Fix AsyncLocal leak in SystemTransactionContext (#1596)

Fixes #1594

Author: pecanw

src/NHibernate.Test/NHSpecificTest/GH1594/Entity.cs

@@ -0,0 +1,10 @@
+using System;
+
+namespace NHibernate.Test.NHSpecificTest.GH1594
+{
+	class Entity
+	{
+		public virtual Guid Id { get; set; }
+		public virtual string Name { get; set; }
+	}
+}

src/NHibernate.Test/NHSpecificTest/GH1594/ExecutionContextExtensions.cs

@@ -0,0 +1,20 @@
+using System.Collections;
+using System.Threading;
+
+namespace NHibernate.Test.NHSpecificTest.GH1594
+{
+	public static class ExecutionContextExtensions
+	{
+		public static int LocalValuesCount(this ExecutionContext c)
+		{
+#if NETFX
+			const string localValuesFieldName = "_localValues";
+#else			
+			const string localValuesFieldName = "m_localValues";
+#endif
+			var f = typeof(ExecutionContext).GetField(localValuesFieldName, System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance);
+			var d = (IDictionary) f.GetValue(c);
+			return d?.Count ?? 0;
+		}
+	}
+}

src/NHibernate.Test/NHSpecificTest/GH1594/Fixture.cs

@@ -0,0 +1,72 @@
+using System.Linq;
+using System.Threading;
+using System.Transactions;
+using NHibernate.Engine;
+using NUnit.Framework;
+
+namespace NHibernate.Test.NHSpecificTest.GH1594
+{
+	[TestFixture]
+	public class Fixture : BugTestCase
+	{
+		protected override bool AppliesTo(ISessionFactoryImplementor factory) =>
+			factory.ConnectionProvider.Driver.SupportsSystemTransactions;
+
+		protected override void OnSetUp()
+		{
+			using (var session = OpenSession())
+			using (var transaction = session.BeginTransaction())
+			{
+				var e1 = new GH1594.Entity {Name = "Bob"};
+				session.Save(e1);
+
+				var e2 = new GH1594.Entity {Name = "Sally"};
+				session.Save(e2);
+
+				transaction.Commit();
+			}
+		}
+
+		protected override void OnTearDown()
+		{
+			using (var session = OpenSession())
+			using (var transaction = session.BeginTransaction())
+			{
+				// The HQL delete does all the job inside the database without loading the entities, but it does
+				// not handle delete order for avoiding violating constraints if any. Use
+				// session.Delete("from System.Object");
+				// instead if in need of having NHbernate ordering the deletes, but this will cause
+				// loading the entities in the session.
+				session.CreateQuery("delete from System.Object").ExecuteUpdate();
+
+				transaction.Commit();
+			}
+		}
+
+		[Test]
+		public void ExecutionContextLocalValuesLeak()
+		{
+			using (var session = OpenSession())
+			{
+				RunInTransaction(session);
+				var localValuesCountAfterFirstCall = ExecutionContext.Capture().LocalValuesCount();
+				RunInTransaction(session);
+				var localValuesCountAfterSecondCall = ExecutionContext.Capture().LocalValuesCount();
+				Assert.AreEqual(localValuesCountAfterFirstCall, localValuesCountAfterSecondCall);
+			}
+		}
+
+		private void RunInTransaction(ISession session)
+		{
+			using (var ts = new TransactionScope())
+			{
+				var result = from e in session.Query<GH1594.Entity>()
+							 where e.Name == "Bob"
+							 select e;
+
+				result.ToList();
+				ts.Complete();
+			}
+		}
+	}
+}

src/NHibernate.Test/NHSpecificTest/GH1594/Mappings.hbm.xml

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="NHibernate.Test"
+                   namespace="NHibernate.Test.NHSpecificTest.GH1594">
+
+  <class name="Entity">
+    <id name="Id" generator="guid.comb"/>
+    <property name="Name"/>
+  </class>
+
+</hibernate-mapping>

src/NHibernate/Transaction/AdoNetWithSystemTransactionFactory.cs

@@ -187,7 +187,7 @@ public class SystemTransactionContext : ITransactionContext, IEnlistmentNotifica
 			private readonly ManualResetEventSlim _lock = new ManualResetEventSlim(true);
 			private volatile bool _needCompletionLocking = true;
 			// Required for not locking the completion phase itself when locking session usages from concurrent threads.
-			private readonly AsyncLocal<bool> _bypassLock = new AsyncLocal<bool>();
+			private static readonly AsyncLocal<bool> _bypassLock = new AsyncLocal<bool>();
 			private readonly int _systemTransactionCompletionLockTimeout;
 
 			/// <summary>
@@ -269,6 +269,7 @@ protected virtual void Lock()
 			protected virtual void Unlock()
 			{
 				_lock.Set();
+				_bypassLock.Value = false;
 			}
 
 			/// <summary>