fix: username onboarding on third party login/link

[lfleischmann]

Description

If there is an existing user with a username and that user decides to link a third party account and username onboarding on login is enabled then logging in via third party results in a username onboarding prompt but if the user enters a username, then this leads to an error: if it is the same username it results in a "username already taken" error, if it is a different username it results in a technical error (unique constraint violation on the DB level because there already is a username for this user). The username prompt should not appear if the username is already set.

Implementation

There's already a check in place for ensuring that a username is set but the persister used to retrieve the identity does not preload the username, hence the check fails.

How to test

Do a before/after of:

• Configure third party provider of your choice, configure it to allow linking
• Enable usernames, set acquiring usernames on login to true
• Create a user, set a username
• Log in with third party provider
  • before: username is prompted for and the above mentioned errors occur when trying to set a username
  • after: username is not prompted for