Lua __gc终结器：只有 userdata 认领的钩子

-- 错误：5.1 表的 __gc 不执行，资源永漏
local res = {name = "render"}
setmetatable(res, {__gc = function()
    doClose(res.name)
end})

local handles = {}
local function openRes(name)
    local h = {name = name, live = true}
    handles[h] = true
    return h
end
local function closeRes(h)
    if h ~= nil and h.live then
        h.live = false
        handles[h] = nil
        doClose(h.name)
    end
end
local r = openRes("render")
closeRes(r)
closeRes(r)