Fix bug in `/sync` response for archived rooms (#16932)

This PR fixes a very, very niche edge-case, but I've got some more work
coming which will otherwise make the problem worse.

The bug happens when the syncing user leaves a room, and has a sync
filter which includes "left" rooms, but sets the timeline limit to 0. In
that case, the state returned in the `state` section is calculated
incorrectly.

The fix is to pass a token corresponding to the point that the user
leaves the room through to `compute_state_delta`.

1 files changed, 11 insertions, 7 deletions

diff --git a/tests/rest/client/utils.py b/tests/rest/client/utils.py
index daa68d78b9..fe00afe198 100644
--- a/tests/rest/client/utils.py
+++ b/tests/rest/client/utils.py
@@ -170,8 +170,8 @@ class RestHelper:
         targ: Optional[str] = None,
         expect_code: int = HTTPStatus.OK,
         tok: Optional[str] = None,
-    ) -> None:
-        self.change_membership(
+    ) -> JsonDict:
+        return self.change_membership(
             room=room,
             src=src,
             targ=targ,
@@ -189,8 +189,8 @@ class RestHelper:
         appservice_user_id: Optional[str] = None,
         expect_errcode: Optional[Codes] = None,
         expect_additional_fields: Optional[dict] = None,
-    ) -> None:
-        self.change_membership(
+    ) -> JsonDict:
+        return self.change_membership(
             room=room,
             src=user,
             targ=user,
@@ -242,8 +242,8 @@ class RestHelper:
         user: Optional[str] = None,
         expect_code: int = HTTPStatus.OK,
         tok: Optional[str] = None,
-    ) -> None:
-        self.change_membership(
+    ) -> JsonDict:
+        return self.change_membership(
             room=room,
             src=user,
             targ=user,
@@ -282,7 +282,7 @@ class RestHelper:
         expect_code: int = HTTPStatus.OK,
         expect_errcode: Optional[str] = None,
         expect_additional_fields: Optional[dict] = None,
-    ) -> None:
+    ) -> JsonDict:
         """
         Send a membership state event into a room.
 
@@ -298,6 +298,9 @@ class RestHelper:
                 using an application service access token in `tok`.
             expect_code: The expected HTTP response code
             expect_errcode: The expected Matrix error code
+
+        Returns:
+            The JSON response
         """
         temp_id = self.auth_user_id
         self.auth_user_id = src
@@ -356,6 +359,7 @@ class RestHelper:
                 )
 
         self.auth_user_id = temp_id
+        return channel.json_body
 
     def send(
         self,