Lua对象遍历快照：先拷键后改值

-- 错误：边遍历边删，后半段键随机漏访
for k in pairs(state) do
    if expired(state[k]) then
        state[k] = nil
    end
end

local keys = {}
for k in pairs(state) do
    keys[#keys + 1] = k
end
for _, k in ipairs(keys) do
    if expired(state[k]) then
        state[k] = nil
    end
end