[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.

3 files changed, 106 insertions, 38 deletions

diff --git a/modules/webrtc/doc_classes/WebRTCMultiplayerPeer.xml b/modules/webrtc/doc_classes/WebRTCMultiplayerPeer.xml
index 927888fe21..0b42c6ed35 100644
--- a/modules/webrtc/doc_classes/WebRTCMultiplayerPeer.xml
+++ b/modules/webrtc/doc_classes/WebRTCMultiplayerPeer.xml
@@ -6,7 +6,7 @@
 	<description>
 		This class constructs a full mesh of [WebRTCPeerConnection] (one connection for each peer) that can be used as a [member MultiplayerAPI.multiplayer_peer].
 		You can add each [WebRTCPeerConnection] via [method add_peer] or remove them via [method remove_peer]. Peers must be added in [constant WebRTCPeerConnection.STATE_NEW] state to allow it to create the appropriate channels. This class will not create offers nor set descriptions, it will only poll them, and notify connections and disconnections.
-		[signal MultiplayerPeer.connection_succeeded] and [signal MultiplayerPeer.server_disconnected] will not be emitted unless [code]server_compatibility[/code] is [code]true[/code] in [method initialize]. Beside that data transfer works like in a [MultiplayerPeer].
+		[signal MultiplayerPeer.connection_succeeded] and [signal MultiplayerPeer.server_disconnected] will not be emitted unless the peer is created using [method create_client]. Beside that data transfer works like in a [MultiplayerPeer].
 		[b]Note:[/b] When exporting to Android, make sure to enable the [code]INTERNET[/code] permission in the Android export preset before exporting the project or using one-click deploy. Otherwise, network communication of any kind will be blocked by Android.
 	</description>
 	<tutorials>
@@ -28,6 +28,31 @@
 				Close all the add peer connections and channels, freeing all resources.
 			</description>
 		</method>
+		<method name="create_client">
+			<return type="int" enum="Error" />
+			<param index="0" name="peer_id" type="int" />
+			<param index="1" name="channels_config" type="Array" default="[]" />
+			<description>
+				Initialize the multiplayer peer as a client with the given [code]peer_id[/code] (must be between 2 and 2147483647). In this mode, you should only call [method add_peer] once and with [code]peer_id[/code] of [code]1[/code]. This mode enables [method MultiplayerPeer.is_server_relay_supported], allowing the upper [MultiplayerAPI] layer to perform peer exchange and packet relaying.
+				You can optionally specify a [code]channels_config[/code] array of [enum MultiplayerPeer.TransferMode] which will be used to create extra channels (WebRTC only supports one transfer mode per channel).
+			</description>
+		</method>
+		<method name="create_mesh">
+			<return type="int" enum="Error" />
+			<param index="0" name="peer_id" type="int" />
+			<param index="1" name="channels_config" type="Array" default="[]" />
+			<description>
+				Initialize the multiplayer peer as a mesh (i.e. all peers connect to each other) with the given [code]peer_id[/code] (must be between 1 and 2147483647).
+			</description>
+		</method>
+		<method name="create_server">
+			<return type="int" enum="Error" />
+			<param index="0" name="channels_config" type="Array" default="[]" />
+			<description>
+				Initialize the multiplayer peer as a server (with unique ID of [code]1[/code]). This mode enables [method MultiplayerPeer.is_server_relay_supported], allowing the upper [MultiplayerAPI] layer to perform peer exchange and packet relaying.
+				You can optionally specify a [code]channels_config[/code] array of [enum MultiplayerPeer.TransferMode] which will be used to create extra channels (WebRTC only supports one transfer mode per channel).
+			</description>
+		</method>
 		<method name="get_peer">
 			<return type="Dictionary" />
 			<param index="0" name="peer_id" type="int" />
@@ -48,18 +73,6 @@
 				Returns [code]true[/code] if the given [code]peer_id[/code] is in the peers map (it might not be connected though).
 			</description>
 		</method>
-		<method name="initialize">
-			<return type="int" enum="Error" />
-			<param index="0" name="peer_id" type="int" />
-			<param index="1" name="server_compatibility" type="bool" default="false" />
-			<param index="2" name="channels_config" type="Array" default="[]" />
-			<description>
-				Initialize the multiplayer peer with the given [code]peer_id[/code] (must be between 1 and 2147483647).
-				If [code]server_compatibilty[/code] is [code]false[/code] (default), the multiplayer peer will be immediately in state [constant MultiplayerPeer.CONNECTION_CONNECTED] and [signal MultiplayerPeer.connection_succeeded] will not be emitted.
-				If [code]server_compatibilty[/code] is [code]true[/code] the peer will suppress all [signal MultiplayerPeer.peer_connected] signals until a peer with id [constant MultiplayerPeer.TARGET_PEER_SERVER] connects and then emit [signal MultiplayerPeer.connection_succeeded]. After that the signal [signal MultiplayerPeer.peer_connected] will be emitted for every already connected peer, and any new peer that might connect. If the server peer disconnects after that, signal [signal MultiplayerPeer.server_disconnected] will be emitted and state will become [constant MultiplayerPeer.CONNECTION_CONNECTED].
-				You can optionally specify a [code]channels_config[/code] array of [enum MultiplayerPeer.TransferMode] which will be used to create extra channels (WebRTC only supports one transfer mode per channel).
-			</description>
-		</method>
 		<method name="remove_peer">
 			<return type="void" />
 			<param index="0" name="peer_id" type="int" />
diff --git a/modules/webrtc/webrtc_multiplayer_peer.cpp b/modules/webrtc/webrtc_multiplayer_peer.cpp
index e03b6b2473..5ea81d5a1b 100644
--- a/modules/webrtc/webrtc_multiplayer_peer.cpp
+++ b/modules/webrtc/webrtc_multiplayer_peer.cpp
@@ -34,7 +34,9 @@
 #include "core/os/os.h"
 
