clear cookie if failing

cmd/checkout/pool-server.go

@@ -210,7 +210,14 @@ func (s *CheckoutPoolServer) StartCheckoutHandler(w http.ResponseWriter, r *http
 	}
 
 	// Create checkout with same ID as cart
-	checkoutId := checkout.CheckoutId(cartId)
+	var checkoutId checkout.CheckoutId = cart.MustNewCartId()
+	cookie, err := r.Cookie(checkoutCookieName)
+	if err == nil {
+		parsed, ok := cart.ParseCartId(cookie.Value)
+		if ok {
+			checkoutId = parsed
+		}
+	}
 
 	// Initialize checkout with cart state wrapped in Any
 	cartStateAny := &messages.InitializeCheckout{
@@ -225,13 +232,14 @@ func (s *CheckoutPoolServer) StartCheckoutHandler(w http.ResponseWriter, r *http
 
 	result, err := s.ApplyLocal(r.Context(), checkoutId, cartStateAny)
 	if err != nil {
+		setCheckoutCookie(w, 0, r.TLS != nil)
 		logger.Error("failed to initialize checkout", "error", err)
 		http.Error(w, "failed to initialize checkout", http.StatusInternalServerError)
 		return
 	}
 
 	// Set checkout cookie
-	setCheckoutCookie(w, result.Result.Id, r.TLS != nil)
+	setCheckoutCookie(w, checkoutId, r.TLS != nil)
 
 	if err := s.WriteResult(w, result.Result); err != nil {
 		logger.Error("failed to write result", "error", err)