If a user implements the user-auth-service trigger in a plugin and that plugin does not echo the passed in app(s) on stdout, the app is assumed to not exist. This mirrors the functionality for applications in regards to auth filtering.
This may still need auditing to ensure it covers everything and doesn't cause issues, but local testing implies that everything is working as expected.
Previously they would just 'disappear', resulting in broken applications until the app was relinked to the service in question.
Closesdokku/dokku-redis#142