Pass join errors through internal API boundaries

Required for certain invite sytests. We will need to think of a
better way of handling this going forwards.

clientapi/routing/joinroom.go

@@ -15,6 +15,7 @@
 package routing
 
 import (
+	"errors"
 	"net/http"
 
 	"github.com/matrix-org/dendrite/clientapi/httputil"
@@ -62,11 +63,32 @@ func JoinRoomByIDOrAlias(
 	}
 
 	// Ask the roomserver to perform the join.
-	if err := rsAPI.PerformJoin(req.Context(), &joinReq, &joinRes); err != nil {
+	err = rsAPI.PerformJoin(req.Context(), &joinReq, &joinRes)
+	// Handle known errors first, if this is 0 then there will be no matches (eg on success)
+	switch joinRes.Error {
+	case roomserverAPI.JoinErrorBadRequest:
 		return util.JSONResponse{
 			Code: http.StatusBadRequest,
-			JSON: jsonerror.Unknown(err.Error()),
+			JSON: jsonerror.Unknown(joinRes.ErrMsg),
 		}
+	case roomserverAPI.JoinErrorNoRoom:
+		return util.JSONResponse{
+			Code: http.StatusNotFound,
+			JSON: jsonerror.NotFound(joinRes.ErrMsg),
+		}
+	case roomserverAPI.JoinErrorNotAllowed:
+		return util.JSONResponse{
+			Code: http.StatusForbidden,
+			JSON: jsonerror.Forbidden(joinRes.ErrMsg),
+		}
+	}
+	// this is always populated on generic errors
+	if joinRes.ErrMsg != "" {
+		return util.ErrorResponse(errors.New(joinRes.ErrMsg))
+	}
+	// this is set on network errors in polylith mode
+	if err != nil {
+		return util.ErrorResponse(err)
 	}
 
 	return util.JSONResponse{

roomserver/api/perform.go

@@ -5,6 +5,17 @@ import (
 	"github.com/matrix-org/util"
 )
 
+type JoinError int
+
+const (
+	// JoinErrorNotAllowed means the user is not allowed to join this room (e.g join_rule:invite or banned)
+	JoinErrorNotAllowed JoinError = 1
+	// JoinErrorBadRequest means the request was wrong in some way (invalid user ID, wrong server, etc)
+	JoinErrorBadRequest JoinError = 2
+	// JoinErrorNoRoom means that the room being joined doesn't exist.
+	JoinErrorNoRoom JoinError = 3
+)
+
 type PerformJoinRequest struct {
 	RoomIDOrAlias string                         `json:"room_id_or_alias"`
 	UserID        string                         `json:"user_id"`
@@ -13,7 +24,12 @@ type PerformJoinRequest struct {
 }
 
 type PerformJoinResponse struct {
+	// The room ID, populated on success.
 	RoomID string `json:"room_id"`
+	// The reason why the join failed. Can be blank.
+	Error JoinError `json:"error"`
+	// Debugging description of the error. Always present on failure.
+	ErrMsg string `json:"err_msg"`
 }
 
 type PerformLeaveRequest struct {

roomserver/internal/perform_join.go

@@ -2,6 +2,7 @@ package internal
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"strings"
 	"time"
@@ -21,9 +22,11 @@ func (r *RoomserverInternalAPI) PerformJoin(
 ) error {
 	_, domain, err := gomatrixserverlib.SplitID('@', req.UserID)
 	if err != nil {
+		res.Error = api.JoinErrorBadRequest
 		return fmt.Errorf("Supplied user ID %q in incorrect format", req.UserID)
 	}
 	if domain != r.Cfg.Matrix.ServerName {
+		res.Error = api.JoinErrorBadRequest
 		return fmt.Errorf("User %q does not belong to this homeserver", req.UserID)
 	}
 	if strings.HasPrefix(req.RoomIDOrAlias, "!") {
@@ -32,6 +35,7 @@ func (r *RoomserverInternalAPI) PerformJoin(
 	if strings.HasPrefix(req.RoomIDOrAlias, "#") {
 		return r.performJoinRoomByAlias(ctx, req, res)
 	}
+	res.Error = api.JoinErrorBadRequest
 	return fmt.Errorf("Room ID or alias %q is invalid", req.RoomIDOrAlias)
 }
 
@@ -99,6 +103,7 @@ func (r *RoomserverInternalAPI) performJoinRoomByID(
 	// Get the domain part of the room ID.
 	_, domain, err := gomatrixserverlib.SplitID('!', req.RoomIDOrAlias)
 	if err != nil {
+		res.Error = api.JoinErrorBadRequest
 		return fmt.Errorf("Room ID %q is invalid", req.RoomIDOrAlias)
 	}
 	req.ServerNames = append(req.ServerNames, domain)
@@ -198,6 +203,10 @@ func (r *RoomserverInternalAPI) performJoinRoomByID(
 			}
 			inputRes := api.InputRoomEventsResponse{}
 			if err = r.InputRoomEvents(ctx, &inputReq, &inputRes); err != nil {
+				var notAllowed *gomatrixserverlib.NotAllowed
+				if errors.As(err, &notAllowed) {
+					res.Error = api.JoinErrorNotAllowed
+				}
 				return fmt.Errorf("r.InputRoomEvents: %w", err)
 			}
 		}
@@ -207,6 +216,7 @@ func (r *RoomserverInternalAPI) performJoinRoomByID(
 		// room. If it is then there's nothing more to do - the room just
 		// hasn't been created yet.
 		if domain == r.Cfg.Matrix.ServerName {
+			res.Error = api.JoinErrorNoRoom
 			return fmt.Errorf("Room ID %q does not exist", req.RoomIDOrAlias)
 		}
 

roomserver/inthttp/server.go

@@ -34,7 +34,7 @@ func AddRoutes(r api.RoomserverInternalAPI, internalAPIMux *mux.Router) {
 				return util.MessageResponse(http.StatusBadRequest, err.Error())
 			}
 			if err := r.PerformJoin(req.Context(), &request, &response); err != nil {
-				return util.ErrorResponse(err)
+				response.ErrMsg = err.Error()
 			}
 			return util.JSONResponse{Code: http.StatusOK, JSON: &response}
 		}),