Add option to allow registrations that begin with '_'

changelog.d/18262.feature

@@ -0,0 +1 @@
+Add option to allow registrations that begin with `_`. Contributed by `_` (@hex5f).

docs/usage/configuration/config_documentation.md

@@ -2887,6 +2887,20 @@ Example configuration:
 inhibit_user_in_use_error: true
 ```
 ---
+### `allow_underscore_prefixed_registration`
+
+Whether users are allowed to register with a underscore-prefixed localpart.
+By default, AppServices use prefixes like `_example` to namespace their
+associated ghost users. If turned on, this may result in clashes or confusion.
+Useful when provisioning users from an external identity provider.
+
+Defaults to false.
+
+Example configuration:
+```yaml
+allow_underscore_prefixed_registration: false
+```
+---
 ## User session management
 ---
 ### `session_lifetime`

synapse/config/registration.py

@@ -162,6 +162,10 @@ def read_config(
             "disable_msisdn_registration", False
         )
 
+        self.allow_underscore_prefixed_localpart = config.get(
+            "allow_underscore_prefixed_localpart", False
+        )
+
         session_lifetime = config.get("session_lifetime")
         if session_lifetime is not None:
             session_lifetime = self.parse_duration(session_lifetime)

synapse/handlers/register.py

@@ -159,7 +159,10 @@ async def check_username(
         if not localpart:
             raise SynapseError(400, "User ID cannot be empty", Codes.INVALID_USERNAME)
 
-        if localpart[0] == "_":
+        if (
+            localpart[0] == "_"
+            and not self.hs.config.registration.allow_underscore_prefixed_localpart
+        ):
             raise SynapseError(
                 400, "User ID may not begin with _", Codes.INVALID_USERNAME
             )

tests/handlers/test_register.py

@@ -588,6 +588,29 @@ def test_register_not_support_user(self) -> None:
         d = self.store.is_support_user(user_id)
         self.assertFalse(self.get_success(d))
 
+    def test_underscore_localpart_rejected_by_default(self) -> None:
+        for invalid_user_id in ("_", "_prefixed"):
+            with self.subTest(invalid_user_id=invalid_user_id):
+                self.get_failure(
+                    self.handler.register_user(localpart=invalid_user_id),
+                    SynapseError,
+                )
+
+    @override_config(
+        {
+            "allow_underscore_prefixed_localpart": "true",
+        }
+    )
+    def test_underscore_localpart_allowed_if_configured(self) -> None:
+        for valid_user_id in ("_", "_prefixed"):
+            with self.subTest(valid_user_id=valid_user_id):
+                user_id = self.get_success(
+                    self.handler.register_user(
+                        localpart=valid_user_id,
+                    ),
+                )
+                self.assertEqual(user_id, f"@{valid_user_id}:test")
+
     def test_invalid_user_id(self) -> None:
         invalid_user_id = "^abcd"
         self.get_failure(