Refs #2465 -- Introduce unique identifiers for authentication backends (#2466)

doc/development/api/auth.rst

@@ -20,20 +20,31 @@ Basically, three pre-defined flows are supported:
 * Authentication mechanisms that rely on **redirection**, e.g. to an OAuth provider. These can be implemented by
   supplying a ``authentication_url`` method and implementing a custom return view.
 
-Authentication backends are *not* collected through a signal. Instead, they must explicitly be set through the
-``auth_backends`` directive in the ``pretix.cfg`` :ref:`configuration file <config>`.
+For security reasons, authentication backends are *not* automatically discovered through a signal. Instead, they must
+explicitly be set through the ``auth_backends`` directive in the ``pretix.cfg`` :ref:`configuration file <config>`.
 
-In each of these methods (``form_authenticate``, ``request_authenticate`` or your custom view) you are supposed to
-either get an existing :py:class:`pretix.base.models.User` object from the database or create a new one. There are a
-few rules you need to follow:
+In each of these methods (``form_authenticate``, ``request_authenticate``, or your custom view) you are supposed to
+use ``User.objects.get_or_create_for_backend`` to get a :py:class:`pretix.base.models.User` object from the database
+or create a new one.
 
-* You **MUST** only return users with the ``auth_backend`` attribute set to the ``identifier`` value of your backend.
+There are a few rules you need to follow:
 
-* You **MUST** create new users with the ``auth_backend`` attribute set to the ``identifier`` value of your backend.
+* You **MUST** have some kind of identifier for a user that is globally unique and **SHOULD** never change, even if the
+  user's name or email address changes. This could e.g. be the ID of the user in an external database. The identifier
+  must not be longer than 190 characters. If you worry your backend might generated longer identifiers, consider
+  using a hash function to trim them to a constant length.
+
+* You **SHOULD** not allow users created by other authentication backends to log in through your code, and you **MUST**
+  only create, modify or return users with ``auth_backend`` set to your backend.
 
 * Every user object **MUST** have an email address. Email addresses are globally unique. If the email address is
   already registered to a user who signs in through a different backend, you **SHOULD** refuse the login.
 
+``User.objects.get_or_create_for_backend`` will follow these rules for you automatically. It works like this:
+
+.. autoclass:: pretix.base.models.UserManager
+   :members: get_or_create_for_backend
+
 The backend interface
 ---------------------
 
@@ -59,6 +70,7 @@ The backend interface
 
    .. automethod:: authentication_url
 
+
 Logging users in
 ----------------
 
@@ -68,3 +80,45 @@ recommend that you use the following utility method to correctly set session val
 authentication (if activated):
 
 .. autofunction:: pretix.control.views.auth.process_login
+
+A custom view that is called after a redirect from an external identity provider could look like this::
+
+   from django.contrib import messages
+   from django.shortcuts import redirect
+   from django.urls import reverse
+
+   from pretix.base.models import User
+   from pretix.base.models.auth import EmailAddressTakenError
+   from pretix.control.views.auth import process_login
+
+
+   def return_view(request):
+       # Verify validity of login with the external provider's API
+       api_response = my_verify_login_function(
+           code=request.GET.get('code')
+       )
+
+       try:
+           u = User.objects.get_or_create_for_backend(
+               'my_backend_name',
+               api_response['userid'],
+               api_response['email'],
+               set_always={
+                   'fullname': '{} {}'.format(
+                       api_response.get('given_name', ''),
+                       api_response.get('family_name', ''),
+                   ),
+               },
+               set_on_creation={
+                   'locale': api_response.get('locale').lower()[:2],
+                   'timezone': api_response.get('zoneinfo', 'UTC'),
+               }
+           )
+       except EmailAddressTakenError:
+           messages.error(
+               request, _('We cannot create your user account as a user account in this system '
+                          'already exists with the same email address.')
+           )
+           return redirect(reverse('control:auth.login'))
+       else:
+           return process_login(request, u, keep_logged_in=False)