ROUTER socket reports error when message cannot be routed

Till now, message was silently dropped if it was sent to
a non-existent peer. Now, ECANTROUTE error is returned.
Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

doc/zmq_send.txt

@@ -79,6 +79,9 @@ The provided 'socket' was invalid.
 *EINTR*::
 The operation was interrupted by delivery of a signal before the message was
 sent.
+*ECANTROUTE*::
+Message cannot be routed to the destination specified as the peer is either
+dead or disconnected. This error makes sense only with ZMQ_ROUTER socket.
 
 
 EXAMPLE

include/zmq.h

@@ -107,6 +107,7 @@ ZMQ_EXPORT void zmq_version (int *major, int *minor, int *patch);
 #define ENOCOMPATPROTO (ZMQ_HAUSNUMERO + 52)
 #define ETERM (ZMQ_HAUSNUMERO + 53)
 #define EMTHREAD (ZMQ_HAUSNUMERO + 54)
+#define ECANTROUTE (ZMQ_HAUSNUMERO + 55)
 
 /*  This function retrieves the errno as it is known to 0MQ library. The goal */
 /*  of this function is to make the code 100% portable, including where 0MQ   */

src/router.cpp

@@ -137,12 +137,18 @@ int zmq::router_t::xsend (msg_t *msg_, int flags_)
             more_out = true;
 
             //  Find the pipe associated with the peer ID stored in the prefix.
-            //  If there's no such pipe just silently ignore the message.
-            zmq_assert (msg_->size () == 4);
+            if (unlikely (msg_->size () != 4)) {
+                errno = ECANTROUTE;
+                return -1;
+            }
             uint32_t peer_id = get_uint32 ((unsigned char*) msg_->data ());
             outpipes_t::iterator it = outpipes.find (peer_id);
+            if (unlikely (it == outpipes.end ())) {
+                errno = ECANTROUTE;
+                return -1;
+            }
 
-            if (it != outpipes.end ()) {
+            //  Check whether the pipe is available for writing.
             current_out = it->second.pipe;
             msg_t empty;
             int rc = empty.init ();
@@ -155,7 +161,6 @@ int zmq::router_t::xsend (msg_t *msg_, int flags_)
             rc = empty.close ();
             errno_assert (rc == 0);
         }
-        }
 
         int rc = msg_->close ();
         errno_assert (rc == 0);