今天在 ubuntu 机器上执行：sudo apt-get update 的时候，提示错误如下：

.......
Fetched 3,192 B in 3s (848 B/s)
Reading package lists... Done
W: GPG error: http://archive.ubuntukylin.com:10006 xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8D5A09DC9B929006

看上去是某个公钥找不到，解决方法就是导入这个公钥

sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 8D5A09DC9B929006