Method call serial number

[howey]

libdbus provides a unique serial number to each method call in order for a client to, for example, distinguish multiple in-flight asynchronous method calls, see dbus_message_get_serial

Is there a way to get a similar feature using sdbus-cpp bindings? Possible implementation using sd_bus_message_get_seqnum:

uint64_t Message::getSeqnum() const
{   
  uint64_t seqnum;
  auto r =  sd_bus_message_get_seqnum((sd_bus_message*)msg_, &seqnum);
  SDBUS_THROW_ERROR_IF(r < 0, "Failed to get seqnum", -r);
  return seqnum;
}

[sangelovic]

@howey Would you submit a PR? I would merge it right away.

[howey]

Actually, the sequence number is not working the way I was expecting. sd_bus_message_get_seqnum returns -ENODATA no matter what.

  auto callback = [](sdbus::MethodReply reply, std::optional<sdbus::Error> error) {
    try {
      auto n = reply.getSeqnum();
      std::cerr << "reply seqnum " << n << std::endl;
    } catch (const sdbus::Error& e) {
      std::cout << "seqnum (3) error: " << e.what() << std::endl;
    }
  };

  auto method = proxy->createMethodCall(interfaceName, methodName);

  try {
    auto n = method.getSeqnum();
    std::cout << "method seqnum " << n << std::endl;
  } catch (const sdbus::Error& e) {
    std::cout << "seqnum (1) error: " << e.what() << std::endl;
  }

  proxy->callMethodAsync(method, callback);

  try {
    auto n = method.getSeqnum();
    std::cout << "method seqnum " << n << std::endl;
  } catch (const sdbus::Error& e) {
    std::cout << "seqnum (2) error: " << e.what() << std::endl;
  }

seqnum (1) error: [System.Error.ENODATA] Failed to get seqnum (No data available)
seqnum (2) error: [System.Error.ENODATA] Failed to get seqnum (No data available)
seqnum (3) error: [System.Error.ENODATA] Failed to get seqnum (No data available)

What I am trying to accomplish is to get some unique identifier at the time an asynchronous method call is sent which can then be compared at the response to distinguish calls of the same method.

I'm sure I could do this another way, maybe by creating my own ID on the client and passing that to the callback lambda. But that would get pretty heavy since I would need to create and maintain a new callback object every method call.

[sangelovic]

@howey I think what you are looking for is sd_bus_message_get_cookie() and sd_bus_message_get_reply_cookie(), respectively. Their man pages, as well as their implementation (e.g. here), suggest that this is the message serial number you may be looking for.

[howey]

Awesome, that is exactly what I was looking for!

Let me work on a pull request