man-db trigger for apt is excessively slow

[Thayne]

Whenever I install a package that includes a man page, apt-install gets stuck for multiple minutes on the step "Processing triggers for man-db".

In many cases, this means that the "man-db" trigger takes significantly longer the rest of the installation combined.

This slowness is enough of a nuisance that I eventually disabled the man-db update by running

```
echo "set man-db/auto-update false" | sudo debconf-communicate 0 value set
```

However, many users probably won't go to the effort of figuring out how to do this, and will just be frustrated by installations taking a long time.

According to some of my co-workers this started happening a few weeks ago (on 24.04).

ProblemType: Bug
DistroRelease: Ubuntu 24.04
Package: man-db 2.12.0-4build2
ProcVersionSignature: Ubuntu 6.8.0-38.38-generic 6.8.8
Uname: Linux 6.8.0-38-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.28.1-0ubuntu3
Architecture: amd64
CasperMD5CheckResult: pass
CurrentDesktop: ubuntu:GNOME
Date: Mon Jul 22 12:20:30 2024
InstallationDate: Installed on 2024-07-19 (3 days ago)
InstallationMedia: Ubuntu 24.04 LTS "Noble Numbat" - Release amd64 (20240424)
SourcePackage: man-db
UpgradeStatus: No upgrade log present (probably fresh install)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Thayne (thayne-u)]

I've made an interesting discovery with respect to this.

If I remove the `percona-toolkit` package, then mandb runs much faster (2 seconds instead of 2 minutes).

I think there is something wrong with the man pages in that package (although I am using the download from https://www.percona.com/percona-toolkit to get a newer version of it, not sure if that would make a difference).

However, ideally, mandb would be able to handle this more gracefully. And in 20.04 this wasn't a problem.

[alxndrsn (alxndrsn)]

In my experience this issue has been caused by a dependency of Playwright (https://playwright.dev/).

I made a github project to test which dependencies are slow: https://github.com/alxndrsn/man-db-update-tests/actions

The obvious culprit there `session-migration`.

[alxndrsn (alxndrsn)]

Actually now I've run those tests a few times, things aren't too predictable: https://github.com/alxndrsn/man-db-update-tests/actions/runs/19230260299/usage

Some packages which took over 1 minute to process triggers:

* percona-toolkit (1 of 5 runs)
* glib-networking (1 of 5 runs)
* session-migration (2 of 5 runs)
* gsettings-desktop-schemas (1 of 5 runs)