Assertion failed at tcp_listener.cpp:58

Imagin we have lost connection to other side. libzmq starts reconnect process.

First connection attempt (made by zmq_connecter_t::start_connecting) calls tcp_connecter_t::open. Call to ::connect was unsuccessfull (for example, we've got WSAECONNREFUSED - port is closed). In this case execution will pass to the end of tcp_connecter_t::open and we will have ECONNREFUSED in errno variable.

In zmq_connecter_t::start_connecting, where we will return from tcp_connecter_t::open, execution will also pass to the end of it and reconnect timer will be activated.

When time arrives zmq_connecter_t::timer_event will be called, that itself will call zmq_connecter_t::start_connecting from which tcp_connecter.open will be called. But this time we will have actual socked ID in s variable because it was not closed after previous connection attempt!

In 3.x branch there is no such error because at the end of start_connections (just before enabling reconnect timer) close() method is called. Same should be done in 2.x branch.