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/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/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

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

 gpg: key 467B942D3A79BD29: public key "MySQL Release Engineering <mysql-build@oss.oracle>" 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 --recv-keys 467B942D3A79BD29

 gpg: key 467B942D3A79BD29: "MySQL Release Engineering <mysql-build@oss.oracle>" not changed
 gpg: Total number processed: 1
 gpg:              unchanged: 1

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

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 <mysql-build@oss.oracle>
sub   rsa4096 2021-12-14 [E] [expires: 2023-12-14]

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