# ── 查询当前商品队列（服务层函数）──────────────────────
function get_queue(product_id):
    return SELECT id, buyer_name, phone, created_at
           FROM intent
           WHERE product_id = product_id AND status = 'pending'
           ORDER BY created_at ASC, id ASC        # 位次 = 结果集行号

# ── 买家撤销意向（一个事务）───────────────────────────
function cancel_intent(token_code):
    BEGIN
        row = SELECT * FROM intent WHERE token_code = token_code
                                   AND status = 'pending'   # 精确匹配 + 仅排队中可撤销
        assert row 存在
        UPDATE intent SET status = 'cancelled' WHERE id = row.id
        # 队列按 ORDER BY 实时计算，删除即前移，无需更新其余行
        # 商品状态不变（on_sale 或 frozen，取决于撤销时机）
    COMMIT

# ── 卖家选定队首进入交易（一个事务）─────────────────────
function start_trade(product_id):
    BEGIN
        assert product.status == 'on_sale'
        head = get_queue(product_id) 的第一行
        assert head 存在
        UPDATE intent SET status = 'trading' WHERE id = head.id
        UPDATE product SET status = 'frozen'      # 自动冻结
    COMMIT

# ── 标记交易失败（一个事务，触发递补）────────────────────
function mark_failed(product_id):
    BEGIN
        cur = SELECT * FROM intent
              WHERE product_id = product_id AND status = 'trading'
        UPDATE intent SET status = 'failed' WHERE id = cur.id
        UPDATE product SET status = 'on_sale'     # ① 商品恢复在售
        head = get_queue(product_id) 的第一行
        IF head 存在 THEN                          # ② 队列非空
            UPDATE intent SET status = 'trading' WHERE id = head.id
            UPDATE product SET status = 'frozen'   # ③ 新队首进入交易，商品再冻结
        ELSE                                       # ④ 队列为空
            商品停留在 on_sale，重新接收新意向
        END IF
    COMMIT

# ── 标记交易成功（一个事务，殃及整个队列）─────────────────
function mark_success(product_id):
    BEGIN
        cur = 当前 trading 意向
        UPDATE intent SET status = 'success' WHERE id = cur.id
        UPDATE intent SET status = 'failed'
              WHERE product_id = product_id AND status = 'pending'
              # 商品已下架，队列中其余排队意向失去意义，全部转 failed
        UPDATE product SET status = 'delisted',
                           trade_result = 'success',
                           traded_at = 当前毫秒时间戳
    COMMIT