man-db: Configure a systemd .service to be triggered from the man-db trigger

[ermo]

The man-db trigger is currently considered a tx trigger, which is quite literally nuts, because all mandb does is that it indexes man-pages under /usr/share/man and then creates whatis and apropos gdbm format files under /var/cache/man/.

From man mandb:

mandb is used to initialise or manually update index database caches.
The caches contain information relevant  to  the  current  state of the manual page system
and the information stored within them is used by the man‐db utilities to enhance
their speed and functionality.

Given that these caches are not critical for watching man pages via man n foo, it is thus reasonable to shove their creation off to a systemd-controlled service that refreshes the releated indices in the background.

One way of efficiently doing this is to have the man-db trigger simply touch a file that is watched by a listening, systemd-controlled man-db.service (or .socket unit).

Example:
https://serverfault.com/questions/1175044/is-there-a-way-to-activate-a-service-unit-whenever-a-file-gets-updated

[jplatte]

Would there be any functional difference between the trigger touching a file that then does socket activation for a systemd one-shot service, vs. the trigger directly telling systemd to run said one-shot service? Maybe I'm misunderstanding the suggestion here.

[jplatte]

Looked at some docs, by using systemctl --no-block start one can get pretty good fire-and-forget speed (about 10ms on my machine) without going the roundabout socket activation. (without no-block, it seems at least 10x slower)