With a modifier, the close method is called on the XAConnection object, with no modifier it's called on the Connection object.

With the postgresql driver, you have to close the XAConnection.

Here's my example, showing the leak :

public class ConnectionLeakLauncher {

    public static void main(String[] args) throws Exception {
        setupDS();

        for (int i = 0; i < 100; i++) {
            Connection connection = DriverManager.getConnection("jdbc:arjuna:java:comp/env/datasources/XaPgDS", "postgres", "azer");

            try (PreparedStatement statement = connection.prepareStatement("SELECT COUNT(*) FROM pg_stat_activity")) {
                ResultSet resultSet = statement.executeQuery();
                if (resultSet.next()) {
                    System.out.println(resultSet.getLong(1));
                }
            } finally {
                connection.close();
            }
        }

    }

    private static void setupDS() throws NamingException, ClassNotFoundException {
        PGXADataSource xaPgDS = new PGXADataSource();
        xaPgDS.setServerName("127.0.0.1");
        xaPgDS.setPortNumber(5432);
        xaPgDS.setDatabaseName("postgres");
        xaPgDS.setUser("postgres");
        xaPgDS.setPassword("azer");
        InitialContext ctx = new InitialContext();
        ctx.bind("datasources/XaPgDS", xaPgDS);

        Class.forName("com.arjuna.ats.jdbc.TransactionalDriver");
        // ModifierFactory.putModifier("postgresql native driver", -1, -1, PsqlConnectionModifier.class.getName());

    }
}

To be honest, I've no idea of the goal of the modifiers. I've just seen that it could solve my problem.

At my first attempt, I've used the IsSameRMModifier class, but having a internal package in my imports hurted me. So I duplicated the class with the name of PsqlConnectionModifier.

I've created the issue : [JBTM-2667] JDBC Connection leak with Postgres - JBoss Issue Tracker

And I can prepare a PR if it can help.