OAuth: Automatically Refresh Token on Expiry

[n7studios]

Summary

Reduces the likelihood of 401 unauthorized requests being sent to the Kit API by scheduling a WordPress cron event to refresh an expired access token on its expiry.

Presently, the Plugin relies on logic to check if an API request returns a 401 error with the message The access token expired. On lower traffic sites - for example, where there are not many customers subscribing or purchases being sent to Kit - this isn't a reliable method, as a call to the API won't always be triggered.

Testing

• testCronEventCreatedWhenAccessTokenObtained: Test that the convertkit_refresh_token event is scheduled when OAuth first completes and the Plugin receives access and refresh tokens.
• testCronEventCreatedWhenTokenRefreshed: Test that the convertkit_refresh_token event is scheduled when the access token is refreshed.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

[n7studios]

Integration tests weren't running on GitHub actions; this ensures they run.

[noelherrick]

Why are you switching to this? Wouldn't this set the expiration to longer than it actually is?

[n7studios]

expires_in is the number of seconds before the access token expires, not a timestamp of when the access token expires, and the API has never returned a fixed value. Adding it to created_at results in the exact issue reported in this PR. Adding it to time() results in the correct calculation of the token's expiry. But if I'm missing something obvious, let me know.

[n7studios]

@noelherrick Let me know if this needs another review. It's the same principle as the main Kit Plugin: Kit/convertkit-wordpress#840

[noelherrick]

Thanks for the reminder!