Add peer visibility to MultiplayerSynchronizer.

MultiplayerSynchronizers can now be configured to limit their visibility
to a subset of the connected peers, if the synchronized node was spawned
by a MultiplayerSpawner (either automatically or via custom spawn) the
given node will also be despawned remotely.

The replication system doesn't have the logic to handle subspawn
directly, but it is possible to handle them appropriately by manually
updating the visibility of the parent before changing the one of the
nested spawns via the "update_visibility" function.

The visibility of each MultiplayerSynchronizer can be controlled by
adding or remove filters via "[add|remove]_visibility_filter(callable)".

To further optimize the network code, visibility filters can be configured
to be automatically updated during idle or physics frame, or set to always
require manual update (via the "update_visibility" function).

1 files changed, 2 insertions, 4 deletions

diff --git a/scene/multiplayer/scene_rpc_interface.cpp b/scene/multiplayer/scene_rpc_interface.cpp
index 84700a82f3..144a10c665 100644
--- a/scene/multiplayer/scene_rpc_interface.cpp
+++ b/scene/multiplayer/scene_rpc_interface.cpp
@@ -302,12 +302,9 @@ void SceneRPCInterface::_send_rpc(Node *p_from, int p_to, uint16_t p_rpc_id, con
 		ERR_FAIL_MSG("Attempt to call RPC with unknown peer ID: " + itos(p_to) + ".");
 	}
 
-	NodePath from_path = multiplayer->get_root_path().rel_path_to(p_from->get_path());
-	ERR_FAIL_COND_MSG(from_path.is_empty(), "Unable to send RPC. Relative path is empty. THIS IS LIKELY A BUG IN THE ENGINE!");
-
 	// See if all peers have cached path (if so, call can be fast).
 	int psc_id;
-	const bool has_all_peers = multiplayer->send_object_cache(p_from, from_path, p_to, psc_id);
+	const bool has_all_peers = multiplayer->send_object_cache(p_from, p_to, psc_id);
 
 	// Create base packet, lots of hardcode because it must be tight.
 
@@ -414,6 +411,7 @@ void SceneRPCInterface::_send_rpc(Node *p_from, int p_to, uint16_t p_rpc_id, con
 		// Not all verified path, so send one by one.
 
 		// Append path at the end, since we will need it for some packets.
+		NodePath from_path = multiplayer->get_root_path().rel_path_to(p_from->get_path());
 		CharString pname = String(from_path).utf8();
 		int path_len = encode_cstring(pname.get_data(), nullptr);
 		MAKE_ROOM(ofs + path_len);