fix: FetchForWritng() async projection returns null if there are no additional events to apply

src/EventSourcingTests/Bugs/Bug_9999_fetch_for_writing_async_projection_with_no_additional_events_should_return_projection.cs

@@ -0,0 +1,61 @@
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using Marten.Events;
+using Marten.Events.Projections;
+using Marten.Schema;
+using Marten.Testing.Harness;
+using Xunit;
+
+namespace EventSourcingTests.Bugs;
+
+public class Bug_9999_fetch_for_writing_async_projection_with_no_additional_events_should_return_projection : BugIntegrationContext
+{
+    // ! IMPORTANT - this does not fail on the bug - for some reason the dynamic codegen doesn't produce the error.
+    // TODO: Rework this to properly highlight the error
+    [Fact]
+    public async Task override_the_optimistic_concurrency_on_projected_document()
+    {
+        StoreOptions(opts =>
+        {
+            opts.Events.StreamIdentity = StreamIdentity.AsString;
+            opts.Projections.Snapshot<TestProjection>(SnapshotLifecycle.Async);
+        });
+
+        var streamKey = Guid.NewGuid().ToString();
+
+        theSession.Events.StartStream(streamKey, new EventA("foo"), new EventB());
+        await theSession.SaveChangesAsync();
+
+        using var daemon = await theStore.BuildProjectionDaemonAsync();
+        await daemon.RebuildProjectionAsync<TestProjection>(CancellationToken.None);
+
+        var result = await theSession.Events.FetchForWriting<TestProjection>(streamKey);
+
+        Assert.Equal(2, result.CurrentVersion);
+        Assert.Equal(2, result.StartingVersion);
+        Assert.NotNull(result.Aggregate);
+        Assert.Equal(streamKey, result.Aggregate.StreamKey);
+        // TODO: There is a weird bug here where ~25% of the time this is set to null. Seems to happen intermittently across all frameworks. No idea why.
+        Assert.Equal("foo", result.Aggregate.Name);
+    }
+
+    public record EventA(string Name);
+    public record EventB;
+}
+
+// put it here to avoid a bug with rebuilding projections whose types are nested classes
+// TODO: Write up this bug
+public record TestProjection
+{
+    [Identity]
+    public string StreamKey { get; set; } = null!;
+
+    public string Name { get; set; } = null!;
+
+    public static TestProjection Create(EventA @event) =>
+        new TestProjection
+        {
+            Name = @event.Name,
+        };
+}

src/Marten/Events/Aggregation/GeneratedAggregateProjectionBase.CodeGen.cs

@@ -280,14 +280,11 @@ private void buildLiveAggregationType(GeneratedAssembly assembly)
 
         buildMethod.DerivedVariables.Add(Variable.For<IQuerySession>("(IQuerySession)session"));
 
-        buildMethod.Frames.Code("if (!events.Any()) return null;");
-
-        buildMethod.Frames.Add(new DeclareAggregateFrame(typeof(T)));
-
-        var callCreateAggregateFrame = new CallCreateAggregateFrame(_createMethods);
-
         // This is the existing snapshot passed into the LiveAggregator
         var snapshot = buildMethod.Arguments.Single(x => x.VariableType == typeof(T));
+        buildMethod.Frames.Code($"if (!events.Any()) return {snapshot.Usage};");
+
+        var callCreateAggregateFrame = new CallCreateAggregateFrame(_createMethods);
         callCreateAggregateFrame.CoalesceAssignTo(snapshot);
 
         buildMethod.Frames.Add(callCreateAggregateFrame);