use pointers for querying sender ID

performinvite.go

@@ -96,8 +96,8 @@ func PerformInvite(ctx context.Context, input PerformInviteInput, fedClient Fede
 		return nil, err
 	}
 
-	if invitedSenderID != "" {
-		err = abortIfAlreadyJoined(ctx, input.RoomID, invitedSenderID, input.MembershipQuerier)
+	if invitedSenderID != nil {
+		err = abortIfAlreadyJoined(ctx, input.RoomID, *invitedSenderID, input.MembershipQuerier)
 		if err != nil {
 			return nil, err
 		}

performinvite_test.go

@@ -13,8 +13,9 @@ import (
 	"golang.org/x/crypto/ed25519"
 )
 
-func SenderIDForUserTest(roomID spec.RoomID, userID spec.UserID) (spec.SenderID, error) {
-	return spec.SenderID(userID.String()), nil
+func SenderIDForUserTest(roomID spec.RoomID, userID spec.UserID) (*spec.SenderID, error) {
+	senderID := spec.SenderID(userID.String())
+	return &senderID, nil
 }
 
 func CreateSenderID(ctx context.Context, userID spec.UserID, roomID spec.RoomID, roomVersion string) (spec.SenderID, ed25519.PrivateKey, error) {

spec/senderid.go

@@ -23,7 +23,7 @@
 type SenderID string
 
 type UserIDForSender func(roomID RoomID, senderID SenderID) (*UserID, error)
-type SenderIDForUser func(roomID RoomID, userID UserID) (SenderID, error)
+type SenderIDForUser func(roomID RoomID, userID UserID) (*SenderID, error)
 
 // CreateSenderID is a function used to create the pseudoID private key.
 type CreateSenderID func(ctx context.Context, userID UserID, roomID RoomID, roomVersion string) (SenderID, ed25519.PrivateKey, error)
@@ -42,3 +42,14 @@
 	}
 	return res, nil
 }
+
+func (s SenderID) IsUserID() bool {
+	// Key is base64, @ is not a valid base64 char
+	// So if string starts with @, then this sender ID must
+	// be a user ID
+	return string(s)[0] == '@'
+}
+
+func (s SenderID) IsPseudoID() bool {
+	return !s.IsUserID()
+}