Add a shortcut to reduce Transaction.Current reads

src/NHibernate.Test/Async/SystemTransactions/SystemTransactionFixture.cs

@@ -10,15 +10,16 @@
 
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Linq;
 using System.Threading;
 using System.Transactions;
 using NHibernate.Cfg;
 using NHibernate.Driver;
 using NHibernate.Engine;
-using NHibernate.Linq;
 using NHibernate.Test.TransactionTest;
 using NUnit.Framework;
+using NHibernate.Linq;
 
 namespace NHibernate.Test.SystemTransactions
 {

src/NHibernate.Test/SystemTransactions/SystemTransactionFixture.cs

@@ -1,12 +1,12 @@
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Linq;
 using System.Threading;
 using System.Transactions;
 using NHibernate.Cfg;
 using NHibernate.Driver;
 using NHibernate.Engine;
-using NHibernate.Linq;
 using NHibernate.Test.TransactionTest;
 using NUnit.Framework;
 
@@ -522,6 +522,44 @@ public void AdditionalJoinDoesNotThrow()
 				Assert.DoesNotThrow(() => s.JoinTransaction());
 			}
 		}
+
+		[Theory, Explicit("Bench")]
+		public void BenchTransactionAccess(bool inTransaction)
+		{
+			var currentTransaction = System.Transactions.Transaction.Current;
+			using (inTransaction ? new TransactionScope() : null)
+			using (var s = OpenSession())
+			{
+				var impl = s.GetSessionImplementation();
+				var transactionContext = impl.TransactionContext;
+				if (inTransaction)
+					s.JoinTransaction();
+
+				// warm-up
+				for (var i = 0; i < 10; i++)
+					currentTransaction = System.Transactions.Transaction.Current;
+				for (var i = 0; i < 10; i++)
+					transactionContext = impl.TransactionContext;
+
+				var sw = new Stopwatch();
+				for (var j = 0; j < 4; j++)
+				{
+					sw.Restart();
+					for (var i = 0; i < 10000; i++)
+						currentTransaction = System.Transactions.Transaction.Current;
+					sw.Stop();
+					Assert.That(currentTransaction, inTransaction ? Is.Not.Null : Is.Null);
+
+					Console.WriteLine($"Current transaction reads have taken {sw.Elapsed}");
+					sw.Restart();
+					for (var i = 0; i < 10000; i++)
+						transactionContext = impl.TransactionContext;
+					sw.Stop();
+					Assert.That(transactionContext, inTransaction ? Is.Not.Null : Is.Null);
+					Console.WriteLine($"Transaction context reads have taken {sw.Elapsed}");
+				}
+			}
+		}
 	}
 
 	[TestFixture]

src/NHibernate/Async/Transaction/AdoNetWithSystemTransactionFactory.cs

@@ -16,7 +16,6 @@
 using NHibernate.AdoNet;
 using NHibernate.Engine;
 using NHibernate.Engine.Transaction;
-using NHibernate.Impl;
 using NHibernate.Util;
 
 namespace NHibernate.Transaction

src/NHibernate/Transaction/AdoNetWithSystemTransactionFactory.cs

@@ -6,7 +6,6 @@
 using NHibernate.AdoNet;
 using NHibernate.Engine;
 using NHibernate.Engine.Transaction;
-using NHibernate.Impl;
 using NHibernate.Util;
 
 namespace NHibernate.Transaction
@@ -47,14 +46,20 @@ public override void EnlistInSystemTransactionIfNeeded(ISessionImplementor sessi
 			if (session == null)
 				throw new ArgumentNullException(nameof(session));
 
-			if (!session.ConnectionManager.ShouldAutoJoinTransaction)
-			{
+			if (!ShouldAutoJoinSystemTransaction(session))
 				return;
-			}
 
 			JoinSystemTransaction(session, System.Transactions.Transaction.Current);
 		}
 
+		private static bool ShouldAutoJoinSystemTransaction(ISessionImplementor session)
+		{
+			var connectionManager = session.ConnectionManager;
+			return connectionManager.ShouldAutoJoinTransaction &&
+				// Shortcut for avoiding accessing Transaction.Current, which is costly.
+				(session.TransactionContext == null || connectionManager.ProcessingFromSystemTransaction);
+		}
+
 		/// <inheritdoc />
 		public override void ExplicitJoinSystemTransaction(ISessionImplementor session)
 		{