Make ZMQ interoperate with ZMQ 2.x SUB sockets

Since ZMQ 2.x does not support subscription forwarding, it's not
possible to use ZMQ 2.x SUB socket to receive messages from a PUB
socket.

This patch adds some compatibility layer so that ZMQ 2.x SUB socket
receives messages from PUB socket.

src/stream_engine.cpp

@@ -401,6 +401,14 @@ bool zmq::stream_engine_t::handshake ()
         //  Make sure the decoder sees the data we have already received.
         inpos = greeting;
         insize = greeting_bytes_read;
+
+        //  To allow for interoperability with peers that do not forward
+        //  their subscriptions, we inject a phony subsription
+        //  message into the incomming message stream. To put this
+        //  message right after the identity message, we temporarily
+        //  divert the message stream from session to ourselves.
+        if (options.type == ZMQ_PUB || options.type == ZMQ_XPUB)
+            decoder.set_msg_sink (this);
     }
 
     // Start polling for output if necessary.
@@ -414,6 +422,30 @@ bool zmq::stream_engine_t::handshake ()
     return true;
 }
 
+int zmq::stream_engine_t::push_msg (msg_t *msg_)
+{
+    zmq_assert (options.type == ZMQ_PUB || options.type == ZMQ_XPUB);
+
+    //  The first message is identity.
+    //  Let the session process it.
+    int rc = session->push_msg (msg_);
+    errno_assert (rc == 0);
+
+    //  Inject the subscription message so that the ZMQ 2.x peer
+    //  receives our messages.
+    rc = msg_->init_size (1);
+    errno_assert (rc == 0);
+    *(unsigned char*) msg_->data () = 1;
+    rc = session->push_msg (msg_);
+    session->flush ();
+
+    //  Once we have injected the subscription message, we can
+    //  Divert the message flow back to the session.
+    decoder.set_msg_sink (session);
+
+    return rc;
+}
+
 void zmq::stream_engine_t::error ()
 {
     zmq_assert (session);

src/stream_engine.hpp

@@ -26,6 +26,7 @@
 
 #include "fd.hpp"
 #include "i_engine.hpp"
+#include "i_msg_sink.hpp"
 #include "io_object.hpp"
 #include "encoder.hpp"
 #include "decoder.hpp"
@@ -41,7 +42,7 @@ namespace zmq
     //  This engine handles any socket with SOCK_STREAM semantics,
     //  e.g. TCP socket or an UNIX domain socket.
 
-    class stream_engine_t : public io_object_t, public i_engine
+    class stream_engine_t : public io_object_t, public i_engine, public i_msg_sink
     {
     public:
 
@@ -55,6 +56,9 @@ namespace zmq
         void activate_in ();
         void activate_out ();
 
+        //  i_msg_sink interface implementation.
+        virtual int push_msg (msg_t *msg_);
+
         //  i_poll_events interface implementation.
         void in_event ();
         void out_event ();