Suggestions for using a custom pre-login signal to perform custom validation?

[Stodge]

I want to add a custom pre-login signal (user_logging_in) that fires just before Django's login function is called. This is a snippet from our custom account app that may explain what I want:

from django.contrib.auth import login as real_login
def custom_login(....)
    # Allow other apps to perform pre-login validation.
    from . import signals
    signals.user_logging_in.send(sender=None, request=request, user=user)

    # Perform the login using Django's code.
    real_login(request, user)

We have a custom licencing engine that we call to in our custom login function to validate the user's licence:

    return validate_license(user, request, license, **kwargs)

So, if the user's licence has expired then they aren't actually logged in and are re-directed back to the login page. This validate_license function can return HTTP statuses as required.

The licence engine is currently tightly coupled into our custom account app. 

I'm trying to work out if I can get the Licence engine to subscribe to the above custom signal so that I can decouple it from our account app. However I don't understand how I would get the Licence engine to influence the response sent to the browser.

Any suggestions greatly appreciated.

Thanks