Add support for via query parameter from MSC4156 (#17322)

This adds support for the `via` query parameter from
matrix-org/matrix-spec-proposals#4156.

changelog.d/17322.feature

@@ -0,0 +1 @@
+Add support for via query parameter from MSC415.

synapse/config/experimental.py

@@ -439,3 +439,6 @@ def read_config(self, config: JsonDict, **kwargs: Any) -> None:
 
         # MSC4151: Report room API (Client-Server API)
         self.msc4151_enabled: bool = experimental.get("msc4151_enabled", False)
+
+        # MSC4156: Migrate server_name to via
+        self.msc4156_enabled: bool = experimental.get("msc4156_enabled", False)

synapse/rest/client/knock.py

@@ -53,6 +53,7 @@ def __init__(self, hs: "HomeServer"):
         super().__init__()
         self.room_member_handler = hs.get_room_member_handler()
         self.auth = hs.get_auth()
+        self._support_via = hs.config.experimental.msc4156_enabled
 
     async def on_POST(
         self,
@@ -74,6 +75,13 @@ async def on_POST(
             remote_room_hosts = parse_strings_from_args(
                 args, "server_name", required=False
             )
+            if self._support_via:
+                remote_room_hosts = parse_strings_from_args(
+                    args,
+                    "org.matrix.msc4156.via",
+                    default=remote_room_hosts,
+                    required=False,
+                )
         elif RoomAlias.is_valid(room_identifier):
             handler = self.room_member_handler
             room_alias = RoomAlias.from_string(room_identifier)

synapse/rest/client/room.py

@@ -417,6 +417,7 @@ def __init__(self, hs: "HomeServer"):
         super().__init__(hs)
         super(ResolveRoomIdMixin, self).__init__(hs)  # ensure the Mixin is set up
         self.auth = hs.get_auth()
+        self._support_via = hs.config.experimental.msc4156_enabled
 
     def register(self, http_server: HttpServer) -> None:
         # /join/$room_identifier[/$txn_id]
@@ -435,6 +436,13 @@ async def _do(
         # twisted.web.server.Request.args is incorrectly defined as Optional[Any]
         args: Dict[bytes, List[bytes]] = request.args  # type: ignore
         remote_room_hosts = parse_strings_from_args(args, "server_name", required=False)
+        if self._support_via:
+            remote_room_hosts = parse_strings_from_args(
+                args,
+                "org.matrix.msc4156.via",
+                default=remote_room_hosts,
+                required=False,
+            )
         room_id, remote_room_hosts = await self.resolve_room_id(
             room_identifier,
             remote_room_hosts,