Stabilize support for MSC4326: Device masquerading for appservices (element-hq#19033)

Note: the code references MSC3202, which is what MSC4326 was split off
from. Only MSC4326 was accepted, MSC3202 wasn't yet.

Author: tulir

changelog.d/19033.feature

@@ -0,0 +1 @@
+Stabilized support for [MSC4326](https://github.com/matrix-org/matrix-spec-proposals/pull/4326): Device masquerading for appservices. Contributed by @tulir @ Beeper.

synapse/api/auth/base.py

@@ -302,12 +302,9 @@ async def get_appservice_user(
           (the user_id URI parameter allows an application service to masquerade
           any applicable user in its namespace)
         - what device the application service should be treated as controlling
-          (the device_id[^1] URI parameter allows an application service to masquerade
+          (the device_id URI parameter allows an application service to masquerade
           as any device that exists for the relevant user)
 
-        [^1] Unstable and provided by MSC3202.
-             Must use `org.matrix.msc3202.device_id` in place of `device_id` for now.
-
         Returns:
             the application service `Requester` of that request
 
@@ -319,7 +316,8 @@ async def get_appservice_user(
         - The returned device ID, if present, has been checked to be a valid device ID
           for the returned user ID.
         """
-        DEVICE_ID_ARG_NAME = b"org.matrix.msc3202.device_id"
+        # TODO: We can drop unstable support after 2026-01-01 (couple months after stable support)
+        UNSTABLE_DEVICE_ID_ARG_NAME = b"org.matrix.msc3202.device_id"
 
         app_service = self.store.get_app_service_by_token(access_token)
         if app_service is None:
@@ -341,13 +339,11 @@ async def get_appservice_user(
         else:
             effective_user_id = app_service.sender
 
-        effective_device_id: Optional[str] = None
-
-        if (
-            self.hs.config.experimental.msc3202_device_masquerading_enabled
-            and DEVICE_ID_ARG_NAME in request.args
-        ):
-            effective_device_id = request.args[DEVICE_ID_ARG_NAME][0].decode("utf8")
+        effective_device_id_args = request.args.get(
+            b"device_id", request.args.get(UNSTABLE_DEVICE_ID_ARG_NAME)
+        )
+        if effective_device_id_args:
+            effective_device_id = effective_device_id_args[0].decode("utf8")
             # We only just set this so it can't be None!
             assert effective_device_id is not None
             device_opt = await self.store.get_device(
@@ -359,6 +355,8 @@ async def get_appservice_user(
                     f"Application service trying to use a device that doesn't exist ('{effective_device_id}' for {effective_user_id})",
                     Codes.UNKNOWN_DEVICE,
                 )
+        else:
+            effective_device_id = None
 
         return create_requester(
             effective_user_id, app_service=app_service, device_id=effective_device_id

synapse/config/experimental.py

@@ -412,11 +412,6 @@ def read_config(
             "msc2409_to_device_messages_enabled", False
         )
 
-        # The portion of MSC3202 which is related to device masquerading.
-        self.msc3202_device_masquerading_enabled: bool = experimental.get(
-            "msc3202_device_masquerading", False
-        )
-
         # The portion of MSC3202 related to transaction extensions:
         # sending device list changes, one-time key counts and fallback key
         # usage to application services.

tests/api/test_auth.py

@@ -42,7 +42,6 @@
 from synapse.util.clock import Clock
 
 from tests import unittest
-from tests.unittest import override_config
 from tests.utils import mock_getRawHeaders
 
 
@@ -237,7 +236,6 @@ def test_get_user_by_req_appservice_valid_token_bad_user_id(self) -> None:
         request.requestHeaders.getRawHeaders = mock_getRawHeaders()
         self.get_failure(self.auth.get_user_by_req(request), AuthError)
 
-    @override_config({"experimental_features": {"msc3202_device_masquerading": True}})
     def test_get_user_by_req_appservice_valid_token_valid_device_id(self) -> None:
         """
         Tests that when an application service passes the device_id URL parameter
@@ -264,15 +262,14 @@ def test_get_user_by_req_appservice_valid_token_valid_device_id(self) -> None:
         request.getClientAddress.return_value.host = "127.0.0.1"
         request.args[b"access_token"] = [self.test_token]
         request.args[b"user_id"] = [masquerading_user_id]
-        request.args[b"org.matrix.msc3202.device_id"] = [masquerading_device_id]
+        request.args[b"device_id"] = [masquerading_device_id]
         request.requestHeaders.getRawHeaders = mock_getRawHeaders()
         requester = self.get_success(self.auth.get_user_by_req(request))
         self.assertEqual(
             requester.user.to_string(), masquerading_user_id.decode("utf8")
         )
         self.assertEqual(requester.device_id, masquerading_device_id.decode("utf8"))
 
-    @override_config({"experimental_features": {"msc3202_device_masquerading": True}})
     def test_get_user_by_req_appservice_valid_token_invalid_device_id(self) -> None:
         """
         Tests that when an application service passes the device_id URL parameter
@@ -299,7 +296,7 @@ def test_get_user_by_req_appservice_valid_token_invalid_device_id(self) -> None:
         request.getClientAddress.return_value.host = "127.0.0.1"
         request.args[b"access_token"] = [self.test_token]
         request.args[b"user_id"] = [masquerading_user_id]
-        request.args[b"org.matrix.msc3202.device_id"] = [masquerading_device_id]
+        request.args[b"device_id"] = [masquerading_device_id]
         request.requestHeaders.getRawHeaders = mock_getRawHeaders()
 
         failure = self.get_failure(self.auth.get_user_by_req(request), AuthError)