This is embarrassing. I should know by now how to fix something like this, but I feel like this is still a problem for the Newbie Corner.
I should preface this by saying that the situation is very similar to what is described here: https://forum.antergos.com/topic/11172/can-t-log-in-to-antergos-kde-plasma
…with the exception that I don’t know how to create a new user. I suspect trying to do so would not work in any case, as neither of the users already on the system work. They both quit back to the greeter.
Next, I figured that I’ll just go in through the textual interface and update my packages. I reasoned that I was probably seeing the results of a regression that came in with the last update. At least I know how to get to the tty’s!
Sure enough, I was able to log in. But then I got this:
error: could not open file /var/lib/pacman/local/accountsservice-0.6.54+2+g204a4ab-1/desc: No such file or directory
The system then asserts that, since there is missing or corrupt metadata, it will not under any circumstances make changes of any sort to any package on the system. Naturally, there is also no obvious way to simply replace a package’s missing metadata with default values from the remote copy (assuming I understand the context of ‘package metadata’ correctly).
Fortunately, that isn’t my primary machine; I’m sending this from my Ubuntu desktop.