页面标价和结算价对不上：促销价格的计算时机、缓存一致与防篡改实践

# price_service.py —— 全站唯一价格计算入口
def calc_price(product_id, qty, coupon_id=None, user_id=None):
    # 1. 取商品基础价（数据库，不缓存）
    product = db.fetchone("SELECT base_price FROM product WHERE id=%s", (product_id,))
    # 2. 取当前生效促销（带活动时间窗过滤）
    promo = db.fetchone(
        "SELECT rule FROM promo_rule "
        "WHERE product_id=%s AND status='ACTIVE' AND now() BETWEEN start_at AND end_at",
        (product_id,),
    )
    # 3. 计算：满减/折扣由规则引擎统一处理
    price = apply_promo(product["base_price"], qty, promo, coupon_id)
    # 4. 返回明细（含促销ID与版本号，落单时校验用）
    return {"unit_price": price, "promo_id": promo["id"] if promo else None,
            "promo_version": promo["version"] if promo else None}

def get_product_page_price(product_id):
    # 促销版本号变化时，缓存自动失效
    ver = redis.get(f"promo:ver:{product_id}") or "0"
    key = f"price:page:{product_id}:{ver}"
    cached = redis.get(key)
    if cached:
        return json.loads(cached)
    price = calc_price(product_id, 1)
    redis.setex(key, 300, json.dumps(price))  # 5分钟，版本号变化即失效
    return price

def update_promo_rule(product_id):
    # 运营改完促销，立刻 bump 版本号
    redis.incr(f"promo:ver:{product_id}")

def create_order(user_id, items, coupon_id=None):
    total = 0
    for it in items:
        # 后台重新计算，忽略前端传来的任何价格字段
        p = calc_price(it["product_id"], it["qty"], coupon_id, user_id)
        total += p["unit_price"] * it["qty"]
        # 记录促销快照，售后/对账时能还原"当时按什么规则算的"
        db.execute(
            "INSERT INTO order_item_snapshot(order_no, product_id, qty, unit_price, promo_id, promo_version, created_at) "
            "VALUES(%s,%s,%s,%s,%s,%s,NOW())",
            (order_no, it["product_id"], it["qty"], p["unit_price"], p["promo_id"], p["promo_version"]),
        )
    # 落库金额以后台重算为准
    db.execute("UPDATE `order` SET total_amount=%s WHERE order_no=%s", (total, order_no))
    return total

// 前端加购/结算时携带已看到的促销版本
function addToCart(productId, qty) {
  const price = await api.calcPrice(productId, qty);
  cart.push({
    productId,
    qty,
    promoVersion: price.promoVersion,  // 用户"看到"的版本
  });
}

def check_promo_version(item, current_version):
    # 用户看到的版本和当前版本不一致 → 返回"价格已更新，请确认"
    if item["promo_version"] != current_version:
        raise PriceChangedError(
            f"促销活动已更新，请刷新后重新确认价格",
            old=item["promo_version"], new=current_version,
        )