Plugin.InAppBilling InAppBillingPurchase.PurchaseToken is sometimes null in production - xamarin

Question

I'm using James Montemagno's Plugin.InAppBilling library for Xamarin to do iOS and Android in-app subscriptions. It has worked mostly correctly so far, except that once in a while, on iOS only, InAppBillingPurchase.PurchaseToken comes back null from calls to PurchaseAsync and GetPurchasesAsync.
For example, in my Restore Purchases logic, I have code similar to this:
var purchases = await CrossInAppBilling.Current.GetPurchasesAsync(ItemType.Subscription);
// Sometimes we receive purchases with no PurchaseToken.
// Can't verify the purchase without a token.
var verifiable = purchases.Where(p => !string.IsNullOrWhiteSpace(p.PurchaseToken));
At this point, verifiable sometimes has a different count (0) than purchases (1).
So far, this has seemed to happen when the user purchased using either store credit or a gift card, and only on iOS.
Also, I'm not sure if this bears on the problem, but I am not using the overloads of PurchaseAsync and GetPurchasesAsync which take IInAppBillingVerifyPurchase, because I use server-side verification exclusively (no client-side verification). My workflow is to make the purchase, add the resulting InAppPurchase object to a queue for processing, and then send it to our server as a separate step, for validation and association with the user's account. However, if this is not a valid workflow or if it is known that sometimes PurchaseToken data will be available via IInAppBillingVerifyPurchase.VerifyPurchase but not attached to the InAppBillingPurchase objects returned from the methods above, I would certainly like to know about that. (For what it's worth, I've read the documentation and don't see anything that suggests this.)
Thanks in advance for any help.

Answer 1

Okay, I think I've learned enough to provide useful information for anyone else dealing with this issue.
First, I've figured out what Apple means by "iOS 6 style" and "iOS 7 style" receipts. These do not refer to the version of iOS that creates the receipts. (My up-to-date iOS 12 device still can and does generate "iOS 6 style" receipts.) Instead, these refer to two different receipt formats that were introduced in the respective iOS versions.
iOS 6 style receipts come from SKPaymentTransaction.transactionReceipt and contain information about one specific transaction. This field is now deprecated by Apple.
iOS 7 style receipts come from the app bundle, via the location named in NSBundle.mainBundle.appStoreReceiptUrl. These receipts contain a full manifest of all purchases ever made by the user. The receipts don't expire either -- you can always send them to Apple for validation (though, obviously, individual transactions contained within might show as expired in the response). These are the receipts you should prefer.
The reason this is important is that if you're using the Plugin.InAppBilling library, the InAppBillingPurchase object you get from calling something like PurchaseAsync contains the deprecated iOS 6 style receipt in its PurchaseToken field.
I'm still not sure why it is sometimes present and sometimes null, but seeing as the underlying source of the data is deprecated, it's probably safe to assume that this can and will happen. Therefore, it probably makes sense to cut over to the iOS 7 style receipts as soon as possible.
Note that when you call PurchaseAsync, if you specify an implementation of IInAppBillingVerifyPurchase, your IInAppBillingVerifyPurchase.VerifyPurchase method will receive the newer iOS7 receipt instead. However, the InAppBillingPurchase object returned by PurchaseAsync still gets an iOS 6 style receipt (if it gets anything at all).
Personally, I like the InAppBillingPurchase object itself. It has useful information packed into a convenient package. Since I want to keep the serialized InAppBillingPurchase objects in a queue so that I can retry validation if there are problems with our servers, connectivity, etc, what I'm doing is immediately replacing the PurchaseToken property with the iOS 7 style receipt that I manually retrieve from the bundle.
If you do this, be sure that your code handles the slightly different formats of the iOS 6 and iOS 7 receipts properly. (Our earlier attempts had some errors stemming from not properly understanding what these terms meant.)
I hope this is useful to someone out there. Good luck!