Avoid loading Django user models before apps are ready

[francoisfreitag]

To register a custom Provider, users subclass allauth.socialaccount.providers.base.ProviderAccount. Their subclass is then registered using allauth.socialaccount.providers.registry.register(MyProvider).

The registration must happen before the allauth.socialaccount app models are ready, as the SocialApp.provider choices come from the registry. Otherwise, a Django migration is created because at the time of the import of allauth.socialaccount.models, the registry was empty, but when the models are ready (and Django can check migrations) the registry contains the custom provider.

Below is an example stack trace to illustrate the issue:

  File "/home/freitafr/dev/itou/itou/allauth_adapters/peamu/provider.py", line 1, in <module>
    from allauth.socialaccount.providers.base import ProviderAccount
  File "/home/freitafr/dev/django-allauth/allauth/socialaccount/providers/base/__init__.py", line 2, in <module>
    from .provider import Provider, ProviderAccount, ProviderException  # noqa
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/freitafr/dev/django-allauth/allauth/socialaccount/providers/base/provider.py", line 3, in <module>
    from allauth.account.models import EmailAddress
  File "/home/freitafr/dev/django-allauth/allauth/account/models.py", line 12, in <module>
    from .adapter import get_adapter
  File "/home/freitafr/dev/django-allauth/allauth/account/adapter.py", line 16, in <module>
    from django.contrib.auth.models import AbstractUser
  File "/home/freitafr/dev/itou/.venv/lib/python3.11/site-packages/django/contrib/auth/models.py", line 3, in <module>
    from django.contrib.auth.base_user import AbstractBaseUser, BaseUserManager
  File "/home/freitafr/dev/itou/.venv/lib/python3.11/site-packages/django/contrib/auth/base_user.py", line 57, in <module>
    class AbstractBaseUser(models.Model):
  File "/home/freitafr/dev/itou/.venv/lib/python3.11/site-packages/django/db/models/base.py", line 129, in __new__
    app_config = apps.get_containing_app_config(module)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/freitafr/dev/itou/.venv/lib/python3.11/site-packages/django/apps/registry.py", line 260, in get_containing_app_config
    self.check_apps_ready()
  File "/home/freitafr/dev/itou/.venv/lib/python3.11/site-packages/django/apps/registry.py", line 138, in check_apps_ready
    raise AppRegistryNotReady("Apps aren't loaded yet.")
django.core.exceptions.AppRegistryNotReady: Apps aren't loaded yet.

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[coveralls]

coverage: 91.324% (+0.007%) from 91.317% when pulling cb05d5d on francoisfreitag:provider into ef12bd6 on pennersr:main.