报错：The following signatures couldn‘t be verified because the public key is not available: NO_PUBKEY

apt update 最后报错

$ sudo apt update
报错

......
91 packages can be upgraded. Run 'apt list --upgradable' to see them.
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://repo.mysql.com/apt/ubuntu focal InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 467B942D3A79BD29
W: Failed to fetch http://repo.mysql.com/apt/ubuntu/dists/focal/InRelease  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 467B942D3A79BD29

重点：
NO_PUBKEY 467B942D3A79BD29

缺少公钥？

重新获取公钥并加入到 apt 信任密钥

1. 获取 gpg 公钥
   $ sudo gpg --keyserver keyserver.ubuntu.com --recv-keys 467B942D3A79BD29

    gpg: key 467B942D3A79BD29: public key "MySQL Release Engineering " imported
    gpg: Total number processed: 1
    gpg:               imported: 1
   

   重复执行多次不会改变，也不影响
   $ sudo gpg --keyserver keyserver.ubuntu.com --recv-keys 467B942D3A79BD29

    gpg: key 467B942D3A79BD29: "MySQL Release Engineering " not changed
    gpg: Total number processed: 1
    gpg:              unchanged: 1
   

2. 导出公钥，加入到 apt 信任密钥
   $ sudo gpg --export --armor 467B942D3A79BD29 | sudo apt-key add -
   OK

3. 再来 update
   $ sudo apt update

    ......
    Reading package lists... Done
    Building dependency tree       
    Reading state information... Done
    98 packages can be upgraded. Run 'apt list --upgradable' to see them.
   

   不再报错了！

4 确认：查询一下被信任密钥

$ apt-key list

......
pub   rsa4096 2021-12-14 [SC] [expires: 2023-12-14]
      859B E8D7 C586 F538 430B  19C2 467B 942D 3A79 BD29
uid           [ unknown] MySQL Release Engineering 
sub   rsa4096 2021-12-14 [E] [expires: 2023-12-14]