Try to process local restricted joins

roomserver/internal/input/input.go

@@ -214,8 +214,7 @@ func (r *Inputer) InputRoomEvents(
 	for _, task := range tasks {
 		if task.err != nil {
 			response.ErrMsg = task.err.Error()
-			_, rejected := task.err.(*gomatrixserverlib.NotAllowed)
-			response.NotAllowed = rejected
+			_, response.NotAllowed = task.err.(*gomatrixserverlib.NotAllowed)
 			return
 		}
 	}

roomserver/internal/perform/perform_join.go

@@ -16,6 +16,7 @@ package perform
 
 import (
 	"context"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"strings"
@@ -149,7 +150,7 @@ func (r *Joiner) performJoinRoomByAlias(
 	return r.performJoinRoomByID(ctx, req)
 }
 
-// TODO: Break this function up a bit
+// nolint:gocyclo
 func (r *Joiner) performJoinRoomByID(
 	ctx context.Context,
 	req *rsAPI.PerformJoinRequest,
@@ -240,6 +241,23 @@ func (r *Joiner) performJoinRoomByID(
 		return req.RoomIDOrAlias, joinedVia, err
 	}
 
+	// Check if the room is a restricted room. If so, update the event
+	// builder content.
+	if restricted, roomIDs, rerr := r.checkIfRestrictedJoin(ctx, req); rerr != nil {
+		return "", "", fmt.Errorf("r.performRestrictedJoinChecks: %w", rerr)
+	} else if restricted {
+		success := false
+		for _, roomID := range roomIDs {
+			if err = r.attemptRestrictedJoinUsingRoomID(ctx, req, roomID, &eb); err != nil {
+				continue
+			}
+			success = true
+		}
+		if !success {
+			return "", "", fmt.Errorf("restricted join failed")
+		}
+	}
+
 	// Try to construct an actual join event from the template.
 	// If this succeeds then it is a sign that the room already exists
 	// locally on the homeserver.
@@ -318,6 +336,81 @@ func (r *Joiner) performJoinRoomByID(
 	return req.RoomIDOrAlias, r.Cfg.Matrix.ServerName, nil
 }
 
+func (r *Joiner) checkIfRestrictedJoin(
+	ctx context.Context,
+	req *rsAPI.PerformJoinRequest,
+) (bool, []string, error) {
+	// Look up the join rules event for the room, so we can check if it is a
+	// restricted room or not.
+	joinRuleEvent, err := r.DB.GetStateEvent(ctx, req.RoomIDOrAlias, gomatrixserverlib.MRoomJoinRules, "")
+	if err != nil {
+		return false, nil, fmt.Errorf("r.DB.GetStateEvent: %w", err)
+	}
+	joinRuleContent := &gomatrixserverlib.JoinRuleContent{
+		JoinRule: gomatrixserverlib.Public,
+	}
+	if err = json.Unmarshal(joinRuleEvent.Content(), &joinRuleContent); err != nil {
+		return false, nil, fmt.Errorf("json.Unmarshal: %w", err)
+	}
+	roomIDs := make([]string, 0, len(joinRuleContent.Allow))
+	for _, allowed := range joinRuleContent.Allow {
+		if allowed.Type != gomatrixserverlib.MRoomMembership {
+			continue
+		}
+		roomIDs = append(roomIDs, allowed.RoomID)
+	}
+	return joinRuleContent.JoinRule != gomatrixserverlib.Restricted, roomIDs, nil
+}
+
+func (r *Joiner) attemptRestrictedJoinUsingRoomID(
+	ctx context.Context,
+	req *rsAPI.PerformJoinRequest,
+	roomID string,
+	eb *gomatrixserverlib.EventBuilder,
+) error {
+	roomInfo, err := r.DB.RoomInfo(ctx, roomID)
+	if err != nil {
+		return fmt.Errorf("r.DB.RoomInfo: %w", err)
+	}
+	powerLevelEvent, err := r.DB.GetStateEvent(ctx, roomID, gomatrixserverlib.MRoomPowerLevels, "")
+	if err != nil {
+		return fmt.Errorf("r.DB.GetStateEvent: %w", err)
+	}
+	powerLevels, err := powerLevelEvent.PowerLevels()
+	if err != nil {
+		return fmt.Errorf("powerLevelEvent.PowerLevels: %w", err)
+	}
+	eventNIDs, err := r.DB.GetMembershipEventNIDsForRoom(ctx, roomInfo.RoomNID, true, true)
+	if err != nil {
+		return fmt.Errorf("r.DB.GetMembershipEventNIDsForRoom: %w", err)
+	}
+	events, err := r.DB.Events(ctx, eventNIDs)
+	if err != nil {
+		return fmt.Errorf("r.DB.Events: %w", err)
+	}
+	for _, event := range events {
+		userID := *event.StateKey()
+		if userID == req.UserID {
+			continue
+		}
+		_, domain, err := gomatrixserverlib.SplitID('@', userID)
+		if err != nil || domain != r.ServerName {
+			continue
+		}
+		if powerLevels.UserLevel(userID) < powerLevels.Invite {
+			continue
+		}
+		if err := eb.SetContent(map[string]string{
+			"membership":                       gomatrixserverlib.Join,
+			"join_authorised_via_users_server": userID,
+		}); err != nil {
+			return fmt.Errorf("eb.SetContent: %w", err)
+		}
+		return nil
+	}
+	return fmt.Errorf("no suitable users found in the room")
+}
+
 func (r *Joiner) performFederatedJoinRoomByID(
 	ctx context.Context,
 	req *rsAPI.PerformJoinRequest,