->GetError()) != S3Backend::Minio) {
        // HEAD already returned a 404, nothing more to do
        return false;
      }
    }

    // We come here in one of two situations:
    // - we don't know the backend and there is no previous outcome
    // - the backend is Minio
    S3Model::HeadObjectRequest req;
    req.SetBucket(ToAwsString(bucket));
    if (backend_ && *backend_ == S3Backend::Minio) {
      // Minio wants a slash at the end, Amazon doesn't
      req.SetKey(ToAwsString(key) + kSep);
    } else {
      req.SetKey(ToAwsString(key));
    }

    auto outcome = client_lock.Move()->HeadObject(req);
    if (outcome.IsSuccess()) {
      return true;
    }
    if (!backend_) {
      if (GetOrSetBackend(outcome.GetError()) == S3Backend::Minio) {
        // Try again with separator-terminated key (see above)
        return IsEmptyDirectory(bucket, key);
      }
    }
    if (IsNotFound(outcome.GetError())) {
      return false;
    }
    return ErrorToStatus(std::forward_as_tuple("When reading information for key '", key,
                                               "' in bucket '", bucket, "': "),
                         "HeadObject", outcome.GetError());
  }

  Result<bool> IsEmptyDirectory(
      const S3Path& path, const S3Model::HeadObjectOutcome* previous_outcome = nullptr) {