fix: Client-Side NetworkEvent.Disconnect can throw an exception in UnityTransport (#1884)

* fix MTT-3359

The fix for the bug.

* test

The integration test to verify the fix

* style

* style and update

* Update CHANGELOG.md

Author: NoelStephensUnity

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -5,6 +5,16 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/) and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 Additional documentation and release notes are available at [Multiplayer Documentation](https://docs-multiplayer.unity3d.com).
+## Unreleased
+
+### Added
+
+### Changed
+
+### Removed
+
+### Fixed
+- Fixed client throwing an exception if it has messages in the outbound queue when processing the NetworkEvent.Disconnect event and is using UTP. (#1884)
 
 ## [1.0.0-pre.7] - 2022-04-06
 

com.unity.netcode.gameobjects/Runtime/Core/NetworkManager.cs

@@ -1446,7 +1446,11 @@ private void HandleRawTransportPoll(NetworkEvent networkEvent, ulong clientId, A
                     }
                     else
                     {
-                        Shutdown();
+                        // We must pass true here and not process any sends messages
+                        // as we are no longer connected and thus there is no one to
+                        // send any messages to and this will cause an exception within
+                        // UnityTransport as the client ID is no longer valid.
+                        Shutdown(true);
                     }
 #if DEVELOPMENT_BUILD || UNITY_EDITOR
                     s_TransportDisconnect.End();

testproject/Assets/Tests/Runtime/ServerDisconnectsClientTest.cs

@@ -0,0 +1,57 @@
+using System.Collections;
+using NUnit.Framework;
+using UnityEngine;
+using UnityEngine.TestTools;
+using Unity.Netcode;
+using Unity.Netcode.TestHelpers.Runtime;
+
+
+namespace TestProject.RuntimeTests
+{
+    public class ServerDisconnectsClientTest : NetcodeIntegrationTest
+    {
+        protected override int NumberOfClients => 1;
+        protected override void OnCreatePlayerPrefab()
+        {
+            m_PlayerPrefab.AddComponent<ClientSendRpcUponDisconnect>();
+            base.OnCreatePlayerPrefab();
+        }
+
+        [UnityTest]
+        public IEnumerator ServerDisconnectsClient()
+        {
+            m_ServerNetworkManager.DisconnectClient(m_ClientNetworkManagers[0].LocalClientId);
+            yield return WaitForConditionOrTimeOut(() => !m_ClientNetworkManagers[0].IsConnectedClient && !m_ClientNetworkManagers[0].IsListening);
+            Assert.IsFalse(s_GlobalTimeoutHelper.TimedOut, "Timed out waiting for client to disconnect!");
+        }
+        public class ClientSendRpcUponDisconnect : NetworkBehaviour
+        {
+            public override void OnNetworkSpawn()
+            {
+                if (!IsServer)
+                {
+                    NetworkManager.OnClientDisconnectCallback += NetworkManager_OnClientDisconnectCallback;
+                }
+                base.OnNetworkSpawn();
+            }
+
+            [ServerRpc(RequireOwnership = false)]
+            public void ClientToServerRpc()
+            {
+                Debug.Log($"Received {nameof(ClientToServerRpc)}");
+            }
+
+            private void NetworkManager_OnClientDisconnectCallback(ulong obj)
+            {
+                NetworkManager.OnClientDisconnectCallback -= NetworkManager_OnClientDisconnectCallback;
+                // To simulate that there were already messages to be sent this frame in the send queue,
+                // we just send a few packets to the server even though we are considered disconnected
+                // at this point.
+                for (int i = 0; i < 5; i++)
+                {
+                    ClientToServerRpc();
+                }
+            }
+        }
+    }
+}