Addresses zeromq/libzmq#1116.

This patch  adds a new member of type deque to the
xpub class that contains pointers to metadata_t.
This deque is then used (alongside pending_data
and pending_flags) to preserve metadata when
copying messages.

src/xpub.cpp

@@ -92,6 +92,7 @@ void zmq::xpub_t::xread_activated (pipe_t *pipe_)
 			{
 				last_pipe = pipe_;
 				pending_data.push_back(blob_t(data, size));
+                pending_metadata.push_back(sub.metadata());
 				pending_flags.push_back(0);				
 			}
 			else
@@ -108,6 +109,7 @@ void zmq::xpub_t::xread_activated (pipe_t *pipe_)
 				if (options.type == ZMQ_XPUB && (unique || (*data == 1 && verbose_subs) ||
 						(*data == 0 && verbose_unsubs && verbose_subs))) {
 					pending_data.push_back(blob_t(data, size));
+                    pending_metadata.push_back(sub.metadata());
 					pending_flags.push_back(0);
 				}
 			}
@@ -115,6 +117,7 @@ void zmq::xpub_t::xread_activated (pipe_t *pipe_)
         else {
             //  Process user message coming upstream from xsub socket
             pending_data.push_back (blob_t (data, size));
+            pending_metadata.push_back (sub.metadata ());
             pending_flags.push_back (sub.flags ());
         }
         sub.close ();
@@ -241,8 +244,10 @@ int zmq::xpub_t::xrecv (msg_t *msg_)
     memcpy (msg_->data (),
         pending_data.front ().data (),
         pending_data.front ().size ());
+    msg_->set_metadata (pending_metadata.front ());
     msg_->set_flags (pending_flags.front ());
     pending_data.pop_front ();
+    pending_metadata.pop_front ();
     pending_flags.pop_front ();
     return 0;
 }
@@ -265,6 +270,7 @@ void zmq::xpub_t::send_unsubscription (unsigned char *data_, size_t size_,
         if (size_ > 0)
             memcpy (&unsub [1], data_, size_);
         self->pending_data.push_back (unsub);
+        self->pending_metadata.push_back (NULL);
         self->pending_flags.push_back (0);
     }
 }

src/xpub.hpp

@@ -109,6 +109,7 @@ namespace zmq
         //  applied to the trie, but not yet received by the user.
         typedef std::basic_string <unsigned char> blob_t;
         std::deque <blob_t> pending_data;
+        std::deque <metadata_t*> pending_metadata;
         std::deque <unsigned char> pending_flags;
 
         xpub_t (const xpub_t&);