[Bug]: /var/tmp/flatpak-cache-* directories are not garbage collected by systemd, because they are constantly touched by flatpak

[gentoo-root]

Checklist

• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the issue tracker for a bug that matches the one I want to file, without success.
• If this is an issue with a particular app, I have tried filing it in the appropriate issue tracker for the app (e.g. under https://github.com/flathub/) and determined that it is an issue with Flatpak itself.
• This issue is not a report of a security vulnerability (see here if you need to report a security issue).

Flatpak version

1.15.4

What Linux distribution are you using?

Arch Linux

Linux distribution version

rolling

What architecture are you using?

x86_64

How to reproduce

1. Use flatpak for more than 30 days.
2. Observe /var/tmp/flatpak-cache-* directories not being cleaned up by the systemd-tmpfiles rule, even though most of them are empty.

Expected Behavior

I expect that /var/tmp/flatpak-cache-* directories do not accumulate indefinitely and are garbage collected eventually.

Actual Behavior

I have a few dozens of stale and empty /var/tmp/flatpak-cache-* directories that are no longer used, but are not garbage collected:

...
drwxr-xr-x 1 max   max        0 Sep 10  2022 flatpak-cache-RTU1R1
drwxr-xr-x 1 max   max        0 Feb  9 12:02 flatpak-cache-SQN5Z1
drwxr-xr-x 1 max   max        0 Nov 26 17:58 flatpak-cache-T5YIT1
...

Additional Information

Running SYSTEMD_LOG_LEVEL=7 systemd-tmpfiles --clean revealed the reason why these unused directories don't get garbage collected: it's because the access time (atime) of each directory is updated regularly, supposedly, by flatpak itself:

...
Directory "/var/tmp/flatpak-cache-RTU1R1": access time Thu 2023-04-06 01:16:53.247647 EEST is too new.
Directory "/var/tmp/flatpak-cache-977DS1": access time Thu 2023-04-06 01:16:53.247647 EEST is too new.
Directory "/var/tmp/flatpak-cache-OYDWR1": access time Thu 2023-04-06 01:16:53.247647 EEST is too new.
...

The atimes of all flatpak-cache directories are within one second, so I suppose flatpak goes over all of them, updating atimes, when it performs some action.

Possible ways to solve it:

1. Flatpak could stop iterating and bumping atimes of flatpak-cache directories that are no longer in use. This way, systemd-tmpfiles could garbage collect them.
2. Alternatively, if the previous way is not possible for some reason, flatpak could ship with a tmpfiles config that tells systemd to ignore atimes on flatpak-cache directories, something like this, not tested yet:

e /var/tmp/flatpak-cache-* - - - 30d:abcmBM

[matejak]

Could this be related to #1119? Interestingly, problems with the cache seem to be correlated with the Arch Linux distribution. Interestingly, e.g. Fedora doesn't seem to do any tmp-related magic when creating the package - see https://src.fedoraproject.org/rpms/flatpak/blob/f38/f/flatpak.spec

[fewaltix]

@matejak: I can confirm that the same problem plagues OpenSuse as well.