adapters: truncate username to the field's max_length (#56482)

mellon/adapters.py

@@ -27,6 +27,7 @@ import requests
 import requests.exceptions
 from atomicwrites import atomic_write
 from django.contrib import auth, messages
+from django.contrib.auth import get_user_model
 from django.contrib.auth.models import Group
 from django.core.exceptions import FieldDoesNotExist, PermissionDenied
 from django.core.files.storage import default_storage
@@ -56,6 +57,8 @@ def display_truncated_list(l, max_length=10):
 
 
 class DefaultAdapter:
+    user_class = get_user_model()
+
     def __init__(self, request=None):
         self.request = request
 
@@ -275,7 +278,7 @@ class DefaultAdapter:
         username_template = utils.get_setting(idp, 'USERNAME_TEMPLATE')
         try:
             username = force_text(username_template).format(realm=realm, attributes=saml_attributes, idp=idp)[
-                :30
+                : self.user_class._meta.get_field('username').max_length
             ]
         except ValueError:
             logger.error('mellon: invalid username template %r', username_template)

tests/test_default_adapter.py

@@ -68,9 +68,9 @@ def jane(db):
 def test_format_username(settings, idp, saml_attributes):
     adapter = DefaultAdapter()
     assert adapter.format_username(idp, {}) is None
-    assert adapter.format_username(idp, saml_attributes) == ('x' * 32 + '@saml')[:30]
+    assert adapter.format_username(idp, saml_attributes) == ('x' * 32 + '@saml')
     settings.MELLON_USERNAME_TEMPLATE = '{attributes[name_id_content]}'
-    assert adapter.format_username(idp, saml_attributes) == ('x' * 32)[:30]
+    assert adapter.format_username(idp, saml_attributes) == ('x' * 32)
     settings.MELLON_USERNAME_TEMPLATE = '{attributes[username][0]}'
     assert adapter.format_username(idp, saml_attributes) == 'foobar'
 
@@ -127,7 +127,7 @@ def test_provision_user_attributes(settings, django_user_model, idp, saml_attrib
         'last_name': '{attributes[last_name][0]}',
     }
     user = SAMLBackend().authenticate(saml_attributes=saml_attributes)
-    assert user.username == 'x' * 30
+    assert user.username == 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx@saml'
     assert user.first_name == 'Foo'
     assert user.last_name == 'Bar'
     assert user.email == 'test@example.net'