Return Restricted to determine if the room was restricted or not

federationapi/routing/join.go

@@ -386,18 +386,36 @@ func checkRestrictedJoin(
 	if err := rsAPI.QueryRestrictedJoinAllowed(httpReq.Context(), req, res); err != nil {
 		return nil, err
 	}
+
 	switch {
+	case !res.Restricted:
+		// The join rules for the room don't restrict membership.
+		return nil, nil
+
 	case !res.Resident:
+		// The join rules restrict membership but our server isn't currently
+		// joined to all of the allowed rooms, so we can't actually decide
+		// whether or not to allow the user to join. This error code should
+		// tell the joining server to try joining via another resident server
+		// instead.
 		return &util.JSONResponse{
 			Code: http.StatusBadRequest,
 			JSON: jsonerror.UnableToAuthoriseJoin("This server cannot authorise the join."),
 		}, nil
+
 	case !res.Allowed:
+		// The join rules restrict membership, our server is in the relevant
+		// rooms and the user wasn't joined to join any of the allowed rooms
+		// and therefore can't join this room.
 		return &util.JSONResponse{
 			Code: http.StatusForbidden,
 			JSON: jsonerror.Forbidden("You are not joined to any matching rooms."),
 		}, nil
+
 	default:
+		// The join rules restrict membership, our server is in the relevant
+		// rooms and the user was allowed to join because they belong to one
+		// of the allowed rooms.
 		return nil, nil
 	}
 }

roomserver/api/query.go

@@ -354,6 +354,7 @@ type QueryRestrictedJoinAllowedRequest struct {
 }
 
 type QueryRestrictedJoinAllowedResponse struct {
+	Restricted bool `json:"restricted"` // Is the room membership restricted?
 	Resident   bool `json:"resident"`   // Is our homeserver in the relevant rooms?
 	Allowed    bool `json:"allowed"`    // Is the join allowed by the rules?
 }

roomserver/internal/query/query.go

@@ -787,7 +787,8 @@ func (r *Queryer) QueryRestrictedJoinAllowed(ctx context.Context, req *api.Query
 		return fmt.Errorf("json.Unmarshal: %w", err)
 	}
 	// If the join rule isn't "restricted" then there's nothing more to do.
-	if joinRules.JoinRule != gomatrixserverlib.Restricted {
+	res.Restricted = joinRules.JoinRule == gomatrixserverlib.Restricted
+	if !res.Restricted {
 		return nil
 	}
 	// Start off by populating the "resident" flag in the response. If we