Problem: HWM in ZMQ_DGRAM socket does not respect multipart message #3268
Description
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Issue description
When sending a lot of messages over a ZMQ_DGRAM socket, I hit an assertion in: Assertion failed: check () (/home/gabm/zeromq/src/msg.cpp:347).
Environment
- libzmq version (commit hash if unreleased):
- OS:
Minimal test code / Steps to reproduce the issue
auto ctx = zmq_ctx_new();
auto sender = zmq_socket(ctx, ZMQ_DGRAM);
int hwm = 3;
zmq_setsockopt(sender, ZMQ_SNDHWM, &hwm, sizeof(hwm));
zmq_bind(sender, "udp://127.0.0.1:5556");
std::string remote = "127.0.0.1:5555";
for (int i = 0; i < 100000; i++) {
zmq_send(sender, remote.c_str(), remote.length(), ZMQ_SNDMORE);
zmq_msg_t msg;
zmq_msg_init_size(&msg, 8000);
zmq_msg_send(&msg, sender, 0);
zmq_msg_close(&msg);
}
zmq_close(sender);
zmq_ctx_destroy(ctx);
What's the actual result? (include assertion message & call stack if applicable)
The callstack is:
1 raise 0x7ffff7705d7f
2 abort 0x7ffff76f0672
3 zmq::zmq_abort err.cpp 88 0x7ffff7d2170e
4 zmq::msg_t::size msg.cpp 347 0x7ffff7d2ce6e
5 zmq::udp_engine_t::out_event udp_engine.cpp 380 0x7ffff7d80db6
6 zmq::epoll_t::loop epoll.cpp 202 0x7ffff7d2085c
7 zmq::worker_poller_base_t::worker_routine poller_base.cpp 139 0x7ffff7d3d50d
8 thread_routine thread.cpp 182 0x7ffff7d6736e
9 start_thread 0x7ffff7bd3a9d
10 clone 0x7ffff77c9a43
So it happens in udp_engine.cpp at line 380. The rc value of the second pull_msg call is -1.
What's the expected result?
that it works 😄
Cause
Digging a bit deeper I found out that it happens when hitting the HWM. I started my program a very small HWM and it happens immediatly. The DGRAM socket interface requires two messages to be sent: the destination address and the body message. I think that one of these two gets discarded when hitting the HWM, the other survives. Then when processing the messages in the udp_engine, we cannot assume to get "both messages", because one might be dropped due to the HWM.