main BUGFIX fix empty rpc-reply

When an RPC output has a container and the RPC callback returns nothing,
the np2srv_rpc_cb does not add the 'ok' empty leaf. The rpc-reply is
empty.
The client receives an error when it reads the RPC response with
nc_recv_reply, because it expects either 'ok' or 'data'.
Look at the content of the reply and add the 'ok' leaf if no child with
the default flag is found.

Fixes: 3936132 ("server UPDATE call NC RPC directly, not using sysrepo")
Link: https://datatracker.ietf.org/doc/html/rfc7950#section-7.14.4
Link: https://datatracker.ietf.org/doc/html/rfc6241#section-4.4

Author: jeremie6wind

src/main.c

@@ -401,9 +401,24 @@ np2srv_rpc_cb(struct lyd_node *rpc, struct nc_session *ncs)
             /* error reply */
             reply = np_reply_err_sr(user_sess->sess, LYD_NAME(rpc));
         } else if (output) {
-            /* data reply */
-            reply = np_reply_success(rpc, output->tree);
-            output->tree = NULL;
+            struct lyd_node *child = NULL;
+            int has_non_default = 0;
+
+            LY_LIST_FOR(lyd_child(output->tree), child) {
+                if (!(child->flags & LYD_DEFAULT)) {
+                    has_non_default = 1;
+                    break;
+                }
+            }
+
+            if (has_non_default) {
+                /* data reply */
+                reply = np_reply_success(rpc, output->tree);
+                output->tree = NULL;
+            } else {
+                /* OK reply */
+                reply = np_reply_success(rpc, NULL);
+            }
         } else {
             /* OK reply */
             reply = np_reply_success(rpc, NULL);