[인앱 결제] 구글/애플 영수증 검증부터 상품 지급까지의 시스템 구축

public void Purchase(string productId)
{
    PopupLoading.ShowLoading(LOADING_KEY_PURCHASING);
    m_StoreController.InitiatePurchase(product);
}

public PurchaseProcessingResult ProcessPurchase(PurchaseEventArgs args)
{
    // 영수증을 로컬에 pending 상태로 저장
    _purchasingProductLocal.receipt = args.purchasedProduct.receipt;
    SavePendingProductListToLocal();
    
    // 서버에 검증 요청
    Purchase_Success(itemType, productId, receipt);
    
    return PurchaseProcessingResult.Pending;
}

{
  "Store": "GooglePlay",
  "Payload": {
    "json": "{\"orderId\":\"GPA.xxxx\", \"productId\":\"gem_100\", \"purchaseToken\":\"xxx\"}",
    "signature": "Base64 RSA 서명"
  }
}

public async Task<(GooglePurchaseValidateType, ProductPurchase?)> GetProductPurchase(
    string productId, 
    string purchaseToken)
{
    try
    {
        var request = _androidPublisherService.Purchases.Products.Get(
            _androidPublisherService.ApplicationName,
            productId, 
            purchaseToken);
        var res = await request.ExecuteAsync();
        return ((GooglePurchaseValidateType)res.PurchaseState!, res);
    }
    catch (Exception)
    {
        return (GooglePurchaseValidateType.BadReceipt, null);
    }
}

switch (status)
{
    case GooglePurchaseValidateType.Complete:
        // 상품 지급 진행
        break;
    case GooglePurchaseValidateType.InProgress:
        throw new PaymentRetryException();
    default:
        throw new InvalidReceiptException();
}

private string MakeToken()
{
    var handler = new JsonWebTokenHandler();
    
    return handler.CreateToken(new SecurityTokenDescriptor
    {
        Issuer = _clientBase.IssuerId,
        Audience = "appstoreconnect-v1",
        NotBefore = DateTime.Now,
        Expires = DateTime.Now.AddSeconds(_tokenLifeSecond),
        Claims = new Dictionary<string, object> { { "bid", _clientBase.BundleId } },
        SigningCredentials = new SigningCredentials(
            GetEcdsaSecurityKey(), 
            SecurityAlgorithms.EcdsaSha256)
    });
}

public async Task<(HttpStatusCode, TransactionInfo?)> GetTransactionInfo(string transactionId)
{
    httpClient.DefaultRequestHeaders.Authorization = 
        new AuthenticationHeaderValue("Bearer", MakeToken());
 
    // Production 환경에서 먼저 시도
    var result = await httpClient.GetAsync(
        $"https://api.storekit.itunes.apple.com/inApps/v1/transactions/{transactionId}");
    
    // 404면 Sandbox 환경에서 재시도
    if (result.StatusCode == HttpStatusCode.NotFound)
    {
        result = await httpClient.GetAsync(
            $"https://api.storekit-sandbox.itunes.apple.com/inApps/v1/transactions/{transactionId}");
    }
 
    return (result.StatusCode, /* 파싱된 정보 */);
}

private async Task ValidatePurchaseStateAlreadyComplete(
    string orderId, 
    string productId)
{
    var purchaseState = await _purchaseRepository.FindByOrderId(orderId);
 
    if (purchaseState?.PurchaseProcess == PurchaseProcess.COMPLETE)
    {
        // 이미 지급 완료된 결제 - 에러 처리
        await _slackNotifier.SendAlert("중복 결제 시도 감지", orderId);
        throw new AlreadyPurchasedException();
    }
}

public async Task ProductPurchaseProcess(...)
{
    // 1. 상품 정보 조회
    var productInfo = await GetProductInfo(productId);
    
    // 2. 중복 지급 체크
    await ValidatePurchaseStateAlreadyComplete(orderId, productId);
    
    // 3. 우편으로 상품 지급
    await SendRewardMail(user, productInfo.Rewards);
    
    // 4. 구매 기록 저장 (COMPLETE 상태)
    await SavePurchaseRecord(orderId, productId, PurchaseProcess.COMPLETE);
}

case GooglePurchaseValidateType.InProgress:
    // 클라이언트에게 재시도 요청
    return new PaymentResponse { ShouldRetry = true };

try
{
    await ProductPurchaseProcess(...);
}
catch (Exception ex)
{
    // 트랜잭션 롤백
    await transaction.RollbackAsync();
    
    // 알림 발송 (수동 처리 필요)
    await _slackNotifier.SendCriticalAlert("상품 지급 실패", orderId, ex);
    
    throw;
}

public void PurchaseResponseResult(ResultCode resultCode, ...)
{
    if (resultCode == ResultCodes.Success)
    {
        // 1. 스토어에 결제 확정 알림
        ConfirmPendingPurchase(productId);
        
        // 2. UI에 아이템 지급 표시
        ProcessPurchase_ReceiveItem(productItemType, productId, response);
    }
}
 
private void ConfirmPendingPurchase(string productId)
{
    // 로컬 pending 목록에서 제거
    _pendingProductLocalList.Remove(_purchasingProductLocal);
    
    // Unity IAP에 확정 알림
    m_StoreController.ConfirmPendingPurchase(product);
}