Detect invalid combination of XA and non-XA transactions.

Author: bgrainger

src/MySqlConnector/MySql.Data.MySqlClient/MySqlConnection.cs

@@ -108,6 +108,8 @@ public override void EnlistTransaction(System.Transactions.Transaction transacti
 					ImplicitTransactionBase implicitTransaction;
 					if (m_connectionSettings.UseXaTransactions)
 					{
+						if (!(existingConnection?.m_connectionSettings.UseXaTransactions ?? true))
+							throw new NotSupportedException("Cannot start an XA transaction when there is an existing non-XA transaction.");
 						implicitTransaction = new XaImplicitTransaction(this);
 					}
 					else

tests/SideBySide/TransactionScopeTests.cs

@@ -555,6 +555,42 @@ public void TwoDifferentConnectionStringsThrowsWithNonXaTransactions()
 			}
 		}
 
+#if !BASELINE
+		[Fact]
+		public void CannotMixXaAndNonXaTransactions()
+		{
+			using (new TransactionScope())
+			{
+				using (var conn1 = new MySqlConnection(AppConfig.ConnectionString))
+				{
+					conn1.Open();
+
+					using (var conn2 = new MySqlConnection(AppConfig.ConnectionString + ";UseXaTransactions=False"))
+					{
+						Assert.Throws<NotSupportedException>(() => conn2.Open());
+					}
+				}
+			}
+		}
+
+		[Fact]
+		public void CannotMixNonXaAndXaTransactions()
+		{
+			using (new TransactionScope())
+			{
+				using (var conn1 = new MySqlConnection(AppConfig.ConnectionString + ";UseXaTransactions=False"))
+				{
+					conn1.Open();
+
+					using (var conn2 = new MySqlConnection(AppConfig.ConnectionString))
+					{
+						Assert.Throws<NotSupportedException>(() => conn2.Open());
+					}
+				}
+			}
+		}
+#endif
+
 		DatabaseFixture m_database;
 	}
 }