fix multiplayer snapshot disconnect race

Game/scripts/network_manager.gd

@@ -71,6 +71,7 @@ var is_server := false
 var is_client := false
 var _peer: MultiplayerPeer   # keep a strong ref alongside multiplayer.multiplayer_peer
 var active_transport := ""
+var _invalidated_peer_ids: Dictionary = {}
 
 var rtt_ms := -1.0            # min-RTT sample currently in the window; -1 = no sample yet
 var clock_offset_ms := 0.0    # add to a local Time.get_ticks_msec() reading to estimate the server's clock
@@ -127,6 +128,27 @@ func poll() -> void:
 	multiplayer.poll()
 
 
+# A peer can be removed from the transport while Godot is still draining the
+# same poll batch. During that interval get_peers() may still contain it, but
+# an RPC send already fails because ENet has torn down its channels.
+func invalidate_peer(peer_id: int) -> void:
+	_invalidated_peer_ids[peer_id] = true
+
+
+func can_send_to_peer(peer_id: int) -> bool:
+	if _invalidated_peer_ids.has(peer_id):
+		return false
+	if _peer == null or _peer is OfflineMultiplayerPeer:
+		return false
+	# A listening server's peer status is transport/version-specific; the
+	# authoritative server is valid as soon as it owns a peer and the target
+	# appears in get_peers(). Clients, however, must not dispatch while their
+	# connection is still handshaking.
+	if not is_server and _peer.get_connection_status() != MultiplayerPeer.CONNECTION_CONNECTED:
+		return false
+	return peer_id in multiplayer.get_peers()
+
+
 func available_transports() -> PackedStringArray:
 	var transports := PackedStringArray([TRANSPORT_ENET])
 	if SteamTransportScript.new().is_available():
@@ -185,6 +207,7 @@ func shutdown() -> void:
 		peer.close()
 	multiplayer.multiplayer_peer = OfflineMultiplayerPeer.new()
 	_peer = null
+	_invalidated_peer_ids.clear()
 	active_transport = ""
 	is_server = false
 	is_client = false