通过连续两次调用boost::asio::read来检索正确的数据

我目前正在与Boost Asio实现网络协议。域类已经存在，我可以

• 将数据包写入std::istream
• 从std::ostream读取数据包。

一个网络包包含一个网络包头。报头以Packet Length字段开头，大小为两个字节(std::uint16_t)。

我使用TCP/IPv4作为传输层，因此我尝试实现以下内容:

1. 读取报文长度，了解报文的总长度。这意味着从套接字中读取两个字节。
2. 读取报文的其余部分。这意味着从套接字读取kActualPacketLength - sizeof(PacketLengthFieldType)字节。
3. 连接两个读二进制数据。

因此，我需要至少两次调用boost::asio::read(我正在同步启动!)。

如果我硬编码预期的长度，我可以通过一次调用boost::asio::read来读取数据包:

Packet const ReadPacketFromSocket() {
    boost::asio::streambuf stream_buffer;    
    boost::asio::streambuf::mutable_buffers_type buffer{
        stream_buffer.prepare(Packet::KRecommendedMaximumSize)};
    std::size_t const kBytesTransferred{boost::asio::read(
        this->socket_,
        buffer,
        // TODO: Remove hard-coded value.
        boost::asio::transfer_exactly(21))};
    stream_buffer.commit(kBytesTransferred);
    std::istream input_stream(&stream_buffer);
    PacketReader const kPacketReader{MessageReader::GetInstance()};
    return kPacketReader.Read(input_stream);
  }

立即读取完整的数据包数据并返回Packet实例。这是有效的，所以这个概念是有效的。

到目前为止一切顺利。现在我的问题是:

如果我用同一个boost::asio::streambuf连续两次呼叫boost::asio::read，我不能让它工作。

代码如下:

Packet const ReadPacketFromSocket() {
  std::uint16_t constexpr kPacketLengthFieldSize{2};
  boost::asio::streambuf stream_buffer;    
  boost::asio::streambuf::mutable_buffers_type buffer{
      stream_buffer.prepare(Packet::KRecommendedMaximumSize)};
  std::size_t const kBytesTransferred{boost::asio::read(
      // The stream from which the data is to be read.
      this->socket_,
      // One or more buffers into which the data will be read.
      buffer,
      // The function object to be called to determine whether the read
      // operation is complete.
      boost::asio::transfer_exactly(kPacketLengthFieldSize))};
  // The received data is "committed" (moved) from the output sequence to the
  // input sequence.
  stream_buffer.commit(kBytesTransferred);
  BOOST_LOG_TRIVIAL(debug) << "bytes transferred: " << kBytesTransferred;
  BOOST_LOG_TRIVIAL(debug) << "size of stream_buffer: " << stream_buffer.size();
  std::uint16_t packet_size;
  // This does seem to modify the streambuf!
  std::istream istream(&stream_buffer);
  istream.read(reinterpret_cast<char *>(&packet_size), sizeof(packet_size));
  BOOST_LOG_TRIVIAL(debug) << "size of stream_buffer: " << stream_buffer.size();
  BOOST_LOG_TRIVIAL(debug) << "data of stream_buffer: " << std::to_string(packet_size);
  std::size_t const kBytesTransferred2{
      boost::asio::read(
          this->socket_,
          buffer,
          boost::asio::transfer_exactly(packet_size - kPacketLengthFieldSize))};
  stream_buffer.commit(kBytesTransferred2);
  BOOST_LOG_TRIVIAL(debug) << "bytes transferred: " << kBytesTransferred2;
  BOOST_LOG_TRIVIAL(debug) << "size of stream_buffer: " << stream_buffer.size();
  // Create an input stream with the data from the stream buffer.
  std::istream input_stream(&stream_buffer);
  PacketReader const kPacketReader{MessageReader::GetInstance()};
  return kPacketReader.Read(input_stream);
}

我有以下问题:

1. 在第一次套接字读取之后从boost::asio::streambuf读取数据包长度似乎从boost::asio::streambuf删除了数据。
2. 如果我使用两个不同的boost::asio::streambuf实例，我不知道如何"连接"/"追加"它们。

在一天结束时，我需要一个std::istream与从套接字获得的正确数据。

谁能给我指个正确的方向?我已经试了好几个小时了……

也许这种方法不是最好的，所以我愿意接受建议来改进我的设计。

谢谢!

Packet const ReadPacketFromSocket() {
  ByteVector const kPacketLengthData{this->ReadPacketLengthFromSocket()};
  PacketHeader::PacketLengthType kPacketLength{
      static_cast<PacketHeader::PacketLengthType>(
          (kPacketLengthData[1] << 8) | kPacketLengthData[0])};
  ByteVector rest_packet_data(Packet::KRecommendedMaximumSize);
  boost::asio::read(
      this->socket_,
      boost::asio::buffer(rest_packet_data),
      boost::asio::transfer_exactly(
          kPacketLength - sizeof(PacketHeader::PacketLengthType)));
  ByteVector data{
      VectorUtils::GetInstance().Concatenate(
          kPacketLengthData,
          rest_packet_data)};
  // Create an input stream from the vector.
  std::stringstream input_stream;
  input_stream.rdbuf()->pubsetbuf(
      reinterpret_cast<char *>(&data[0]), data.size());
  PacketReader const kPacketReader{MessageReader::GetInstance()};
  return kPacketReader.Read(input_stream);
}
ByteVector ReadPacketLengthFromSocket() {
  ByteVector data_holder(sizeof(PacketHeader::PacketLengthType));
  boost::asio::read(
      this->socket_,
      boost::asio::buffer(data_holder),
      boost::asio::transfer_exactly(sizeof(PacketHeader::PacketLengthType)));
  return data_holder;
}