Function ReadMessageInternal

cpp/src/arrow/ipc/message.cc:455–472

Common helper for the two ReadMessage overloads that take a file + offset. When body_length is provided, metadata and body are read in a single IO. When body_length is absent, metadata is read first, then the body is read separately.

// When body_length is absent, metadata is read first, then the body is read
// separately.
static Result<std::unique_ptr<Message>> ReadMessageInternal(
    int64_t offset, int32_t metadata_length, std::optional<int64_t> body_length,
    io::RandomAccessFile* file, const FieldsLoaderFunction& fields_loader) {
  ReadMessageState state;

  if (metadata_length < state.decoder->next_required_size()) {
    return Status::Invalid("metadata_length should be at least ",
                           state.decoder->next_required_size());
  }
  // When body_length is known, read metadata + body in one IO call.
  // Otherwise, read only metadata first.
  ARROW_ASSIGN_OR_RAISE(std::shared_ptr<Buffer> metadata,
                        file->ReadAt(offset, metadata_length + body_length.value_or(0),
                                     /*allow_short_read=*/false));

  return ReadMessageContinued(offset, metadata_length, body_length, metadata, file,
                              fields_loader, &state);
}

Result<std::unique_ptr<Message>> ReadMessage(int64_t offset, int32_t metadata_length,
                                             io::RandomAccessFile* file,