Require AppserviceRegistrationType (#9548)

This change ensures that the appservice registration behaviour follows the spec. We decided to do this for Dendrite, so it made sense to also make a PR for synapse to correct the behaviour.

synapse/rest/client/v2_alpha/register.py

@@ -13,7 +13,6 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-
 import hmac
 import logging
 import random
@@ -22,7 +21,7 @@ from typing import List, Union
 import synapse
 import synapse.api.auth
 import synapse.types
-from synapse.api.constants import LoginType
+from synapse.api.constants import APP_SERVICE_REGISTRATION_TYPE, LoginType
 from synapse.api.errors import (
     Codes,
     InteractiveAuthIncompleteError,
@@ -430,15 +429,20 @@ class RegisterRestServlet(RestServlet):
                 raise SynapseError(400, "Invalid username")
             desired_username = body["username"]
 
-        appservice = None
-        if self.auth.has_access_token(request):
-            appservice = self.auth.get_appservice_by_req(request)
-
         # fork off as soon as possible for ASes which have completely
         # different registration flows to normal users
 
         # == Application Service Registration ==
-        if appservice:
+        if body.get("type") == APP_SERVICE_REGISTRATION_TYPE:
+            if not self.auth.has_access_token(request):
+                raise SynapseError(
+                    400,
+                    "Appservice token must be provided when using a type of m.login.application_service",
+                )
+
+            # Verify the AS
+            self.auth.get_appservice_by_req(request)
+
             # Set the desired user according to the AS API (which uses the
             # 'user' key not 'username'). Since this is a new addition, we'll
             # fallback to 'username' if they gave one.
@@ -459,6 +463,11 @@ class RegisterRestServlet(RestServlet):
             )
 
             return 200, result
+        elif self.auth.has_access_token(request):
+            raise SynapseError(
+                400,
+                "An access token should not be provided on requests to /register (except if type is m.login.application_service)",
+            )
 
         # == Normal User Registration == (everyone else)
         if not self._registration_enabled: