🐛(backend) manage race condition when creating sandbox document

When a user is created and a sandbox document should be created, we can
have a race condition on the document creation leading to an error for
the user. To avoid this we have to manage this part in a transaction and
locking the document table

src/backend/core/tests/test_models_invitations.py

@@ -114,7 +114,7 @@ def test_models_invitations_new_user_convert_invitations_to_accesses():
     ).exists()  # the other invitation remains
 
 
-def test_models_invitationd_new_user_filter_expired_invitations():
+def test_models_invitations_new_user_filter_expired_invitations():
     """
     Upon creating a new identity, valid invitations should be converted into accesses
     and expired invitations should remain unchanged.
@@ -145,7 +145,7 @@ def test_models_invitationd_new_user_filter_expired_invitations():
 
 
 @pytest.mark.parametrize("num_invitations, num_queries", [(0, 3), (1, 7), (20, 7)])
-def test_models_invitationd_new_userd_user_creation_constant_num_queries(
+def test_models_invitations_new_userd_user_creation_constant_num_queries(
     django_assert_num_queries, num_invitations, num_queries
 ):
     """

src/backend/core/tests/test_models_users.py

@@ -3,6 +3,7 @@ Unit tests for the User model
 """
 
 import uuid
+from concurrent.futures import ThreadPoolExecutor
 from unittest.mock import patch
 
 from django.core.exceptions import ValidationError
@@ -188,9 +189,8 @@ def test_models_users_handle_onboarding_documents_on_restricted_document_is_not_
     """On-boarding document can be used when restricted"""
 
     document = factories.DocumentFactory(link_reach=models.LinkReachChoices.RESTRICTED)
-    settings.USER_ONBOARDING_DOCUMENTS = [str(document.id)]
-
-    user = factories.UserFactory()
+    with override_settings(USER_ONBOARDING_DOCUMENTS=[str(document.id)]):
+        user = factories.UserFactory()
 
     assert not models.LinkTrace.objects.filter(user=user, document=document).exists()
 
@@ -302,3 +302,30 @@ def test_models_users_duplicate_onboarding_sandbox_document_integration_with_oth
         document=sandbox_doc, user=user, role=models.RoleChoices.OWNER
     ).exists()
     assert models.LinkTrace.objects.filter(document=onboarding_doc, user=user).exists()
+
+
+@pytest.mark.django_db(transaction=True)
+def test_models_users_duplicate_onboarding_sandbox_race_condition():
+    """
+    It should be possible to create several documents at the same time
+    without causing any race conditions or data integrity issues.
+    """
+
+    def create_user():
+        return factories.UserFactory()
+
+    template_document = factories.DocumentFactory(title="Getting started with Docs")
+    with (
+        override_settings(
+            USER_ONBOARDING_SANDBOX_DOCUMENT=str(template_document.id),
+        ),
+        ThreadPoolExecutor(max_workers=2) as executor,
+    ):
+        future1 = executor.submit(create_user)
+        future2 = executor.submit(create_user)
+
+        user1 = future1.result()
+        user2 = future2.result()
+
+        assert isinstance(user1, models.User)
+        assert isinstance(user2, models.User)