handle limited quantity

pkg/cart/mutation_change_quantity.go

@@ -48,8 +48,15 @@ func ChangeQuantity(g *CartGrain, m *messages.ChangeQuantity) error {
 		g.UpdateTotals()
 		return nil
 	}
-
-	g.Items[foundIndex].Quantity = int(m.Quantity)
-	g.UpdateTotals()
+	item := g.Items[foundIndex]
+	if item == nil {
+		return fmt.Errorf("ChangeQuantity: item id %d not found", m.Id)
+	}
+	if item.ReservationEndTime != nil {
+		return fmt.Errorf("ChangeQuantity: cannot change quantity of reserved item id %d", m.Id)
+	} else {
+		item.Quantity = int(m.Quantity)
+		g.UpdateTotals()
+	}
 	return nil
 }