Lua缓存过期：TTL时间戳的失效检查

-- 错误：有缓存就一直用，旧榜挂三天
local function getRank()
    if cached ~= nil then
        return cached
    end
    cached = fetchRank()
    return cached
end

local store = {}
local TTL = {rank = 60, cfg = 600}
local function getCache(key, fetch)
    local it = store[key]
    if it and os.time() - it.at < TTL[key] then
        return it.value
    end
    local value = fetch()
    store[key] = {value = value, at = os.time()}
    return value
end