fixes #1565: Allow session to flush commands on an engine error

src/session_base.cpp

@@ -428,6 +428,9 @@ void zmq::session_base_t::engine_error (
 
     if (zap_pipe)
         zap_pipe->check_read ();
+
+    zmq_assert(socket);
+    socket->flush_commands();
 }
 
 void zmq::session_base_t::process_term (int linger_)

src/socket_base.cpp

@@ -1417,6 +1417,13 @@ void zmq::socket_base_t::check_destroy ()
     }
 }
 
+void zmq::socket_base_t::flush_commands ()
+{
+    ENTER_MUTEX();
+    process_commands (0, false);
+    EXIT_MUTEX();
+}
+
 void zmq::socket_base_t::read_activated (pipe_t *pipe_)
 {
     xread_activated (pipe_);

src/socket_base.hpp

@@ -109,6 +109,8 @@ namespace zmq
         void out_event ();
         void timer_event (int id_);
 
+        void flush_commands();
+
         //  i_pipe_events interface implementation.
         void read_activated (pipe_t *pipe_);
         void write_activated (pipe_t *pipe_);