Related to #4567

Changes proposed in this Pull Request:

As per the discussion in #4567, this PR builds on the code from that PR to use WC_Stripe_Database_Cache instead of built-in WordPress transients. I initially kept this work separate as we didn't have a mechanism to clean up stale entries in the database cache, but we addressed that shortcoming in #4609, which was merged earlier this week.

At an implementation level, this PR does the following:

• Change the all payment methods cache to use the database cache instead of a transient
• Implement a new, static WC_Stripe_Customer::clear_customer_cache() method, which clears the transients and the all-payments database cache
• Refactor the WC_Stripe_Customer->clear_cache() method to call the new static function
• Update WC_Stripe_API::attach_payment_method_to_customer() and WC_Stripe_API::detach_payment_method_from_customer() to call the new static cache clear function when the APIs don't return an error
  • We already had a subtle bug/gap where we were calling attach_payment_to_customer() without clearing the cache -- see

    woocommerce-gateway-stripe/includes/payment-methods/class-wc-stripe-upe-payment-gateway.php

    Lines 2517 to 2519 in f06e518

    	if ( WC_Stripe_Payment_Methods::ACH === $selected_payment_type ) {
    	WC_Stripe_API::attach_payment_method_to_customer( $payment_information['customer'], $payment_method_id );
    	}

• Update the attach_source(), delete_source(), and detach_payment_method() methods in WC_Stripe_Customer to clear their local customer data instead of clearing their full cache
  • It's worth noting that the logic in attach_source() was not there before

Concerns

Given that this cache is going to be more durable, I am not 100% sure that we have all the use cases covered for saving a new payment method, especially because the POST /v1/payment_methods API call is made directly from the front-end, and we only get the payment method ID and make a decision about saving the payment method in Woo later.

Testing instructions

The best approach will be to follow the testing instructions from #4567.

• Covered with tests (or have a good reason not to test in description ☝️)
• [N/A] Tested on mobile (or does not apply)

Changelog entry

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Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• Added the needs docs label (or does not apply)
• Included this PR in the Release Thread scope (or does not apply)