Set the ZMQ Receive Buffer High Water Mark only if it changes (#444)

This is to reduce the impact of a bug present in ZMQ 4.2.0 and ZMQ 4.2.1
which led to a bad lwm and hwm calculation when ZMQ_RCVHWM was set after
the bind of the socket. See cppTango#444 for more details

Author: bourtemb

cppapi/client/zmqeventconsumer.cpp

@@ -886,7 +886,17 @@ bool ZmqEventConsumer::process_ctrl(zmq::message_t &received_ctrl,zmq::pollitem_
 
             if (connect_pub == true)
             {
-                event_sub_sock->setsockopt(ZMQ_RCVHWM,&sub_hwm,sizeof(sub_hwm));
+                int current_sub_hwm = SUB_HWM;
+                size_t curr_sub_hw_size = sizeof(current_sub_hwm);
+                event_sub_sock->getsockopt(ZMQ_RCVHWM,&current_sub_hwm, &curr_sub_hw_size);
+                if(sub_hwm != current_sub_hwm)
+                {
+                    // Set the ZMQ Receive Buffer High Water Mark only if it changes
+                    // This is to reduce the impact of a bug present in ZMQ 4.2.0 and ZMQ 4.2.1
+                    // which leads to a bad lwm and hwm calculation when ZMQ_RCVHWM is set after
+                    // the bind of the socket. See cppTango#444 for more details
+                    event_sub_sock->setsockopt(ZMQ_RCVHWM, &sub_hwm, sizeof(sub_hwm));
+                }
 
                 event_sub_sock->connect(endpoint);
                 if (force_connect == 0)