Update zmq_msg_get(ZMQ_SHARED) to return true for type_cmsg messages

doc/zmq_msg_get.txt

@@ -30,8 +30,8 @@ aware that the respective socket might be closed already, reused even.
 Currently only implemented for TCP sockets.
 
 *ZMQ_SHARED*::
-Indicates that a message has been copied and MAY share underlying storage
-with another copy of this message.
+Indicates that a message MAY share underlying storage with another copy of
+this message.
 
 RETURN VALUE
 ------------

src/zmq.cpp

@@ -630,7 +630,8 @@ int zmq_msg_get (zmq_msg_t *msg_, int property_)
             // warning: int64_t to int
             return ((zmq::msg_t*) msg_)->fd ();
         case ZMQ_SHARED:
-            return (((zmq::msg_t*) msg_)->flags () & zmq::msg_t::shared)? 1: 0;
+            return (((zmq::msg_t*) msg_)->is_cmsg ()) ||
+                   (((zmq::msg_t*) msg_)->flags () & zmq::msg_t::shared)? 1: 0;
         default:
             errno = EINVAL;
             return -1;

tests/test_msg_flags.cpp

@@ -65,7 +65,7 @@ int main (void)
     more = zmq_msg_more (&msg);
     assert (more == 0);
 
-    // Test ZMQ_SHARED property
+    // Test ZMQ_SHARED property (case 1, refcounted messages)
     zmq_msg_t msg_a;
     rc = zmq_msg_init_size(&msg_a, 1024); // large enough to be a type_lmsg
     assert (rc == 0);
@@ -85,6 +85,24 @@ int main (void)
     rc = zmq_msg_get(&msg_b, ZMQ_SHARED);
     assert (rc == 1);
 
+    // cleanup
+    rc = zmq_msg_close(&msg_a);
+    assert (rc == 0);
+    rc = zmq_msg_close(&msg_b);
+    assert (rc == 0);
+
+    // Test ZMQ_SHARED property (case 2, constant data messages)
+    rc = zmq_msg_init_data(&msg_a, (void*) "TEST", 5, 0, 0);
+    assert (rc == 0);
+
+    // Message reports as shared
+    rc = zmq_msg_get(&msg_a, ZMQ_SHARED);
+    assert (rc == 1);
+
+    // cleanup
+    rc = zmq_msg_close(&msg_a);
+    assert (rc == 0);
+
     //  Deallocate the infrastructure.
     rc = zmq_close (sc);
     assert (rc == 0);