Fix spaces over federation (#3347)

Fixes #2504

 A few issues with the previous iteration:
- We never returned `inaccessible_children`, which (if I read the code
correctly), made Synapse raise an error and thus not returning the
requested rooms
- For restricted rooms, we didn't return the list of allowed rooms

roomserver/api/api.go

@@ -141,7 +141,12 @@ type QueryRoomHierarchyAPI interface {
 	//
 	// If returned walker is nil, then there are no more rooms left to traverse. This method does not modify the provided walker, so it
 	// can be cached.
-	QueryNextRoomHierarchyPage(ctx context.Context, walker RoomHierarchyWalker, limit int) ([]fclient.RoomHierarchyRoom, *RoomHierarchyWalker, error)
+	QueryNextRoomHierarchyPage(ctx context.Context, walker RoomHierarchyWalker, limit int) (
+		hierarchyRooms []fclient.RoomHierarchyRoom,
+		inaccessibleRooms []string,
+		hierarchyWalker *RoomHierarchyWalker,
+		err error,
+	)
 }
 
 type QueryMembershipAPI interface {

roomserver/api/wrapper.go

@@ -189,7 +189,7 @@ func PopulatePublicRooms(ctx context.Context, roomIDs []string, rsAPI QueryBulkS
 			RoomID: roomID,
 		}
 		joinCount := 0
-		var joinRule, guestAccess string
+		var guestAccess string
 		for tuple, contentVal := range data {
 			if tuple.EventType == spec.MRoomMember && contentVal == "join" {
 				joinCount++
@@ -210,12 +210,12 @@ func PopulatePublicRooms(ctx context.Context, roomIDs []string, rsAPI QueryBulkS
 				pub.WorldReadable = contentVal == "world_readable"
 			// need both of these to determine whether guests can join
 			case joinRuleTuple:
-				joinRule = contentVal
+				pub.JoinRule = contentVal
 			case guestTuple:
 				guestAccess = contentVal
 			}
 		}
-		if joinRule == spec.Public && guestAccess == "can_join" {
+		if pub.JoinRule == spec.Public && guestAccess == "can_join" {
 			pub.GuestCanJoin = true
 		}
 		pub.JoinedMembersCount = joinCount