[MP] Fix broken RPCs after dictionary keys type change

As part of RPCs processing, they need to be sorted reliably across all
peers, so that unique IDs can be assigned to greatly optimize the
network layer.

The RPC configuration nodes are stored in dictionaries which, until
recently, always casted StringName keys to String.

Since method names (keys) in the RPC configuration were StringName,
a side effect of the above change is that sorting the dictionary keys no
longer sort them alphabetically by default (StringName are compared
using their pointers).

This commit changes the RPC processing logic to use sort_custom to
provide a function that can handle the StringName comparison.

modules/multiplayer/scene_rpc_interface.cpp

@@ -73,6 +73,16 @@ int get_packet_len(uint32_t p_node_target, int p_packet_len) {
 	}
 }
 
+bool SceneRPCInterface::_sort_rpc_names(const Variant &p_l, const Variant &p_r) {
+	if (likely(p_l.is_string() && p_r.is_string())) {
+		return p_l.operator String() < p_r.operator String();
+	}
+	bool valid = false;
+	Variant res;
+	Variant::evaluate(Variant::OP_LESS, p_l, p_r, res, valid);
+	return valid ? res.operator bool() : false;
+}
+
 void SceneRPCInterface::_parse_rpc_config(const Variant &p_config, bool p_for_node, RPCConfigCache &r_cache) {
 	if (p_config.get_type() == Variant::NIL) {
 		return;
@@ -80,7 +90,7 @@ void SceneRPCInterface::_parse_rpc_config(const Variant &p_config, bool p_for_no
 	ERR_FAIL_COND(p_config.get_type() != Variant::DICTIONARY);
 	const Dictionary config = p_config;
 	Array names = config.keys();
-	names.sort(); // Ensure ID order
+	names.sort_custom(callable_mp_static(&SceneRPCInterface::_sort_rpc_names)); // Ensure ID order
 	for (int i = 0; i < names.size(); i++) {
 		ERR_CONTINUE(!names[i].is_string());
 		String name = names[i].operator String();

modules/multiplayer/scene_rpc_interface.h

@@ -91,6 +91,8 @@ private:
 #endif
 
 protected:
+	static bool _sort_rpc_names(const Variant &p_l, const Variant &p_r);
+
 	void _process_rpc(Node *p_node, const uint16_t p_rpc_method_id, int p_from, const uint8_t *p_packet, int p_packet_len, int p_offset);
 
 	void _send_rpc(Node *p_from, int p_to, uint16_t p_rpc_id, const RPCConfig &p_config, const StringName &p_name, const Variant **p_arg, int p_argcount);