 void WebRTCMultiplayerPeer::_bind_methods() {
-	ClassDB::bind_method(D_METHOD("initialize", "peer_id", "server_compatibility", "channels_config"), &WebRTCMultiplayerPeer::initialize, DEFVAL(false), DEFVAL(Array()));
+	ClassDB::bind_method(D_METHOD("create_server", "channels_config"), &WebRTCMultiplayerPeer::create_server, DEFVAL(Array()));
+	ClassDB::bind_method(D_METHOD("create_client", "peer_id", "channels_config"), &WebRTCMultiplayerPeer::create_client, DEFVAL(Array()));
+	ClassDB::bind_method(D_METHOD("create_mesh", "peer_id", "channels_config"), &WebRTCMultiplayerPeer::create_mesh, DEFVAL(Array()));
 	ClassDB::bind_method(D_METHOD("add_peer", "peer", "peer_id", "unreliable_lifetime"), &WebRTCMultiplayerPeer::add_peer, DEFVAL(1));
 	ClassDB::bind_method(D_METHOD("remove_peer", "peer_id"), &WebRTCMultiplayerPeer::remove_peer);
 	ClassDB::bind_method(D_METHOD("has_peer", "peer_id"), &WebRTCMultiplayerPeer::has_peer);
@@ -52,6 +54,15 @@ int WebRTCMultiplayerPeer::get_packet_peer() const {
 	return next_packet_peer;
 }
 
