Check if alias already exists

src/github.com/matrix-org/dendrite/clientapi/readers/directory.go

@@ -113,6 +113,13 @@ func SetLocalAlias(
 		return httputil.LogThenError(req, err)
 	}
 
+	if queryRes.AliasExists {
+		return util.JSONResponse{
+			Code: 409,
+			JSON: jsonerror.Unknown("The alias " + alias + " already exists."),
+		}
+	}
+
 	return util.JSONResponse{
 		Code: 200,
 		JSON: struct{}{},

src/github.com/matrix-org/dendrite/roomserver/api/query.go

@@ -111,7 +111,10 @@ type SetRoomAliasRequest struct {
 }
 
 // SetRoomAliasResponse is a response to SetRoomAlias
-type SetRoomAliasResponse struct{}
+type SetRoomAliasResponse struct {
+	// Does the alias already refer to a room?
+	AliasExists bool `json:"alias_exists"`
+}
 
 // RoomserverQueryAPI is used to query information from the room server.
 type RoomserverQueryAPI interface {

src/github.com/matrix-org/dendrite/roomserver/query/query.go

@@ -172,8 +172,19 @@ func (r *RoomserverQueryAPI) SetRoomAlias(
 	request *api.SetRoomAliasRequest,
 	response *api.SetRoomAliasResponse,
 ) error {
+	// Check if the alias isn't already referring to a room
+	roomID, err := r.DB.GetRoomIDFromAlias(request.Alias)
+	if err != nil {
+		return err
+	}
+	if len(roomID) > 0 {
+		// If the alias already exists, stop the process
+		response.AliasExists = true
+		return nil
+	}
+	response.AliasExists = false
+
 	// Save the new alias
-	// TODO: Check if alias already exists
 	if err := r.DB.SetRoomAlias(request.Alias, request.RoomID); err != nil {
 		return err
 	}
@@ -183,7 +194,6 @@ func (r *RoomserverQueryAPI) SetRoomAlias(
 		return err
 	}
 
-	// We don't need to return anything in the response, so we don't edit it
 	return nil
 }