Fix MarkAsComplete on new saga instance

src/NServiceBus.Testing.Tests/Sagas/SagaCompletion.cs

@@ -0,0 +1,43 @@
+namespace NServiceBus.Testing.Tests.Sagas
+{
+    using System;
+    using System.Threading.Tasks;
+    using NUnit.Framework;
+
+    [TestFixture]
+    public class SagaCompletion
+    {
+        [Test]
+        public async Task MarkStartedSagaAsComplete()
+        {
+            var saga = new TestableSaga<TestSaga, TestSagaData>();
+
+            var result = await saga.Handle(new StartSagaMessage() { CorrelationId = Guid.NewGuid() });
+
+            Assert.IsTrue(result.Completed);
+        }
+
+        class TestSaga : NServiceBus.Saga<TestSagaData>, IAmStartedByMessages<StartSagaMessage>
+        {
+            protected override void ConfigureHowToFindSaga(SagaPropertyMapper<TestSagaData> mapper) => mapper
+                .MapSaga(d => d.CorrelationId)
+                .ToMessage<StartSagaMessage>(m => m.CorrelationId);
+
+            public Task Handle(StartSagaMessage message, IMessageHandlerContext context)
+            {
+                MarkAsComplete();
+                return Task.CompletedTask;
+            }
+        }
+
+        class TestSagaData : ContainSagaData
+        {
+            public Guid CorrelationId { get; set; }
+        }
+
+        class StartSagaMessage : IMessage
+        {
+            public Guid CorrelationId { get; set; }
+        }
+    }
+}

src/NServiceBus.Testing/Sagas/TestableSaga.cs

@@ -320,6 +320,11 @@ async Task SaveSagaData(
         {
             if (saga.Completed)
             {
+                if (isNew)
+                {
+                    // Can't update a brand new but already-completed saga
+                    return;
+                }
                 await persister.Complete(saga.Entity, session, contextBag, cancellationToken).ConfigureAwait(false);
             }
             else if (isNew)