Yesterday, I opened Software Sources on Linux Mint and changed the base mirror from archive.ubuntu.com to a different website, hoping this would fix a 0% [waiting for headers] issue I kept running into when I tried installing software. It did fix that issue, but shortly after I applied that fix my computer's keyboard got stuck in Japanese, which freaked me out so I rebooted my computer. I was then greeted by an error message from Ibus about how several of my keyboard layouts apparently didn't exist. Indeed, I suddenly only had a Norwegian and Japanese keyboard to choose from, and Ibus no longer listed my preferred Russian, Esperanto, and Amharic keyboards as options. I soon enough managed to figure out that ibus-m17n had somehow been uninstalled, so I reinstalled it and set up my preferred keyboard layouts again.
Today I changed my base mirror back to archive.ubuntu.com and restarted my computer… annnnnnd now I'm discovering that a bunch of my other shit has just straight up disappeared: Inkscape, Kdenlive, GIMP, these three programs were all apparently uninstalled without my knowledge. If any other programs I use have been uninstalled I am yet to become aware of this. Thankfully, all the programs I noticed were missing were easy to reinstall, and I can jump back into my projects like literally nothing happened. But it's still really damn uncomfortable to suddenly realize that a bunch of my important programs just up and disappeared.
All of this apparently happened (correct me if I'm wrong!) because APT — which is the thingy that handles installing/uninstalling software on Linux Mint — can become convinced that you have obsolete or conflicting packages if you're switching to or from out-of-sync mirrors, or mirrors with bad metadata or whatever; and when APT thinks you have bad packages, it'll just unquestioningly yeet them. In my case the damage this behavior caused seems minimal enough that it's not worth the risk of using Timeshift (I still remember what happened last time I used Timeshift! I ended up breaking initramfs so I couldn't even boot my computer without selecting an older kernel!!), so I'll just reinstall any missing software I notice and take the L. But I'm definitely not going to make that mistake again in case it causes even worse trouble for me the third time, and you should also be advised that changing package mirrors, especially after you've been on Linux Mint for a while, can cause you this sort of trouble.
It sounds like the interim repository you chose wasn't exactly a mirror. It might have been a repository for a different distribution or something. In the future, it might be a safer option to leave the default mirror in
/etc/apt/sources.listand list 3rd party mirrors after it, as alternatives (but it is still essential to make sure the repositories are actually compatible). Also, you should never need to add keys to your APT keyring for mirrors (if you do, they are not mirrors. It makes sense to do this for third party repositories, but not something which is supposed to be an exact copy of the distribution repository).The way APT (and most other package managers) work is by keeping track of a list of software which is has been installed explicitly. Some of these are marked (APT terminology) as "manual" by the distribution maintainers. These are installed when you install the OS. Every time you install a package via
apt-get/apt/synaptic/gnome-sofwareetc. these are also marked as "manual." When software is installed or updated, the real goal is to satisfy the dependencies of these manual-marked packages. Everything else is installed as needed (marked as "auto"), and discarded as soon as no manual-marked packages depend on them any longer.By the sound of it, these relationships got cooked somewhere along the way. A lot of the software which was installed automatically as dependencies suddenly had no dependents, (because the package definitions changed and they probably depended on newer / older / different versions) and so they vanished. It's actually impressive that enough vital things survived to recover this.
Anyway, deep-frying a Linux box now and then is how you learn.