disposer + process exit

a.js

module.exports = function(fn, exit) {
  var close;
  return pg.connectAsync(connString).bind({}).spread(function(client, _close) {
    close = _close;
    return fn(client);
  }).catch(console.log).finally(function() {
    if (close) {
      close();
    }
    if (exit) {
      process.exit();
    }
  });
};