So I upgraded from Debian 10 to Debian 11.
All was fine, except for Postgres.
Postgres went from 11 to 13. The way it’s arranged is that both run at once, with the new Postgres in non-default locations, and there’s a script to migrate.
The script completely fails to work in every possible way, with the Escher-like error messages (“I can’t work - there’s no server”, so you run the server, “I can’t work - there’s a server”).
I tried to run just 11 and point the mediawiki at it. No go - and in fact, we’re in the archetypal open source situation; it was working, now it isn’t, there are no error messages (mediawiki reports “can’t connect to database - unknown error”) and Googling brings up very few, and contradictory, docs on how to configure a mediawiki to use a UNIX socket (and in fact the working config I had is not found in any of those docs).
It looks to me like migrating to 13 properly is the next method to try to get this all working.
What’s happened here is 100% in line with my prior experiences of, and so expectation of, open source software.
Upgrading Debian 10 to 11 broke Mediawiki.
The root cause is because PHP was upgraded from 7.3 to 7.4, but the PHP postgres package, which was installed for 7.3, was not installed for 7.4. This is unexpected behaviour, in fact, it looks to me like an upgrade failure.
The only error message was “cannot connect to database”, so tracking it down from error information was not possible.