+int WebRTCMultiplayerPeer::get_packet_channel() const {
+	return next_packet_channel < CH_RESERVED_MAX ? 0 : next_packet_channel - CH_RESERVED_MAX + 1;
+}
+
+MultiplayerPeer::TransferMode WebRTCMultiplayerPeer::get_packet_mode() const {
+	ERR_FAIL_INDEX_V(next_packet_channel, channels_modes.size(), TRANSFER_MODE_RELIABLE);
+	return channels_modes[next_packet_channel];
+}
+
 bool WebRTCMultiplayerPeer::is_server() const {
 	return unique_id == TARGET_PEER_SERVER;
 }
@@ -113,24 +124,14 @@ void WebRTCMultiplayerPeer::poll() {
 	// Signal newly connected peers
 	for (int &E : add) {
 		// Already connected to server: simply notify new peer.
-		// NOTE: Mesh is always connected.
-		if (connection_status == CONNECTION_CONNECTED) {
-			emit_signal(SNAME("peer_connected"), E);
-		}
-
-		// Server emulation mode suppresses peer_conencted until server connects.
-		if (server_compat && E == TARGET_PEER_SERVER) {
+		if (network_mode == MODE_CLIENT) {
+			ERR_CONTINUE(E != TARGET_PEER_SERVER); // Bug.
 			// Server connected.
 			connection_status = CONNECTION_CONNECTED;
 			emit_signal(SNAME("peer_connected"), TARGET_PEER_SERVER);
 			emit_signal(SNAME("connection_succeeded"));
-			// Notify of all previously connected peers
-			for (const KeyValue<int, Ref<ConnectedPeer>> &F : peer_map) {
-				if (F.key != 1 && F.value->connected) {
-					emit_signal(SNAME("peer_connected"), F.key);
-				}
-			}
-			break; // Because we already notified of all newly added peers.
+		} else {
+			emit_signal(SNAME("peer_connected"), E);
 		}
 	}
 	// Fetch next packet
@@ -150,11 +151,14 @@ void WebRTCMultiplayerPeer::_find_next_peer() {
 			++E;
 			continue;
 		}
+		int idx = 0;
 		for (const Ref<WebRTCDataChannel> &F : E->value->channels) {
 			if (F->get_available_packet_count()) {
+				next_packet_channel = idx;
 				next_packet_peer = E->key;
 				return;
 			}
+			idx++;
 		}
 		++E;
 	}
@@ -165,11 +169,14 @@ void WebRTCMultiplayerPeer::_find_next_peer() {
 			++E;
 			continue;
 		}
+		int idx = 0;
 		for (const Ref<WebRTCDataChannel> &F : E->value->channels) {
 			if (F->get_available_packet_count()) {
+				next_packet_channel = idx;
 				next_packet_peer = E->key;
 				return;
 			}
+			idx++;
 		}
 		if (E->key == (int)next_packet_peer) {
 			break;
@@ -177,6 +184,7 @@ void WebRTCMultiplayerPeer::_find_next_peer() {
 		++E;
 	}
 	// No packet found
+	next_packet_channel = 0;
 	next_packet_peer = 0;
 }
 
@@ -184,11 +192,28 @@ MultiplayerPeer::ConnectionStatus WebRTCMultiplayerPeer::get_connection_status()
 	return connection_status;
 }
 
