如何在luasocket库中检查套接字是否关闭？

我正在使用Lua编程语言编写服务器，网络层基于LuaSocket。

除了尝试从套接字中读取数据（调用时它将返回nil和字符串“close”）之外，我在其参考手册中找不到任何检测套接字是否关闭的方法。

我的代码看起来像这样：

local socket = require 'socket'
local server = socket.tcp()
local port = 9527

server:bind('*', port)
local status, errorMessage = server:listen()
if status == 1 then
    printf('Server is launched successfully on port %u', port)
else
    printf('Server listen failed, error message is %s', errorMessage)
    return
end

local sockets = {server}

while true do
    local results = socket.select(sockets)
    for _, sock in ipairs(results) do
        if sock == server then
            local s = server:accept()

            callback('Connected', s)
            table.insert(sockets, s)

            printf('%s connected', s:getsockname())
        else
            -- trying to detect socket is closed
            if sock:isClosed() then
                callback('Disconnected', sock)

                for i, s in ipairs(sockets) do
                    if s == sock then
                        table.remove(sockets, i)
                        break
                    end
                end

                printf('%s disconnected', sock:getsockname())
            else
                callback('ReadyRead', sock)
            end
        end
    end
end