通过VirtualBox，即使在不同的机器上，我有两个运行着Ubuntu Server 20.04的实例，具体信息如下：
两者都安装了MySQL，并且使用的是我几个月前的相同指令。突然之间，当我在其中一个上执行
问题
- 为什么会发生这种情况？ - 如何解决它？
lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 20.04.3 LTS
Release: 20.04
Codename: focal
两者都安装了MySQL，并且使用的是我几个月前的相同指令。突然之间，当我在其中一个上执行
sudo apt update时，出现了以下情况：Get:2 http://repo.mysql.com/apt/ubuntu focal InRelease [12.9 kB]
Hit:3 http://pe.archive.ubuntu.com/ubuntu focal InRelease
Ign:4 https://packages.erlang-solutions.com/ubuntu focal InRelease
Hit:5 https://packages.erlang-solutions.com/ubuntu focal Release
Get:6 http://pe.archive.ubuntu.com/ubuntu focal-updates InRelease [114 kB]
Err:2 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
Get:9 http://pe.archive.ubuntu.com/ubuntu focal-backports InRelease [108 kB]
Get:10 http://pe.archive.ubuntu.com/ubuntu focal-security InRelease [114 kB]
Hit:7 https://packagecloud.io/rabbitmq/rabbitmq-server/ubuntu focal InRelease
Fetched 349 kB in 3s (133 kB/s)
Reading package lists... Done
Building dependency tree
Reading state information... Done
All packages are up to date.
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
W: Some index files failed to download. They have been ignored, or old ones used instead.
问题
- 为什么会发生这种情况？ - 如何解决它？
sudo apt update都没有任何问题，后来当然也用sudo apt upgrade成功了。为什么突然之间“GPG密钥”就消失了呢？ - Manuel Jordan