REQ socket drop message silently if two message coming together very fast

[ruleil]

This is a reproducible message silently dropped issue with a very small changes to the lbbroker.cpp.

https://github.com/booksbyus/zguide/blob/master/examples/C%2B%2B/lbbroker.cpp

In the lbbroker.cpp file, a worker will be pop out from work_queue when handling a request from client thread. If we modify the line 163 from "std::string worker_addr = worker_queue.front();" to "static std::string worker_addr = worker_queue.front();", as the following picture shows. All request will be forwarded to one worker, then we can see some messages will be dropped by the REQ socket the worker. Is it normal? REQ socket cannot receive other message when the worker logic is running ?