Add default constructor to Akka.Remote.Ack to fix JSON serializer

src/core/Akka.Remote.Tests/Serialization/BugFix4399Spec.cs

@@ -0,0 +1,36 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Akka.Actor;
+using Akka.Serialization;
+using Akka.TestKit;
+using Xunit;
+
+namespace Akka.Remote.Tests.Serialization
+{
+    public class BugFix4399Spec : AkkaSpec
+    {
+        [Fact]
+        public void JsonSerializer_should_be_able_to_serialize_Ack()
+        {
+            var serializer = new NewtonSoftJsonSerializer((ExtendedActorSystem) Sys);
+
+            var nacks = new List<SeqNo>();
+            for (var i = 100; i < 200; ++i)
+            {
+                nacks.Add(new SeqNo(i));
+            }
+
+            var message = new Ack(new SeqNo(666), nacks);
+            var serializedBytes = serializer.ToBinary(message);
+            var deserialized = (Ack)serializer.FromBinary(serializedBytes, typeof(Ack));
+
+            deserialized.CumulativeAck.RawValue.ShouldBe(666);
+            deserialized.Nacks.Count.ShouldBe(100);
+            var seqNos = deserialized.Nacks.Select(ack => (int)ack.RawValue);
+            seqNos.ShouldOnlyContainInOrder(Enumerable.Range(100, 100).ToArray());
+        }
+    }
+}

src/core/Akka.Remote/AckedDelivery.cs

@@ -283,6 +283,10 @@ internal interface IHasSequenceNumber
     /// </summary>
     internal sealed class Ack
     {
+        [Obsolete("Only used for serialization", true)]
+        public Ack()
+        { }
+
         /// <summary>
         /// Class representing an acknowledgement with select negative acknowledgements.
         /// </summary>