-Error WebRTCMultiplayerPeer::initialize(int p_self_id, bool p_server_compat, Array p_channels_config) {
+Error WebRTCMultiplayerPeer::create_server(Array p_channels_config) {
+	return _initialize(1, MODE_SERVER, p_channels_config);
+}
+
+Error WebRTCMultiplayerPeer::create_client(int p_self_id, Array p_channels_config) {
+	ERR_FAIL_COND_V_MSG(p_self_id == 1, ERR_INVALID_PARAMETER, "Clients cannot have ID 1.");
+	return _initialize(p_self_id, MODE_CLIENT, p_channels_config);
+}
+
+Error WebRTCMultiplayerPeer::create_mesh(int p_self_id, Array p_channels_config) {
+	return _initialize(p_self_id, MODE_MESH, p_channels_config);
+}
+
+Error WebRTCMultiplayerPeer::_initialize(int p_self_id, NetworkMode p_mode, Array p_channels_config) {
 	ERR_FAIL_COND_V(p_self_id < 1 || p_self_id > ~(1 << 31), ERR_INVALID_PARAMETER);
 	channels_config.clear();
+	channels_modes.clear();
+	channels_modes.push_back(TRANSFER_MODE_RELIABLE);
+	channels_modes.push_back(TRANSFER_MODE_UNRELIABLE_ORDERED);
+	channels_modes.push_back(TRANSFER_MODE_UNRELIABLE);
 	for (int i = 0; i < p_channels_config.size(); i++) {
-		ERR_FAIL_COND_V_MSG(p_channels_config[i].get_type() != Variant::INT, ERR_INVALID_PARAMETER, "The 'channels_config' array must contain only enum values from 'MultiplayerPeer.Multiplayer::TransferMode'");
+		ERR_FAIL_COND_V_MSG(p_channels_config[i].get_type() != Variant::INT, ERR_INVALID_PARAMETER, "The 'channels_config' array must contain only enum values from 'MultiplayerPeer.TransferMode'");
 		int mode = p_channels_config[i].operator int();
 		// Initialize data channel configurations.
 		Dictionary cfg;
@@ -207,16 +232,17 @@ Error WebRTCMultiplayerPeer::initialize(int p_self_id, bool p_server_compat, Arr
 			case TRANSFER_MODE_RELIABLE:
 				break;
 			default:
-				ERR_FAIL_V_MSG(ERR_INVALID_PARAMETER, vformat("The 'channels_config' array must contain only enum values from 'MultiplayerPeer.Multiplayer::TransferMode'. Got: %d", mode));
+				ERR_FAIL_V_MSG(ERR_INVALID_PARAMETER, vformat("The 'channels_config' array must contain only enum values from 'MultiplayerPeer.TransferMode'. Got: %d", mode));
 		}
 		channels_config.push_back(cfg);
+		channels_modes.push_back((TransferMode)mode);
 	}
 
 	unique_id = p_self_id;
-	server_compat = p_server_compat;
+	network_mode = p_mode;
 
 	// Mesh and server are always connected
-	if (!server_compat || p_self_id == 1) {
+	if (p_mode != MODE_CLIENT) {
 		connection_status = CONNECTION_CONNECTED;
 	} else {
 		connection_status = CONNECTION_CONNECTING;
@@ -224,6 +250,10 @@ Error WebRTCMultiplayerPeer::initialize(int p_self_id, bool p_server_compat, Arr
 	return OK;
 }
 
+bool WebRTCMultiplayerPeer::is_server_relay_supported() const {
+	return network_mode == MODE_SERVER || network_mode == MODE_CLIENT;
+}
+
 int WebRTCMultiplayerPeer::get_unique_id() const {
 	ERR_FAIL_COND_V(connection_status == CONNECTION_DISCONNECTED, 1);
 	return unique_id;
@@ -261,7 +291,10 @@ Dictionary WebRTCMultiplayerPeer::get_peers() {
 }
 
