I'm building an iOS app that will include an IAP with a non-renewing subscription. Apple provides this nugget in their overview of IAP:
You are required to deliver non-renewing subscriptions to all devices owned by the user. Non-renewing subscriptions are not automatically synchronized to all devices by Store Kit; you must implement this infrastructure yourself. For example, most subscriptions are provided by an external server; your server would need to implement a mechanism to identify users and associate subscription purchases with the user who purchased them.
I would like to follow this rule: I want my users to be able to take advantage of the subscription they purchase on one device on other devices as well. So how can I identify the same user across their iPhone and iPad? I understand that you can't use the Apple ID, and you can't rely on a registration method.
I just now found this question; the answer given there doesn't appear to be workable. There must be something more elegant that others have done.