use a decorator for custom config

tests/unittest.py

@@ -168,8 +168,9 @@ class HomeserverTestCase(TestCase):
     * override default_config, to return a modified configuration dictionary for use
       by setup_test_homeserver.
 
-    * On a per-test basis, you can attach an 'extra_config' attribute, with a dictionary
-      containing additional configuration settings to be added to the basic config dict.
+    * On a per-test basis, you can use the @override_config decorator to give a
+      dictionary containing additional configuration settings to be added to the basic
+      config dict.
 
     Attributes:
         servlets (list[function]): List of servlet registration function.
@@ -185,9 +186,9 @@ class HomeserverTestCase(TestCase):
     def __init__(self, methodName, *args, **kwargs):
         super().__init__(methodName, *args, **kwargs)
 
-        # see if we have a custom config method for this test
+        # see if we have any additional config for this test
         method = getattr(self, methodName)
-        self._extra_config = getattr(method, "extra_config", None)
+        self._extra_config = getattr(method, "_extra_config", None)
 
     def setUp(self):
         """
@@ -299,8 +300,8 @@ def default_config(self, name="test"):
         """
         config = default_config(name)
 
-        # apply any additional config which was specified as an extra_config attribute
-        # on the test method..
+        # apply any additional config which was specified via the override_config
+        # decorator.
         if self._extra_config is not None:
             config.update(self._extra_config)
 
@@ -562,3 +563,27 @@ def attempt_wrong_password_login(self, username, password):
         )
         self.render(request)
         self.assertEqual(channel.code, 403, channel.result)
+
+
+def override_config(extra_config):
+    """A decorator which can be applied to test functions to give additional HS config
+
+    For use
+
+    For example:
+
+        class MyTestCase(HomeserverTestCase):
+            @override_config({"enable_registration": False, ...})
+            def test_foo(self):
+                ...
+
+    Args:
+        extra_config(dict): Additional config settings to be merged into the default
+            config dict before instantiating the test homeserver.
+    """
+
+    def decorator(func):
+        func._extra_config = extra_config
+        return func
+
+    return decorator