Show message after redirection

Question

This will be mostly a Drupal question rather may not have any relation to Drupal commerce checkout.

I have an Offsite Checkout redirect payment method in which I have a buildRedirectFrom as stated in https://docs.drupalcommerce.org/commerce2/developer-guide/payments/create-payment-gateway/off-site-gateways/off-site-redirect which will redirect to a vendor site for payment. I have the CompleteUrl which will come back the call to the controller method after payment completion.

I need to show a custom error message after redirecting to the checkout page, when the payment fails or canceled the payment based on the status I receive from vendor API.

But somehow the message is not showing up after the redirection. I tried to show the message first and then redirect too. Is there any solution in Drupal or particularly in commerce for this?

My try

In controller method:

if( $statusFromAPI != 'PAYSUCCESS' ){
        $redirect = new RedirectResponse('/checkout/' .$order_id . '/review');
        $redirect->send();
        if($statusFromAPI == 'ERROR'){
          $this->messenger()->addError("Payment has been failed. Please check the payment details.", TRUE);
        } else if($statusFromAPI == 'PAYSTART'){
          $this->messenger()->addWarning("Please review the payment details again.");
        } else{
          $this->messenger()->addWarning("Please review the payment details.", TRUE);
  }

Answer 1

A controller has to return the result, this includes the redirect response. But before you have to set the message:

$redirect = new RedirectResponse('/checkout/' . $order_id . '/review');
$this->messenger()->addWarning('This is a warning!');
return $redirect;

BTW in Drupal it's not allowed to send responses. If you are in a form you can set it in $form_state, in an event subscriber set it in the event if it is one of the three main Symfony kernel events.