Add a config flag to inhibit `M_USER_IN_USE` during registration (#11743)

This is mostly motivated by the tchap use case, where usernames are automatically generated from the user's email address (in a way that allows figuring out the email address from the username). Therefore, it's an issue if we respond to requests on /register and /register/available with M_USER_IN_USE, because it can potentially leak email addresses (which include the user's real name and place of work).

This commit adds a flag to inhibit the M_USER_IN_USE errors that are raised both by /register/available, and when providing a username early into the registration process. This error will still be raised if the user completes the registration process but the username conflicts. This is particularly useful when using modules (https://github.com/matrix-org/synapse/pull/11790 adds a module callback to set the username of users at registration) or SSO, since they can ensure the username is unique.

More context is available in the PR that introduced this behaviour to synapse-dinsic: matrix-org/synapse-dinsic#48 - as well as the issue in the matrix-dinsic repo: matrix-org/matrix-dinsic#476

1 files changed, 41 insertions, 0 deletions

diff --git a/tests/rest/client/test_register.py b/tests/rest/client/test_register.py
index 6e7c0f11df..407dd32a73 100644
--- a/tests/rest/client/test_register.py
+++ b/tests/rest/client/test_register.py
@@ -726,6 +726,47 @@ class RegisterRestServletTestCase(unittest.HomeserverTestCase):
             {"errcode": "M_UNKNOWN", "error": "Unable to parse email address"},
         )
 
+    @override_config(
+        {
+            "inhibit_user_in_use_error": True,
+        }
+    )
+    def test_inhibit_user_in_use_error(self):
+        """Tests that the 'inhibit_user_in_use_error' configuration flag behaves
+        correctly.
+        """
+        username = "arthur"
+
+        # Manually register the user, so we know the test isn't passing because of a lack
+        # of clashing.
+        reg_handler = self.hs.get_registration_handler()
+        self.get_success(reg_handler.register_user(username))
+
+        # Check that /available correctly ignores the username provided despite the
+        # username being already registered.
+        channel = self.make_request("GET", "register/available?username=" + username)
+        self.assertEquals(200, channel.code, channel.result)
+
+        # Test that when starting a UIA registration flow the request doesn't fail because
+        # of a conflicting username
+        channel = self.make_request(
+            "POST",
+            "register",
+            {"username": username, "type": "m.login.password", "password": "foo"},
+        )
+        self.assertEqual(channel.code, 401)
+        self.assertIn("session", channel.json_body)
+
+        # Test that finishing the registration fails because of a conflicting username.
+        session = channel.json_body["session"]
+        channel = self.make_request(
+            "POST",
+            "register",
+            {"auth": {"session": session, "type": LoginType.DUMMY}},
+        )
+        self.assertEqual(channel.code, 400, channel.json_body)
+        self.assertEqual(channel.json_body["errcode"], Codes.USER_IN_USE)
+
 
 class AccountValidityTestCase(unittest.HomeserverTestCase):