Add via param to hierarchy enpoint (#18070)

### Pull Request Checklist

Implementation of
[MSC4235](matrix-org/matrix-spec-proposals#4235)
as per suggestion in [pull request
17750](#17750 (comment)).

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https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->

* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
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- Feel free to credit yourself, by adding a sentence "Contributed by
@github_username." or "Contributed by [Your Name]." to the end of the
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* [x] [Code
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correct
(run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

---------

Co-authored-by: Quentin Gliech <quenting@element.io>

Author: kfiven

changelog.d/18070.feature

@@ -0,0 +1 @@
+Support for [MSC4235](https://github.com/matrix-org/matrix-spec-proposals/pull/4235): via query param for hierarchy endpoint. Contributed by Krishan (@kfiven).

synapse/config/experimental.py

@@ -561,6 +561,9 @@ def read_config(
         # MSC4076: Add `disable_badge_count`` to pusher configuration
         self.msc4076_enabled: bool = experimental.get("msc4076_enabled", False)
 
+        # MSC4235: Add `via` param to hierarchy endpoint
+        self.msc4235_enabled: bool = experimental.get("msc4235_enabled", False)
+
         # MSC4263: Preventing MXID enumeration via key queries
         self.msc4263_limit_key_queries_to_users_who_share_rooms = experimental.get(
             "msc4263_limit_key_queries_to_users_who_share_rooms",

synapse/handlers/room_summary.py

@@ -111,7 +111,15 @@ def __init__(self, hs: "HomeServer"):
         # If a user tries to fetch the same page multiple times in quick succession,
         # only process the first attempt and return its result to subsequent requests.
         self._pagination_response_cache: ResponseCache[
-            Tuple[str, str, bool, Optional[int], Optional[int], Optional[str]]
+            Tuple[
+                str,
+                str,
+                bool,
+                Optional[int],
+                Optional[int],
+                Optional[str],
+                Optional[Tuple[str, ...]],
+            ]
         ] = ResponseCache(
             hs.get_clock(),
             "get_room_hierarchy",
@@ -126,6 +134,7 @@ async def get_room_hierarchy(
         max_depth: Optional[int] = None,
         limit: Optional[int] = None,
         from_token: Optional[str] = None,
+        remote_room_hosts: Optional[Tuple[str, ...]] = None,
     ) -> JsonDict:
         """
         Implementation of the room hierarchy C-S API.
@@ -143,6 +152,9 @@ async def get_room_hierarchy(
             limit: An optional limit on the number of rooms to return per
                 page. Must be a positive integer.
             from_token: An optional pagination token.
+            remote_room_hosts: An optional list of remote homeserver server names. If defined,
+                each host will be used to try and fetch the room hierarchy. Must be a tuple so
+                that it can be hashed by the `RoomSummaryHandler._pagination_response_cache`.
 
         Returns:
             The JSON hierarchy dictionary.
@@ -162,6 +174,7 @@ async def get_room_hierarchy(
                 max_depth,
                 limit,
                 from_token,
+                remote_room_hosts,
             ),
             self._get_room_hierarchy,
             requester.user.to_string(),
@@ -170,6 +183,7 @@ async def get_room_hierarchy(
             max_depth,
             limit,
             from_token,
+            remote_room_hosts,
         )
 
     async def _get_room_hierarchy(
@@ -180,6 +194,7 @@ async def _get_room_hierarchy(
         max_depth: Optional[int] = None,
         limit: Optional[int] = None,
         from_token: Optional[str] = None,
+        remote_room_hosts: Optional[Tuple[str, ...]] = None,
     ) -> JsonDict:
         """See docstring for SpaceSummaryHandler.get_room_hierarchy."""
 
