So, unwisely, I thought I’d run apt -t bookworm-backports upgrade on a system here at home to grab all the available backports (note to future me: be more selective about this). This had worked on several other machines but failed on this one because of a missing libsystemd-shared.252.so file needed by systemd-machine-id-setup. I’m not immediately sure why (maybe I had never landed on that version on this host?) but wanted to share the solution that worked for me: copying the missing so files from another, as yet-unupgraded, machine’s /usr/lib/x86_64-linux-gnu/systemd dir. That lets systemd-machine-id-setup work for the apt upgrade run.
In case it helps anyone else later searching for this, the error message was:
Setting up systemd (254.26-1~bpo12+1) ...
systemd-machine-id-setup: error while loading shared libraries: libsystemd-shared-252.so: cannot op
en shared object file: No such file or directory
dpkg: error processing package systemd (--configure):
installed systemd package post-installation script subprocess returned error exit status 127
Errors were encountered while processing:
systemd