🐛(backend) fix create document for user when sub does not match

When creating a document on behalf of a user via the server-to-server
API, a special edge case was broken that should should never happen
but happens in our OIDC federation because one of the provider modifies
the users "sub" each time they login.

We end-up with existing users for who the email matches but not the sub.
They were not correctly handled.

I made a few additional fixes and improvements to the endpoint.

src/backend/core/tests/authentication/test_backends.py

@@ -1,5 +1,6 @@
 """Unit tests for the Authentication Backends."""
 
+import random
 import re
 from logging import Logger
 from unittest import mock
@@ -64,7 +65,33 @@ def test_authentication_getter_existing_user_via_email(
     assert user == db_user
 
 
-def test_authentication_getter_existing_user_no_fallback_to_email(
+def test_authentication_getter_email_none(monkeypatch):
+    """
+    If no user is found with the sub and no email is provided, a new user should be created.
+    """
+
+    klass = OIDCAuthenticationBackend()
+    db_user = UserFactory(email=None)
+
+    def get_userinfo_mocked(*args):
+        user_info = {"sub": "123"}
+        if random.choice([True, False]):
+            user_info["email"] = None
+        return user_info
+
+    monkeypatch.setattr(OIDCAuthenticationBackend, "get_userinfo", get_userinfo_mocked)
+
+    user = klass.get_or_create_user(
+        access_token="test-token", id_token=None, payload=None
+    )
+
+    # Since the sub and email didn't match, it should create a new user
+    assert models.User.objects.count() == 2
+    assert user != db_user
+    assert user.sub == "123"
+
+
+def test_authentication_getter_existing_user_no_fallback_to_email_allow_duplicate(
     settings, monkeypatch
 ):
     """
@@ -77,6 +104,7 @@ def test_authentication_getter_existing_user_no_fallback_to_email(
 
     # Set the setting to False
     settings.OIDC_FALLBACK_TO_EMAIL_FOR_IDENTIFICATION = False
+    settings.OIDC_ALLOW_DUPLICATE_EMAILS = True
 
     def get_userinfo_mocked(*args):
         return {"sub": "123", "email": db_user.email}
@@ -93,6 +121,39 @@ def test_authentication_getter_existing_user_no_fallback_to_email(
     assert user.sub == "123"
 
 
+def test_authentication_getter_existing_user_no_fallback_to_email_no_duplicate(
+    settings, monkeypatch
+):
+    """
+    When the "OIDC_FALLBACK_TO_EMAIL_FOR_IDENTIFICATION" setting is set to False,
+    the system should not match users by email, even if the email matches.
+    """
+
+    klass = OIDCAuthenticationBackend()
+    db_user = UserFactory()
+
+    # Set the setting to False
+    settings.OIDC_FALLBACK_TO_EMAIL_FOR_IDENTIFICATION = False
+    settings.OIDC_ALLOW_DUPLICATE_EMAILS = False
+
+    def get_userinfo_mocked(*args):
+        return {"sub": "123", "email": db_user.email}
+
+    monkeypatch.setattr(OIDCAuthenticationBackend, "get_userinfo", get_userinfo_mocked)
+
+    with pytest.raises(
+        SuspiciousOperation,
+        match=(
+            "We couldn't find a user with this sub but the email is already associated "
+            "with a registered user."
+        ),
+    ):
+        klass.get_or_create_user(access_token="test-token", id_token=None, payload=None)
+
+    # Since the sub doesn't match, it should not create a new user
+    assert models.User.objects.count() == 1
+
+
 def test_authentication_getter_existing_user_with_email(
     django_assert_num_queries, monkeypatch
 ):