3 files changed, 49 insertions, 10 deletions
diff --git a/synapse/federation/federation_client.py b/synapse/federation/federation_client.py
index 3b85b135e0..bc3f96c1fc 100644
--- a/synapse/federation/federation_client.py
+++ b/synapse/federation/federation_client.py
@@ -1395,11 +1395,28 @@ class FederationClient(FederationBase):
         async def send_request(
             destination: str,
         ) -> Tuple[JsonDict, Sequence[JsonDict], Sequence[str]]:
-            res = await self.transport_layer.get_room_hierarchy(
-                destination=destination,
-                room_id=room_id,
-                suggested_only=suggested_only,
-            )
+            try:
+                res = await self.transport_layer.get_room_hierarchy(
+                    destination=destination,
+                    room_id=room_id,
+                    suggested_only=suggested_only,
+                )
+            except HttpResponseException as e:
+                # If an error is received that is due to an unrecognised endpoint,
+                # fallback to the unstable endpoint. Otherwise consider it a
+                # legitmate error and raise.
+                if not self._is_unknown_endpoint(e):
+                    raise
+
+                logger.debug(
+                    "Couldn't fetch room hierarchy with the v1 API, falling back to the unstable API"
+                )
+
+                res = await self.transport_layer.get_room_hierarchy_unstable(
+                    destination=destination,
+                    room_id=room_id,
+                    suggested_only=suggested_only,
+                )
 
             room = res.get("room")
             if not isinstance(room, dict):
@@ -1449,6 +1466,10 @@ class FederationClient(FederationBase):
             if e.code != 502:
                 raise
 
+            logger.debug(
+                "Couldn't fetch room hierarchy, falling back to the spaces API"
+            )
+
             # Fallback to the old federation API and translate the results if
             # no servers implement the new API.
             #
diff --git a/synapse/federation/transport/client.py b/synapse/federation/transport/client.py
index 0fea221165..fe29bcfd4b 100644
--- a/synapse/federation/transport/client.py
+++ b/synapse/federation/transport/client.py
@@ -1192,10 +1192,24 @@ class TransportLayerClient:
         )
 
     async def get_room_hierarchy(
-        self,
-        destination: str,
-        room_id: str,
-        suggested_only: bool,
+        self, destination: str, room_id: str, suggested_only: bool
+    ) -> JsonDict:
+        """
+        Args:
+            destination: The remote server
+            room_id: The room ID to ask about.
+            suggested_only: if True, only suggested rooms will be returned
+        """
+        path = _create_v1_path("/hierarchy/%s", room_id)
+
+        return await self.client.get_json(
+            destination=destination,
+            path=path,
+            args={"suggested_only": "true" if suggested_only else "false"},
+        )
+
+    async def get_room_hierarchy_unstable(
+        self, destination: str, room_id: str, suggested_only: bool
     ) -> JsonDict:
         """
         Args:
diff --git a/synapse/federation/transport/server/federation.py b/synapse/federation/transport/server/federation.py
index 2fdf6cc99e..66e915228c 100644
--- a/synapse/federation/transport/server/federation.py
+++ b/synapse/federation/transport/server/federation.py
@@ -611,7 +611,6 @@ class FederationSpaceSummaryServlet(BaseFederationServlet):
 
 
 class FederationRoomHierarchyServlet(BaseFederationServlet):
-    PREFIX = FEDERATION_UNSTABLE_PREFIX + "/org.matrix.msc2946"
     PATH = "/hierarchy/(?P<room_id>[^/]*)"
 
     def __init__(
@@ -637,6 +636,10 @@ class FederationRoomHierarchyServlet(BaseFederationServlet):
         )
 
 
+class FederationRoomHierarchyUnstableServlet(FederationRoomHierarchyServlet):
+    PREFIX = FEDERATION_UNSTABLE_PREFIX + "/org.matrix.msc2946"
+
+
 class RoomComplexityServlet(BaseFederationServlet):
     """
     Indicates to other servers how complex (and therefore likely
@@ -701,6 +704,7 @@ FEDERATION_SERVLET_CLASSES: Tuple[Type[BaseFederationServlet], ...] = (
     RoomComplexityServlet,
     FederationSpaceSummaryServlet,
     FederationRoomHierarchyServlet,
+    FederationRoomHierarchyUnstableServlet,
     FederationV1SendKnockServlet,
     FederationMakeKnockServlet,
 )