【性能调优】弱表缓存失效：缓存击穿的防护

---危险写法：到期即失效，并发全部穿透
-- local cache = {}
-- function Rank.get()
--     local v = cache.top
--     if not v or os.time() - v.at > 60 then
--         v = DB.query("...全量排行查询...")  -- 100 个并发全走到这
--         cache.top = { data = v, at = os.time() }
--     end
--     return v.data
-- end

---防护：逻辑过期，旧值兜底
local cache = { data = nil, at = 0 }
local refreshing = false

function Rank.get()
    local stale = os.time() - cache.at > 60
    if stale and not refreshing then
        refreshing = true
        Rank.reloadAsync(function(data)
            cache.data = data
            cache.at = os.time()
            refreshing = false
        end)
    end
    return cache.data   -- 旧值兜底，永不穿透
end

---定时预热：过期前主动换血
setontimerex(73, 50)
function Rank.preheat()
    -- 缓存年龄超 50 秒即主动重建，不等 60 秒过期
    if os.time() - cache.at > 50 then
        cache.data = DB.query("...全量排行查询...")
        cache.at = os.time()
    end
end