Add test for num_user limit via MLC

tests/oauth/test_users.py

@@ -0,0 +1,48 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+from CTFd.models import Users
+from CTFd.utils import set_config
+from tests.helpers import create_ctfd, destroy_ctfd, login_with_mlc, register_user
+
+
+def test_num_users_oauth_limit():
+    """Only num_users users can be created even via MLC"""
+    app = create_ctfd()
+    app.config.update(
+        {
+            "OAUTH_CLIENT_ID": "ctfd_testing_client_id",
+            "OAUTH_CLIENT_SECRET": "ctfd_testing_client_secret",
+            "OAUTH_AUTHORIZATION_ENDPOINT": "http://auth.localhost/oauth/authorize",
+            "OAUTH_TOKEN_ENDPOINT": "http://auth.localhost/oauth/token",
+            "OAUTH_API_ENDPOINT": "http://api.localhost/user",
+        }
+    )
+    with app.app_context():
+        register_user(app)
+        # There should be the admin and our registered user
+        assert Users.query.count() == 2
+        set_config("num_users", 1)
+
+        # This registration should fail and we should still have 2 users
+        login_with_mlc(
+            app,
+            name="foobarbaz",
+            email="foobarbaz@a.com",
+            oauth_id=111,
+            scope="profile",
+            raise_for_error=False,
+        )
+        assert Users.query.count() == 2
+
+        # We increment num_users to 2 and then login again
+        set_config("num_users", 2)
+        login_with_mlc(
+            app,
+            name="foobarbaz",
+            email="foobarbaz@a.com",
+            oauth_id=111,
+            scope="profile",
+        )
+        # The above login should have succeeded
+        assert Users.query.count() == 3
+    destroy_ctfd(app)