Add option to allow registrations that begin with _

This commit resolves a long-standing question dating back to Jira.

> leonerd: Should this be a configuration option? We don't want to allow
> new user signups beginning with `_` on matrix.org, because of clashes
> with ASes; but maybe other sites wouldn't mind that?

As this may result in clashes with AppService namespaces, it is
disallowed by default. Homeservers which provision users from an
external identity provider may find this useful.

Fixes: SYN-738

Author: hex5f

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(