 Error WebRTCMultiplayerPeer::add_peer(Ref<WebRTCPeerConnection> p_peer, int p_peer_id, int p_unreliable_lifetime) {
-	ERR_FAIL_COND_V(p_peer_id < 0 || p_peer_id > ~(1 << 31), ERR_INVALID_PARAMETER);
+	ERR_FAIL_COND_V(network_mode == MODE_NONE, ERR_UNCONFIGURED);
+	ERR_FAIL_COND_V(network_mode == MODE_CLIENT && p_peer_id != 1, ERR_INVALID_PARAMETER);
+	ERR_FAIL_COND_V(network_mode == MODE_SERVER && p_peer_id == 1, ERR_INVALID_PARAMETER);
+	ERR_FAIL_COND_V(p_peer_id < 1 || p_peer_id > ~(1 << 31), ERR_INVALID_PARAMETER);
 	ERR_FAIL_COND_V(p_unreliable_lifetime < 0, ERR_INVALID_PARAMETER);
 	ERR_FAIL_COND_V(is_refusing_new_connections(), ERR_UNAUTHORIZED);
 	// Peer must be valid, and in new state (to create data channels)
@@ -308,8 +341,12 @@ void WebRTCMultiplayerPeer::remove_peer(int p_peer_id) {
 	if (peer->connected) {
 		peer->connected = false;
 		emit_signal(SNAME("peer_disconnected"), p_peer_id);
-		if (server_compat && p_peer_id == TARGET_PEER_SERVER) {
-			emit_signal(SNAME("server_disconnected"));
+		if (network_mode == MODE_CLIENT && p_peer_id == TARGET_PEER_SERVER) {
+			if (connection_status == CONNECTION_CONNECTING) {
+				emit_signal(SNAME("connection_failed"));
+			} else {
+				emit_signal(SNAME("server_disconnected"));
+			}
 			connection_status = CONNECTION_DISCONNECTED;
 		}
 	}
@@ -403,7 +440,9 @@ void WebRTCMultiplayerPeer::close() {
 	channels_config.clear();
 	unique_id = 0;
 	next_packet_peer = 0;
+	next_packet_channel = 0;
 	target_peer = 0;
+	network_mode = MODE_NONE;
 	connection_status = CONNECTION_DISCONNECTED;
 }
 
diff --git a/modules/webrtc/webrtc_multiplayer_peer.h b/modules/webrtc/webrtc_multiplayer_peer.h
index ea7c60036b..3f608200fd 100644
--- a/modules/webrtc/webrtc_multiplayer_peer.h
+++ b/modules/webrtc/webrtc_multiplayer_peer.h
@@ -48,6 +48,13 @@ private:
 		CH_RESERVED_MAX = 3
 	};
 
+	enum NetworkMode {
+		MODE_NONE,
+		MODE_SERVER,
+		MODE_CLIENT,
+		MODE_MESH,
+	};
+
 	class ConnectedPeer : public RefCounted {
 	public:
 		Ref<WebRTCPeerConnection> connection;
@@ -67,19 +74,25 @@ private:
 	int client_count = 0;
 	ConnectionStatus connection_status = CONNECTION_DISCONNECTED;
 	int next_packet_peer = 0;
-	bool server_compat = false;
+	int next_packet_channel = 0;
+	NetworkMode network_mode = MODE_NONE;
 
 	HashMap<int, Ref<ConnectedPeer>> peer_map;
+	List<TransferMode> channels_modes;
 	List<Dictionary> channels_config;
 
 	void _peer_to_dict(Ref<ConnectedPeer> p_connected_peer, Dictionary &r_dict);
 	void _find_next_peer();
+	Ref<ConnectedPeer> _get_next_peer();
+	Error _initialize(int p_self_id, NetworkMode p_mode, Array p_channels_config = Array());
 
 public:
 	WebRTCMultiplayerPeer() {}
 	~WebRTCMultiplayerPeer();
 
-	Error initialize(int p_self_id, bool p_server_compat = false, Array p_channels_config = Array());
+	Error create_server(Array p_channels_config = Array());
+	Error create_client(int p_self_id, Array p_channels_config = Array());
+	Error create_mesh(int p_self_id, Array p_channels_config = Array());
 	Error add_peer(Ref<WebRTCPeerConnection> p_peer, int p_peer_id, int p_unreliable_lifetime = 1);
 	void remove_peer(int p_peer_id);
 	bool has_peer(int p_peer_id);
@@ -98,8 +111,11 @@ public:
 
 	int get_unique_id() const override;
 	int get_packet_peer() const override;
+	int get_packet_channel() const override;
+	TransferMode get_packet_mode() const override;
 
 	bool is_server() const override;
+	bool is_server_relay_supported() const override;
 
 	void poll() override;