Fix code duplicate bug

Signed-off-by: Asai Neko <sugar@sne.moe>

data/user.go

@@ -165,18 +165,25 @@ func (self *User) FastListUsers(limit, offset int64) (*[]UserSearchDoc, error) {
 }
 
 func (self *User) GenCheckinCode(eventId uuid.UUID) (*string, error) {
-	rng := rand.New(rand.NewSource(time.Now().UnixNano()))
-	randomNumber := rng.Intn(900000) + 100000
-	randNumString := fmt.Sprintf("%06d", randomNumber)
 	ctx := context.Background()
 	ttl := viper.GetDuration("ttl.checkin_code_ttl")
-	Redis.Set(
-		ctx,
-		"checkin_code:"+randNumString,
-		self.UserId.String()+":"+eventId.String(),
-		ttl,
-	)
-	return &randNumString, nil
+	rng := rand.New(rand.NewSource(time.Now().UnixNano()))
+
+	for {
+		code := fmt.Sprintf("%06d", rng.Intn(900000)+100000)
+		ok, err := Redis.SetNX(
+			ctx,
+			"checkin_code:"+code,
+			"user_id:"+self.UserId.String()+":event_id:"+eventId.String(),
+			ttl,
+		).Result()
+		if err != nil {
+			return nil, err
+		}
+		if ok {
+			return &code, nil
+		}
+	}
 }
 
 func (self *User) VerifyCheckinCode(checkinCode string) (*uuid.UUID, error) {