Coupon can't be removed when added to cart via WC_AJAX

[iMicknl]

Prerequisites

• I have searched for similar issues in both open and closed tickets and cannot find a duplicate
• The issue still exists against the latest master branch of WooCommerce
• This is not a usage question (Those should be directed to the community, unless this is a question about a premium plugin in which you should use the helpdesk for official extensions or contact the author of 3rd party extensions)
• I have attempted to find the simplest possible steps to reproduce the issue

I am writing a plugin where I check via an external API if a discount card is valid. If they deserve a discount I create a new coupon which I apply to the their cart via WC()->cart->add_discount(). This works like a charm, however users can't remove the coupon anymore. If I call this function not via Ajax (in footer.php for example) it works without any problems.

Steps to reproduce the issue

1. Call WC()->cart->add_discount( $coupon_code ); via WC_AJAX or WP_AJAX
2. Remove coupon from cart, which results in 'Coupon removed' message.
3. Refresh page. Coupon is still applied.

Expected behavior and actual behavior

I was expecting that the coupon would be removed successfully when I remove it from the cart.

Environment

``` ### WordPress Environment ###

Home URL: https://webshop.dev
Site URL: https://webshop.dev
WC Version: 3.1.1
Log Directory Writable: ✔
WP Version: 4.8
WP Multisite: –
WP Memory Limit: 256 MB
WP Debug Mode: –
WP Cron: ✔
Language: en_US

Server Environment

Server Info: nginx/1.10.2
PHP Version: 7.1.6
PHP Post Max Size: 8 MB
PHP Time Limit: 30
PHP Max Input Vars: 1000
cURL Version: 7.51.0
SecureTransport

SUHOSIN Installed: –
Max Upload Size: 2 MB
Default Timezone is UTC: ✔
fsockopen/cURL: ✔
SoapClient: ✔
DOMDocument: ✔
GZip: ✔
Multibyte String: ✔
Remote Post: ✔
Remote Get: ✔

Database

WC Database Version: 3.1.1
WC Database Prefix: wp_
woocommerce_sessions: ✔
woocommerce_api_keys: ✔
woocommerce_attribute_taxonomies: ✔
woocommerce_downloadable_product_permissions: ✔
woocommerce_order_items: ✔
woocommerce_order_itemmeta: ✔
woocommerce_tax_rates: ✔
woocommerce_tax_rate_locations: ✔
woocommerce_shipping_zones: ✔
woocommerce_shipping_zone_locations: ✔
woocommerce_shipping_zone_methods: ✔
woocommerce_payment_tokens: ✔
woocommerce_payment_tokenmeta: ✔
MaxMind GeoIP Database: ✔

Security

Secure connection (HTTPS): ✔
Hide errors from visitors: ✔

Active Plugins (2)

WooCommerce CJP discount: by codeconverters – 1.0.0
WooCommerce: by Automattic – 3.1.1

Settings

API Enabled: ✔
Force SSL: ✔
Currency: EUR (€)
Currency Position: left
Thousand Separator: ,
Decimal Separator: .
Number of Decimals: 2
Taxonomies: Product Types: external (external)
grouped (grouped)
simple (simple)
variable (variable)

Taxonomies: Product Visibility: exclude-from-catalog (exclude-from-catalog)
exclude-from-search (exclude-from-search)
featured (featured)
outofstock (outofstock)
rated-1 (rated-1)
rated-2 (rated-2)
rated-3 (rated-3)
rated-4 (rated-4)
rated-5 (rated-5)

WC Pages

Shop base: #4 - /
Cart: #5 - /cart/
Checkout: #6 - /checkout/
My account: #7 - /my-account/

Theme

Name: Storefront
Version: 2.2.4
Author URL: https://woocommerce.com/
Child Theme: ❌ – If you are modifying WooCommerce on a parent theme that you did not build personally we recommend using a child theme. See: How to create a child theme
WooCommerce Support: ✔

Templates

Overrides: –

</details>

## Isolating the problem
- [x] This bug happens with only WooCommerce plugin active
- [x] This bug happens with a default WordPress theme active, or [Storefront](https://woocommerce.com/storefront/)
- [x] I can reproduce this bug consistently

[mikejolley]

To test this I added the above code to the AJAX add to cart event. There were no issues removing the coupon code on cart page. There is no difference between a coupon added via AJAX to anywhere else, and the removal process is identical for all.

Which leaves me with this theory :) The code you added to the AJAX events is so greedy it runs on all AJAX events, e.g. the cart update events called after removing a coupon, so it's added right back after removal. Without seeing the exact code it's hard to confirm this though.

[iMicknl]

@mikejolley thanks for your response. I will have a closer look at this issue and update this issue with my resolution / findings. Thanks for sharing your theory.

[iMicknl]

@mikejolley I found the issue. I was using the coupon_id in WC()->cart->add_discount( $coupon_id );, but it should be the coupon_code. If you use the coupon_id everything works, except for removing the coupon from the cart. This wasn't related to WC_AJAX after all.

In my case; in class-wc-cart.php / remove_coupon() the array will be searched based on the coupon_code, while the applied_coupons contains the coupon_id.

		$position     = array_search( $coupon_code, $this->applied_coupons );

		if ( false !== $position ) {
			unset( $this->applied_coupons[ $position ] );
		}

		WC()->session->set( 'applied_coupons', $this->applied_coupons );