Incorrect update_last_login signal handler behavior when using custom User model with an arbitrary last_login attribute (not a Field instance)

[Mikhail Porokhovnichenko]

Please note! It's not a #26823 duplicate, and it's just an issue related this one.

In the current implementation, a signal handler connects with a user model when this model has a last_login field.

if hasattr(get_user_model(), 'last_login'):
    from .models import update_last_login
    user_logged_in.connect(update_last_login, dispatch_uid='update_last_login')

And it would work when there isn't a last_login attribute in the custom user model. But what if I've inherited my custom model from AbstractUser and dropped the last_login by setting it to None?

class User(AbstractBaseUser):
    last_login = None     # Drop ``last_login`` field off

Technically, the model has a last_login attribute, but it's not a field!

Suggesting change the check condition something like that:

last_login = getattr(get_user_model(), 'last_login', None)
if last_login is not None:
    # ...

or even

if isisintance(last_login, models.Field):
    # ...