Bug#1018165: apt-update: make it possible to reliably tell when the apt cache was updated

[Jakub Ružička <jakub.ruzicka@nic.cz>]

Package: apt
Version: 2.2.4
Severity: wishlist
Tags: upstream
X-Debbugs-Cc: jakub.ruzicka@nic.cz

Debian users are supposed manually keep apt cache up to date by running
`apt update` but there is no reliable way to tell when the cache was
last updated.

As with any cache, the age of cached data is a fundamental information.

It's a very reasonable request to "update cache if it's old" but there
seems to be no way to do so reliably on Debian-based systems preventing
smooth automated operation.

This feature is available with most other package managers:

* Fedora's dnf updates cache automatically.
* Arch's pacman can update cache before package install with -y/--refresh.

Furthermore, many people are asking howto tell Debian apt cache age on
the internet:

* https://serverfault.com/questions/20747/find-last-time-update-was-performed-with-apt-get
* https://askubuntu.com/questions/410247/how-to-know-last-time-apt-get-update-was-executed

The best available solution (correct me if I'm wrong) is unreliable -
checking the timestamp of

    /var/lib/apt/lists/

However, this doesn't update when there are no changes to package lists.

Please provide a way to reliably tell the last cache update time on any
distro using apt.

I suggest updating

    /var/lib/apt/lists/last-update

or similar file's timestamp.

I'm happy to attempt to implement this change if someone experienced can
point out a proper way to do this.