NH-2176 - handling null enlistment (and forgotten update of stored ambient transaction).

Author: fredericDelaporte

src/NHibernate/AdoNet/ConnectionManager.cs

@@ -155,6 +155,7 @@ private void CloseConnection()
 		{
 			Factory.ConnectionProvider.CloseConnection(_connection);
 			_connection = null;
+			_connectionAmbientTransaction = null;
 		}
 
 		public DbConnection GetConnection()
@@ -374,10 +375,17 @@ public void EnlistIfRequired()
 			var tran = System.Transactions.Transaction.Current;
 			if (tran == _connectionAmbientTransaction)
 				return;
+
+			if (tran == null && !Factory.Dialect.RequiresNullEnlistment)
+			{
+				_connectionAmbientTransaction = null;
+				return;
+			}
 			// Will fail if the connection is already enlisted in another not yet completed transaction.
 			// Probable case: nested transaction scope. Supporting this could be done by releasing the
 			// connection instead of enlisting.
 			_session.Connection.EnlistTransaction(tran);
+			_connectionAmbientTransaction = tran;
 		}
 
 		private bool RequireConnectionSwapInDtc

src/NHibernate/Dialect/Dialect.cs

@@ -1410,6 +1410,12 @@ public IList<SqlString> GetTokens()
 		/// </summary>
 		public virtual bool SupportsConcurrentWritingConnectionsInSameTransaction => SupportsConcurrentWritingConnections;
 
+		/// <summary>
+		/// Does this dialect requires null enlistment for reusing a connection after a transaction scope, without
+		/// using a new scope?
+		/// </summary>
+		public virtual bool RequiresNullEnlistment => false;
+
 		#endregion
 
 		#region Limit/offset support

src/NHibernate/Dialect/Oracle8iDialect.cs

@@ -464,6 +464,12 @@ public override long TimestampResolutionInTicks
 			}
 		}
 
+		/// <summary>
+		/// Does this dialect requires null enlistment for reusing a connection after a transaction scope, without
+		/// using a new scope?
+		/// </summary>
+		public override bool RequiresNullEnlistment => true;
+
 		#region Overridden informational metadata
 
 		public override bool SupportsEmptyInList