Problem: can no longer send user data from XSUB to XPUB

Solution: fix regression introduced by:
zeromq#3168

Correctly fall back to user message if the first byte is neither 0 nor
1, and add a simple unit test

src/xpub.cpp

@@ -104,13 +104,10 @@ void zmq::xpub_t::xread_activated (pipe_t *pipe_)
  data = static_cast<unsigned char *> (sub.command_body ());
  size = sub.command_body_size ();
  subscribe = sub.is_subscribe ();
- } else if (sub.size () > 0) {
- unsigned char first = *msg_data;
- if (first == 0 || first == 1) {
- data = msg_data + 1;
- size = sub.size () - 1;
- subscribe = first == 1;
- }
+ } else if (sub.size () > 0 && (*msg_data == 0 || *msg_data == 1)) {
+ data = msg_data + 1;
+ size = sub.size () - 1;
+ subscribe = *msg_data == 1;
  } else {
  // Process user message coming upstream from xsub socket
  _pending_data.push_back (blob_t (msg_data, sub.size ()));

tests/test_xpub_manual.cpp

@@ -434,6 +434,28 @@ void test_unsubscribe_cleanup ()
  test_context_socket_close (sub);
 }
 
+void test_user_message ()
+{
+ // Create a publisher
+ void *pub = test_context_socket (ZMQ_XPUB);
+ TEST_ASSERT_SUCCESS_ERRNO (zmq_bind (pub, "inproc://soname"));
+
+ // Create a subscriber
+ void *sub = test_context_socket (ZMQ_XSUB);
+ TEST_ASSERT_SUCCESS_ERRNO (zmq_connect (sub, "inproc://soname"));
+
+ // Send some data that is neither sub nor unsub
+ const char subscription[] = {2, 'A', 0};
+ send_string_expect_success (sub, subscription, 0);
+
+ // Receive subscriptions from subscriber
+ recv_string_expect_success (pub, subscription, 0);
+
+ // Clean up.
+ test_context_socket_close (pub);
+ test_context_socket_close (sub);
+}
+
 int main ()
 {
  setup_test_environment ();
@@ -444,6 +466,7 @@ int main ()
  RUN_TEST (test_xpub_proxy_unsubscribe_on_disconnect);
  RUN_TEST (test_missing_subscriptions);
  RUN_TEST (test_unsubscribe_cleanup);
+ RUN_TEST (test_user_message);
 
  return UNITY_END ();
 }