fix: Don't accept invalid connections in UnityTransport.Send

Author: simon-lemay-unity

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -22,6 +22,7 @@ Additional documentation and release notes are available at [Multiplayer Documen
 
 ### Fixed
 
+- Fixed an issue in `UnityTransport` where the transport would accept sends on invalid connections, leading to a useless memory allocation and confusing error message.
 - Fixed issue where the time delta that interpolators used would not be properly updated during multiple fixed update invocations within the same player loop frame. (#3355)
 - Fixed issue when using a distributed authority network topology and many clients attempt to connect simultaneously the session owner could max-out the maximum in-flight reliable messages allowed, start dropping packets, and some of the connecting clients would fail to fully synchronize. (#3350)
 - Fixed issue when using a distributed authority network topology and scene management was disabled clients would not be able to spawn any new network prefab instances until synchronization was complete. (#3350)

com.unity.netcode.gameobjects/Runtime/Transports/UTP/UnityTransport.cs

@@ -1347,8 +1347,13 @@ public override NetcodeNetworkEvent PollEvent(out ulong clientId, out ArraySegme
         /// <param name="networkDelivery">The delivery type (QoS) to send data with</param>
         public override void Send(ulong clientId, ArraySegment<byte> payload, NetworkDelivery networkDelivery)
         {
-            var pipeline = SelectSendPipeline(networkDelivery);
+            var connection = ParseClientId(clientId);
+            if (!m_Driver.IsCreated || m_Driver.GetConnectionState(connection) != NetworkConnection.State.Connected)
+            {
+                return;
+            }
 
+            var pipeline = SelectSendPipeline(networkDelivery);
             if (pipeline != m_ReliableSequencedPipeline && payload.Count > m_MaxPayloadSize)
             {
                 Debug.LogError($"Unreliable payload of size {payload.Count} larger than configured 'Max Payload Size' ({m_MaxPayloadSize}).");