What happens to a ZeroMQ multipart message when send fails before the final part? - c

Question

I'm intending to use multi-part messages in ZeroMQ, but I need to know what happens to the initially enqueued message parts when I get a send error before the last message part is sent.
For example, lets say I have a PUSH socket and I am sending a two part message because I am collecting the header and body from different sources. What happens if the header sends fine, but there is an error sending the body? Does the header remain in the socket's outbound queue until I attempt to send another message, or does the header get dropped due to the error in the body send?
Perhaps some code will make the question more precise:
int header[] = {1, 2, 3}; // <- Header From Source 1
int body[] = {4, 5, 6, 7, 8, 9}; // <- Body From Source 2
void *ctx = zmq_ctx_new();
void *push = zmq_socket(ctx, ZMQ_PUSH);
zmq_connect(push, "tcp://localhost:7890");
int headerSent = 0;
int bodySent = 0;
headerSent = zmq_send(push, &header[0], sizeof(header), ZMQ_SNDMORE);
if (-1 != headerSent)
{
bodySent = zmq_send(push, &body[0], sizeof(body), 0);
// What if ^-- this fails?
}
// Do some other processing to prepare a new header and body here...
headerSent = zmq_send(push, &header[0], sizeof(header), ZMQ_SNDMORE);
if (-1 != headerSent)
{
bodySent = zmq_send(push, &body[0], sizeof(body), 0);
}
Is it possible for me to have a message with two headers here, or does ZeroMQ discard the initial message parts that were submitted with the ZMQ_SNDMORE flag when a subsequent send fails due to (e.g.) interruption from a signal? I'm hoping that is the case because the documentation on zmq_send promises that all message parts will be delivered, or none of them will be delivered. However, I'm not sure how the library decides which message parts are from the same message. Does it consider two message parts to be from the same message if a send error occurs in between them?

Answer 1

ZeroMQ Guide:
When you send a multipart message, the first part (and all following parts) are only actually sent on the wire when you send
the final part.
You will receive all parts of a message, or none at all.
What you could always do is send single part messages and assemble them and manage failure on the receiving end
Recent update: Pieter Hintjens: The End of ZeroMQ Multipart

Answer 2

I don't know exactly what will happen, however logic dictates one of 3 outcomes:
The 2nd frame, which failed, will have successfully processed the 0 in the multipart parameter instead of ZMQ_SNDMORE. The first message will be dropped, and the 2nd message will start anew.
The 2nd frame, which failed, will not have successfully processed the 0 in the multipart parameter instead of ZMQ_SNDMORE. The first message will still be open, your 2nd "header" will be frame 2, your 2nd "body" will be frame 3, and the message will just send with 3 frames instead of 2, erroneously
ZMQ will have its pants around its ankles, and be in some error state which will respond in unexpected ways.
My expectation is that the 2nd possibility is what will happen, if the call fails that it should produce zero side effects so it will be as if it never happened. This is supported by the info on the man page talking about the several error types that you might encounter. In particular it looks like it should be a very rare/exceptional situation where the first part of a multipart message would be accepted by the 2nd would not.