feat: Notify acks messages in one way messages

If only one peer is sending notify messages,  we still want
the acks to come back,  so if no messages are being sent back,
we will send an empty message with the acks.

Assets/Mirror/Runtime/NetworkConnection.cs

@@ -8,6 +8,10 @@
 
 namespace Mirror
 {
+    [NetworkMessage]
+    public struct NotifyAck
+    {
+    }
 
     /// <summary>
     /// A High level network connection. This is used for connections from client-to-server and for connection from server-to-client.
@@ -83,6 +87,9 @@ public NetworkConnection(IConnection connection)
         {
             Assert.IsNotNull(connection);
             this.connection = connection;
+
+            // a black message to ensure a notify timeout
+            RegisterHandler<NotifyAck>(msg => { });
         }
 
         /// <summary>
@@ -377,13 +384,17 @@ internal struct PacketEnvelope
         }
         const int ACK_MASK_BITS = sizeof(ulong) * 8;
         const int WINDOW_SIZE = 512;
+        // packages will be acked no longer than this time;
+        public float NOTIFY_ACK_TIMEOUT = 0.3f;
 
         private Sequencer sequencer = new Sequencer(16);
         readonly Queue<PacketEnvelope> sendWindow = new Queue<PacketEnvelope>(WINDOW_SIZE);
+        private float lastNotifySentTime;
 
         private ushort receiveSequence;
         private ulong receiveMask;
 
+
 
         /// <summary>
         /// Sends a message, but notify when it is delivered or lost
@@ -398,7 +409,7 @@ public void SendNotify<T>(T msg, object token, int channelId = Channel.Unreliabl
                 NotifyLost?.Invoke(this, token);
                 return;
             }
-
+            
             using (PooledNetworkWriter writer = NetworkWriterPool.GetWriter())
             {
                 var notifyPacket = new NotifyPacket
@@ -418,6 +429,7 @@ public void SendNotify<T>(T msg, object token, int channelId = Channel.Unreliabl
                 MessagePacker.Pack(msg, writer);
                 NetworkDiagnostics.OnSend(msg, channelId, writer.Length, 1);
                 SendAsync(writer.ToArraySegment(), channelId).Forget();
+                lastNotifySentTime = Time.unscaledTime;
             }
 
         }
@@ -449,6 +461,11 @@ internal void ReceiveNotify(NotifyPacket notifyPacket, NetworkReader networkRead
 
             int msgType = MessagePacker.UnpackId(networkReader);
             InvokeHandler(msgType, networkReader, channelId);
+
+            if (Time.unscaledTime - lastNotifySentTime > NOTIFY_ACK_TIMEOUT)
+            {
+                SendNotify(new NotifyAck(), null, channelId);
+            }
         }
 
         // the other end just sent us a message