[MP] Let MultiplayerAPI handle packet relaying and peer signaling.

MultiplayerPeer changes:

- Adds is_server_relay_supported virtual method

Informs the upper MultiplayerAPI layer if it can signal peers connected
to the server to other clients, and perform packet relaying among them.

- Adds get_packet_channel and get_packet_mode virtual methods

Allows the MultiplayerAPI to retrieve the channel and transfer modes to
use when relaying the last received packet.

SceneMultiplayerPeer changes:

- Implement peer signaling and packet relaying when the MultiplayerPeer
  advertise they are supported.

ENet, WebRTC, WebSocket changes:

- Removed custom code for relaying from WebSocket and ENet, and let it
  be handled by the upper layer.
- Update WebRTC to split create_client, create_server, and create_mesh,
  with the latter behaving like the old initialize with
  "server_compatibility = false", and the first two supporting the upper
  layer relaying protocol.

2 files changed, 24 insertions, 0 deletions

diff --git a/scene/main/multiplayer_peer.cpp b/scene/main/multiplayer_peer.cpp
index 9b63118f7c..462dc1babb 100644
--- a/scene/main/multiplayer_peer.cpp
+++ b/scene/main/multiplayer_peer.cpp
@@ -78,6 +78,10 @@ bool MultiplayerPeer::is_refusing_new_connections() const {
 	return refuse_connections;
 }
 
+bool MultiplayerPeer::is_server_relay_supported() const {
+	return false;
+}
+
 void MultiplayerPeer::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("set_transfer_channel", "channel"), &MultiplayerPeer::set_transfer_channel);
 	ClassDB::bind_method(D_METHOD("get_transfer_channel"), &MultiplayerPeer::get_transfer_channel);
@@ -86,6 +90,8 @@ void MultiplayerPeer::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("set_target_peer", "id"), &MultiplayerPeer::set_target_peer);
 
 	ClassDB::bind_method(D_METHOD("get_packet_peer"), &MultiplayerPeer::get_packet_peer);
+	ClassDB::bind_method(D_METHOD("get_packet_channel"), &MultiplayerPeer::get_packet_channel);
+	ClassDB::bind_method(D_METHOD("get_packet_mode"), &MultiplayerPeer::get_packet_mode);
 
 	ClassDB::bind_method(D_METHOD("poll"), &MultiplayerPeer::poll);
 
@@ -96,6 +102,8 @@ void MultiplayerPeer::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("set_refuse_new_connections", "enable"), &MultiplayerPeer::set_refuse_new_connections);
 	ClassDB::bind_method(D_METHOD("is_refusing_new_connections"), &MultiplayerPeer::is_refusing_new_connections);
 
+	ClassDB::bind_method(D_METHOD("is_server_relay_supported"), &MultiplayerPeer::is_server_relay_supported);
+
 	ADD_PROPERTY(PropertyInfo(Variant::BOOL, "refuse_new_connections"), "set_refuse_new_connections", "is_refusing_new_connections");
 	ADD_PROPERTY(PropertyInfo(Variant::INT, "transfer_mode", PROPERTY_HINT_ENUM, "Unreliable,Unreliable Ordered,Reliable"), "set_transfer_mode", "get_transfer_mode");
 	ADD_PROPERTY(PropertyInfo(Variant::INT, "transfer_channel", PROPERTY_HINT_RANGE, "0,255,1"), "set_transfer_channel", "get_transfer_channel");
@@ -177,6 +185,14 @@ bool MultiplayerPeerExtension::is_refusing_new_connections() const {
 	return MultiplayerPeer::is_refusing_new_connections();
 }
 
+bool MultiplayerPeerExtension::is_server_relay_supported() const {
+	bool can_relay;
+	if (GDVIRTUAL_CALL(_is_server_relay_supported, can_relay)) {
+		return can_relay;
+	}
+	return MultiplayerPeer::is_server_relay_supported();
+}
+
 void MultiplayerPeerExtension::_bind_methods() {
 	GDVIRTUAL_BIND(_get_packet, "r_buffer", "r_buffer_size");
 	GDVIRTUAL_BIND(_put_packet, "p_buffer", "p_buffer_size");
diff --git a/scene/main/multiplayer_peer.h b/scene/main/multiplayer_peer.h
index ab7483ece5..63ce66871e 100644
--- a/scene/main/multiplayer_peer.h
+++ b/scene/main/multiplayer_peer.h
@@ -74,10 +74,13 @@ public:
 	virtual TransferMode get_transfer_mode() const;
 	virtual void set_refuse_new_connections(bool p_enable);
 	virtual bool is_refusing_new_connections() const;
+	virtual bool is_server_relay_supported() const;
 
 	virtual void set_target_peer(int p_peer_id) = 0;
 
 	virtual int get_packet_peer() const = 0;
+	virtual TransferMode get_packet_mode() const = 0;
+	virtual int get_packet_channel() const = 0;
 
 	virtual bool is_server() const = 0;
 
@@ -123,12 +126,17 @@ public:
 	virtual bool is_refusing_new_connections() const override;
 	GDVIRTUAL0RC(bool, _is_refusing_new_connections); // Optional.
 
+	virtual bool is_server_relay_supported() const override;
+	GDVIRTUAL0RC(bool, _is_server_relay_supported); // Optional.
+
 	EXBIND1(set_transfer_channel, int);
 	EXBIND0RC(int, get_transfer_channel);
 	EXBIND1(set_transfer_mode, TransferMode);
 	EXBIND0RC(TransferMode, get_transfer_mode);
 	EXBIND1(set_target_peer, int);
 	EXBIND0RC(int, get_packet_peer);
+	EXBIND0RC(TransferMode, get_packet_mode);
+	EXBIND0RC(int, get_packet_channel);
 	EXBIND0RC(bool, is_server);
 	EXBIND0(poll);
 	EXBIND0RC(int, get_unique_id);