fix: ObjectDisposedException on Unity Transport shutdown [backport]

Author: EmandM

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

@@ -1442,6 +1442,11 @@ public override bool StartServer()
         /// </summary>
         public override void Shutdown()
         {
+            if (NetworkManager && !NetworkManager.ShutdownInProgress)
+            {
+                Debug.LogWarning("Directly calling `UnityTransport.Shutdown()` results in unexpected shutdown behaviour. All pending events will be lost. Use `NetworkManager.Shutdown()` instead.");
+            }
+
             if (m_Driver.IsCreated)
             {
                 // Flush all send queues to the network. NGO can be configured to flush its message
@@ -1461,6 +1466,7 @@ public override void Shutdown()
             DisposeInternals();
 
             m_ReliableReceiveQueues.Clear();
+            m_State = State.Disconnected;
 
             // We must reset this to zero because UTP actually re-uses clientIds if there is a clean disconnect
             m_ServerClientId = 0;

com.unity.netcode.gameobjects/Tests/Runtime/Transports/UnityTransportTests.cs

@@ -517,5 +517,42 @@ public IEnumerator ReliablePayloadsCanBeLargerThanMaximum()
 
             yield return null;
         }
+
+        public enum AfterShutdownAction
+        {
+            Send,
+            DisconnectRemoteClient,
+            DisconnectLocalClient,
+        }
+
+        [UnityTest]
+        public IEnumerator DoesNotActAfterShutdown([Values] AfterShutdownAction afterShutdownAction)
+        {
+            InitializeTransport(out m_Server, out m_ServerEvents);
+            InitializeTransport(out m_Client1, out m_Client1Events);
+
+            m_Server.StartServer();
+            m_Client1.StartClient();
+
+            yield return WaitForNetworkEvent(NetworkEvent.Connect, m_Client1Events);
+
+            m_Server.Shutdown();
+
+            if (afterShutdownAction == AfterShutdownAction.Send)
+            {
+                var data = new ArraySegment<byte>(new byte[16]);
+                m_Server.Send(m_Client1.ServerClientId, data, NetworkDelivery.Reliable);
+            }
+            else if (afterShutdownAction == AfterShutdownAction.DisconnectRemoteClient)
+            {
+                m_Server.DisconnectRemoteClient(m_Client1.ServerClientId);
+
+                LogAssert.Expect(LogType.Assert, "DisconnectRemoteClient should be called on a listening server");
+            }
+            else if (afterShutdownAction == AfterShutdownAction.DisconnectLocalClient)
+            {
+                m_Server.DisconnectLocalClient();
+            }
+        }
     }
 }