PGM transport reconciled with subscription forwarding

As PGM is not capable of passing subscriptions upstream,
subscriptions are ignored at sub side and engine subscribes
for all messages on pub side.
Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

src/pgm_receiver.cpp

@@ -68,6 +68,9 @@ void zmq::pgm_receiver_t::plug (io_thread_t *io_thread_, i_engine_sink *sink_)
     set_pollin (socket_handle);
 
     sink = sink_;
+
+    //  If there are any subscriptions already queued in the session, drop them.
+    drop_subscriptions ();
 }
 
 void zmq::pgm_receiver_t::unplug ()
@@ -101,7 +104,7 @@ void zmq::pgm_receiver_t::terminate ()
 
 void zmq::pgm_receiver_t::activate_out ()
 {
-    zmq_assert (false);
+    drop_subscriptions ();
 }
 
 void zmq::pgm_receiver_t::activate_in ()
@@ -255,5 +258,12 @@ void zmq::pgm_receiver_t::timer_event (int token)
     in_event ();
 }
 
+void zmq::pgm_receiver_t::drop_subscriptions ()
+{
+    msg_t msg;
+    while (sink->read (&msg))
+        msg.close ();
+}
+
 #endif
 

src/pgm_receiver.hpp

@@ -64,6 +64,10 @@ namespace zmq
 
     private:
 
+        //  PGM is not able to move subscriptions upstream. Thus, drop all
+        //  the pending subscriptions.
+        void drop_subscriptions ();
+
         //  RX timeout timer ID.
         enum {rx_timer_id = 0xa1};
 

src/pgm_sender.cpp

@@ -88,6 +88,15 @@ void zmq::pgm_sender_t::plug (io_thread_t *io_thread_, i_engine_sink *sink_)
 
     //  Set POLLOUT for downlink_socket_handle.
     set_pollout (handle);
+
+    //  PGM is not able to pass subscriptions upstream, thus we have no idea
+    //  what messages are peers interested in. Because of that we have to
+    //  subscribe for all the messages.
+    msg_t msg;
+    msg.init ();
+    bool ok = sink_->write (&msg);
+    zmq_assert (ok);
+    sink_->flush ();
 }
 
 void zmq::pgm_sender_t::unplug ()