Woocommerce - update_order_review become 502 after add_discount programmatically - wordpress

Question

I need some help. I want to implement, add existing coupon to cart when some conditions are fulfilled. I've got nothing wrong with the condition. But, when the function are going to add discount to coupon, the wc-ajax=update_order_review become 502.
However, if I do another things like changing address or something that can re-run update_order_review, it become 200. update_order_review successfully running and coupon is added to cart.
This is my function code so far, and I've copied some code from stackoverflow forum too
add_action('woocommerce_before_calculate_totals', 'discount_based_on_weight_threshold');
function discount_based_on_weight_threshold( $cart ) {
if ( ! defined( 'DOING_AJAX' ) )
return;
// Your settings
$coupon_code = 'freeongkir8krpx'; // Coupon code
// Initializing variables
$applied_coupons = $cart->get_applied_coupons();
$coupon_code = sanitize_text_field( $coupon_code );
$methods = WC()->session->get( 'chosen_shipping_methods' );
$method = isset( $methods[ 0 ] ) ? $methods[ 0 ] : false;
$kupon = new WC_Coupon('freeongkir10k');
if($method !== 'jne_shipping_test'){
if($cart->get_subtotal() <= $kupon->get_minimum_amount()){
$user = wp_get_current_user();
error_log( 'CHECK USER ROLES');
error_log( print_r($user->roles, TRUE) );
if(in_array( 'administrator', (array) $user->roles )){
// Applying coupon
if( ! in_array($coupon_code, $applied_coupons) ){
$cart->add_discount( $coupon_code );
wc_clear_notices();
}
}
} else {
if( in_array($coupon_code, $applied_coupons) ){
$cart->remove_coupon( $coupon_code );
}
}
} else {
if( in_array($coupon_code, $applied_coupons) ){
$cart->remove_coupon( $coupon_code );
}
}
}
When I disable $cart->add_discount( $coupon_code );, it running successfully, so I pretty sure that adding discount is the trigger.
And the flow that I used is
Go to checkout, conditions fulfilled, add_discount error, another function re-run update_order_review, coupon applied.
Change shipping methods, conditions not fulfilled, applied coupon is deleted
Change shipping methods, conditions fulfilled, add_discount error, update_order_review become 502, no other function re-run update_order_review, infinite loading in checkout order review
So any wrong code or flow do I have? or there are any ways to trick the error since it will applied the code if the update_order_review are re-run
Thank you

Answer 1

Do you get any helpful information in your browser console or the wp error log?
If I remember right 502 on AJAX request means the code fails to give you a valid response.
Hm, I don't yet have a helpful answer.
I refactored your code a bit to improve readability.
This should do exactly what your code does.
add_discount() has been renamed apply_coupon() - maybe try that.
<?php
add_action('woocommerce_before_calculate_totals', 'discount_based_on_weight_threshold');
function discount_based_on_weight_threshold( $cart ) {
if ( ! defined( 'DOING_AJAX' ) )
return;
// Your settings
$coupon_code = 'freeongkir8krpx'; // Coupon code
// Initializing variables
$applied_coupons = $cart->get_applied_coupons();
$coupon_code = sanitize_text_field( $coupon_code );
$methods = WC()->session->get( 'chosen_shipping_methods' );
$method = isset( $methods[ 0 ] ) ? $methods[ 0 ] : false;
$kupon = new WC_Coupon('freeongkir10k');
// remove code if already applied
if( in_array($coupon_code, $applied_coupons) ){
$cart->remove_coupon( $coupon_code );
}
if(
$method !== 'jne_shipping_test') &&
$cart->get_subtotal() <= $kupon->get_minimum_amount()
){
$user = wp_get_current_user();
error_log( 'CHECK USER ROLES');
error_log( print_r($user->roles, TRUE) );
if( in_array( 'administrator', (array) $user->roles ) ){
// Applying coupon
$cart->apply_coupon( $coupon_code );
wc_clear_notices();
}
}
}