@@ -199,7 +214,7 @@ async def _get_room_hierarchy(
 
         if not local_room:
             room_hierarchy = await self._summarize_remote_room_hierarchy(
-                _RoomQueueEntry(requested_room_id, ()),
+                _RoomQueueEntry(requested_room_id, remote_room_hosts or ()),
                 False,
             )
             root_room_entry = room_hierarchy[0]
@@ -240,7 +255,7 @@ async def _get_room_hierarchy(
             processed_rooms = set(pagination_session["processed_rooms"])
         else:
             # The queue of rooms to process, the next room is last on the stack.
-            room_queue = [_RoomQueueEntry(requested_room_id, ())]
+            room_queue = [_RoomQueueEntry(requested_room_id, remote_room_hosts or ())]
 
             # Rooms we have already processed.
             processed_rooms = set()

synapse/rest/client/room.py

@@ -1538,6 +1538,7 @@ def __init__(self, hs: "HomeServer"):
         super().__init__()
         self._auth = hs.get_auth()
         self._room_summary_handler = hs.get_room_summary_handler()
+        self.msc4235_enabled = hs.config.experimental.msc4235_enabled
 
     async def on_GET(
         self, request: SynapseRequest, room_id: str
@@ -1547,13 +1548,23 @@ async def on_GET(
         max_depth = parse_integer(request, "max_depth")
         limit = parse_integer(request, "limit")
 
+        # twisted.web.server.Request.args is incorrectly defined as Optional[Any]
+        remote_room_hosts = None
+        if self.msc4235_enabled:
+            args: Dict[bytes, List[bytes]] = request.args  # type: ignore
+            via_param = parse_strings_from_args(
+                args, "org.matrix.msc4235.via", required=False
+            )
+            remote_room_hosts = tuple(via_param or [])
+
         return 200, await self._room_summary_handler.get_room_hierarchy(
             requester,
             room_id,
             suggested_only=parse_boolean(request, "suggested_only", default=False),
             max_depth=max_depth,
             limit=limit,
             from_token=parse_string(request, "from"),
+            remote_room_hosts=remote_room_hosts,
         )
 
 

tests/handlers/test_room_summary.py

@@ -1080,6 +1080,62 @@ async def get_room_hierarchy(
             self.assertEqual(federation_requests, 2)
             self._assert_hierarchy(result, expected)
 
+    def test_fed_remote_room_hosts(self) -> None:
+        """
+        Test if requested room is available over federation using via's.
+        """
+        fed_hostname = self.hs.hostname + "2"
+        fed_space = "#fed_space:" + fed_hostname
+        fed_subroom = "#fed_sub_room:" + fed_hostname
+
+        remote_room_hosts = tuple(fed_hostname)
+
+        requested_room_entry = _RoomEntry(
+            fed_space,
+            {
+                "room_id": fed_space,
+                "world_readable": True,
+                "join_rule": "public",
+                "room_type": RoomTypes.SPACE,
+            },
+            [
+                {
+                    "type": EventTypes.SpaceChild,
+                    "room_id": fed_space,
+                    "state_key": fed_subroom,
+                    "content": {"via": [fed_hostname]},
+                }
+            ],
+        )
+        child_room = {
+            "room_id": fed_subroom,
+            "world_readable": True,
+            "join_rule": "public",
+        }
+
+        async def summarize_remote_room_hierarchy(
+            _self: Any, room: Any, suggested_only: bool
+        ) -> Tuple[Optional[_RoomEntry], Dict[str, JsonDict], Set[str]]:
+            return requested_room_entry, {fed_subroom: child_room}, set()
+
+        expected = [
+            (fed_space, [fed_subroom]),
+            (fed_subroom, ()),
+        ]
+
+        with mock.patch(
+            "synapse.handlers.room_summary.RoomSummaryHandler._summarize_remote_room_hierarchy",
+            new=summarize_remote_room_hierarchy,
+        ):
+            result = self.get_success(
+                self.handler.get_room_hierarchy(
+                    create_requester(self.user),
+                    fed_space,
+                    remote_room_hosts=remote_room_hosts,
+                )
+            )
+        self._assert_hierarchy(result, expected)
+
 
 class RoomSummaryTestCase(unittest.HomeserverTestCase):
     servlets = [