【网络通信】粘包处理：长度前缀协议的解析器

---发送端：长度前缀加消息体
function Pkt.send(conn, msgId, body)
    local bodyLen = #body
    local head = string.pack('>I2I4', msgId, bodyLen)
    conn:send(head .. body)
end

---接收端：缓冲区的解析循环
function Pkt.parse(conn)
    local buf = conn.buffer
    while true do
        -- 头部不足 6 字节：等待更多数据
        if #buf < 6 then break end
        local msgId, bodyLen = string.unpack('>I2I4', buf)
        local total = 6 + bodyLen
        -- 消息体不足：等待更多数据
        if #buf < total then break end
        local body = buf:sub(7, total)
        buf = buf:sub(total + 1)
        MsgRoute.dispatch(conn, msgId, body)
    end
    conn.buffer = buf
end

---异常的防护：长度字段的合法性校验
local MAX_BODY = 64 * 1024

function Pkt.safeParse(conn)
    local buf = conn.buffer
    while #buf >= 6 do
        local msgId, bodyLen = string.unpack('>I2I4', buf)
        -- 长度超限：连接异常，直接断开
        if bodyLen > MAX_BODY then
            conn:close('非法长度')
            return
        end
        if #buf < 6 + bodyLen then break end
        MsgRoute.dispatch(conn, msgId, buf:sub(7, 6 + bodyLen))
        buf = buf:sub(7 + bodyLen)
    end
    conn.buffer = buf
end