fix: Wrong MTU calculations in UnityTransport (#2731)

Author: simon-lemay-unity

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

@@ -261,7 +261,7 @@ public int FillWriterWithBytes(ref DataStreamWriter writer, int maxBytes = 0)
                 return 0;
             }
 
-            var maxLength = maxBytes == 0 ? writer.Capacity : maxBytes;
+            var maxLength = maxBytes == 0 ? writer.Capacity : Math.Min(maxBytes, writer.Capacity);
             var copyLength = Math.Min(maxLength, Length);
 
             unsafe

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

@@ -784,13 +784,21 @@ private void SendBatchedMessages(SendTarget sendTarget, BatchedSendQueue queue)
             {
                 return;
             }
+
+            var mtu = 0;
+            if (NetworkManager)
+            {
+                var ngoClientId = NetworkManager.ConnectionManager.TransportIdToClientId(sendTarget.ClientId);
+                mtu = NetworkManager.GetPeerMTU(ngoClientId);
+            }
+
             new SendBatchedMessagesJob
             {
                 Driver = m_Driver.ToConcurrent(),
                 Target = sendTarget,
                 Queue = queue,
                 ReliablePipeline = m_ReliableSequencedPipeline,
-                MTU = NetworkManager ? NetworkManager.GetPeerMTU(sendTarget.ClientId) + m_Driver.MaxHeaderSize(sendTarget.NetworkPipeline) : 0,
+                MTU = mtu,
             }.Run();
         }
 

com.unity.netcode.gameobjects/Tests/Editor/Transports/BatchedSendQueueTests.cs

@@ -293,6 +293,23 @@ public void BatchedSendQueue_FillWriterWithBytes_WriterCapacityEqualToLength()
             AssertIsTestMessage(data);
         }
 
+        [Test]
+        public void BatchedSendQueue_FillWriterWithBytes_MaxBytesGreaterThanCapacity()
+        {
+            var dataLength = k_TestMessageSize + BatchedSendQueue.PerMessageOverhead;
+
+            using var q = new BatchedSendQueue(k_TestQueueCapacity);
+            using var data = new NativeArray<byte>(dataLength, Allocator.Temp);
+
+            q.PushMessage(m_TestMessage);
+            q.PushMessage(m_TestMessage);
+
+            var writer = new DataStreamWriter(data);
+            Assert.AreEqual(dataLength, q.FillWriterWithBytes(ref writer, dataLength * 2));
+            AssertIsTestMessage(data);
+            Assert.False(writer.HasFailedWrites);
+        }
+
         [Test]
         public void BatchedSendQueue_Consume_LessThanLength()
         {