fix(batched-query): enlist FetchForExclusiveWriting item before awaiting BeginTransactionAsync (#4589)

src/Marten/Services/BatchQuerying/BatchedQuery.Events.cs

@@ -132,31 +132,46 @@ public Task<IEventStream<T>> FetchForWriting<T>(string key, long expectedVersion
 
     public async Task<IEventStream<T>> FetchForExclusiveWriting<T>(Guid id) where T : class
     {
-        await Parent.BeginTransactionAsync(CancellationToken.None).ConfigureAwait(false);
-
+        // Enlist synchronously BEFORE the first await so the item is in _items
+        // by the time control returns to the caller. A subsequent Execute() is
+        // then guaranteed to see and process the item.
+        //
+        // Previously, `await Parent.BeginTransactionAsync(...)` ran first. Under
+        // concurrency BeginTransactionAsync does not complete synchronously
+        // (AutoClosingLifetime.StartAsync performs a real socket round-trip in
+        // NpgsqlConnection.OpenAsync), so the method yielded before AddItem ran.
+        // The codegen pattern `var t = batch.Events.FetchForExclusiveWriting(id);
+        // await batch.Execute(ct); var s = await t;` then called Execute with an
+        // empty _items list, returned immediately, and the item.Result was never
+        // populated — causing the awaiter on `t` to wedge forever.
         _documentTypes.Add(typeof(IEvent));
         var plan = Parent.Events.As<EventStore>().FindFetchPlan<T, Guid>();
         if (plan.Lifecycle != ProjectionLifecycle.Live)
         {
             _documentTypes.Add(typeof(T));
         }
         var handler = plan.BuildQueryHandler(Parent, id, true);
+        var resultTask = AddItem(handler);
 
-        return await AddItem(handler).ConfigureAwait(false);
+        await Parent.BeginTransactionAsync(CancellationToken.None).ConfigureAwait(false);
+        return await resultTask.ConfigureAwait(false);
     }
 
     public async Task<IEventStream<T>> FetchForExclusiveWriting<T>(string key) where T : class
     {
-        await Parent.BeginTransactionAsync(CancellationToken.None).ConfigureAwait(false);
-
+        // See the Guid overload above for the explanation — enlist synchronously
+        // before the first await to avoid the async-vs-sync-enlistment race.
         _documentTypes.Add(typeof(IEvent));
         var plan = Parent.Events.As<EventStore>().FindFetchPlan<T, string>();
         if (plan.Lifecycle != ProjectionLifecycle.Live)
         {
             _documentTypes.Add(typeof(T));
         }
         var handler = plan.BuildQueryHandler(Parent, key, true);
-        return await AddItem(handler).ConfigureAwait(false);
+        var resultTask = AddItem(handler);
+
+        await Parent.BeginTransactionAsync(CancellationToken.None).ConfigureAwait(false);
+        return await resultTask.ConfigureAwait(false);
     }
 
     public Task<T?> FetchLatest<T>(Guid id) where T : class