Problem: ROUTER loses first frame metadata on message prefetch

Solution: copy metadata on prefetch to the first frame

src/router.cpp

@@ -401,6 +401,8 @@ bool zmq::router_t::xhas_in ()
     errno_assert (rc == 0);
     memcpy (_prefetched_id.data (), routing_id.data (), routing_id.size ());
     _prefetched_id.set_flags (msg_t::more);
+    if (_prefetched_msg.metadata ())
+        _prefetched_id.set_metadata (_prefetched_msg.metadata ());
 
     _prefetched = true;
     _routing_id_sent = false;

tests/test_metadata.cpp

@@ -127,10 +127,61 @@ void test_metadata ()
     zmq_threadclose (zap_thread);
 }
 
+void test_router_prefetch_metadata ()
+{
+    char my_endpoint[MAX_SOCKET_STRING];
+    setup_test_context ();
+
+    //  Spawn ZAP handler
+    //  We create and bind ZAP socket in main thread to avoid case
+    //  where child thread does not start up fast enough.
+    void *handler = zmq_socket (get_test_context (), ZMQ_REP);
+    TEST_ASSERT_NOT_NULL (handler);
+    TEST_ASSERT_SUCCESS_ERRNO (zmq_bind (handler, "inproc://zeromq.zap.01"));
+    void *zap_thread = zmq_threadstart (&zap_handler, handler);
+
+    void *server = test_context_socket (ZMQ_ROUTER);
+    void *client = test_context_socket (ZMQ_REQ);
+    TEST_ASSERT_SUCCESS_ERRNO (
+      zmq_setsockopt (server, ZMQ_ZAP_DOMAIN, "DOMAIN", 6));
+    bind_loopback_ipv4 (server, my_endpoint, sizeof (my_endpoint));
+    TEST_ASSERT_SUCCESS_ERRNO (zmq_connect (client, my_endpoint));
+
+    send_string_expect_success (client, "This is a message", 0);
+
+    msleep (SETTLE_TIME);
+
+    // Check for messages in the ROUTER socket which will trigger a prefetch
+    unsigned long int dummy;
+    size_t dummy_size = sizeof (dummy);
+    zmq_getsockopt (server, ZMQ_EVENTS, &dummy, &dummy_size);
+
+    zmq_msg_t msg;
+
+    // Ensure all frames in the message contain metadata
+    for (int i = 0; i < 3; i++) {
+        zmq_msg_init (&msg);
+        TEST_ASSERT_SUCCESS_ERRNO (zmq_msg_recv (&msg, server, 0));
+        TEST_ASSERT_EQUAL_STRING ("World", zmq_msg_gets (&msg, "Hello"));
+        zmq_msg_close (&msg);
+    }
+
+    test_context_socket_close_zero_linger (client);
+    test_context_socket_close_zero_linger (server);
+
+    //  Shutdown
+    teardown_test_context ();
+
+    //  Wait until ZAP handler terminates
+    zmq_threadclose (zap_thread);
+}
+
+
 int main ()
 {
     setup_test_environment ();
     UNITY_BEGIN ();
     RUN_TEST (test_metadata);
+    RUN_TEST (test_router_prefetch_metadata);
     return UNITY_END ();
 }