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

Solution: fix regression introduced by:
https://github.com/zeromq/libzmq/pull/3168

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

Fixes https://github.com/zeromq/libzmq/issues/3656

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 ();
 }