Bug#862055: apt: Rework daily timers for buster, drop internal timestamps

[Julian Andres Klode <jak@debian.org>]

Package: apt
Severity: normal
Control: tag -1 confirmed

APT's daily cron job and systemd timers currently keep internal
timestamps. systemd keeps its own internal timestamp, leading
to two timestamps: When the systemd timestamp is old enough,
the apt timestamp might not be causing confusion because the
service started and did nothing.

We currently have 6 timestamps:

* update
* download upgradable
* unattended-upgrade
* backup archive/
* autoclean
* clean

I propose to refactor this into three groups:

* download (update, download-upgradable, download u-u upgrades)
* upgrade (run unattended-upgrade)
* maintenance (backup/autoclean/clean)

And the existing options for intervals become booleans for systemd
users, where the intervals are configured by overriding the systemd
timer units apt-daily-download, apt-daily-upgrade, apt-daily-maintenance
instead.

This means systemd systems will have 3 timers with 3 time stamps,
clearly defined behavior when starting the service (always performing
the action), and things will just work in a reasonable way.

This will also allow us to implement retrying on failure efficiently
and generally have a better working solution than now.

-- 
Debian Developer - deb.li/jak | jak-linux.org - free software dev
                  |  Ubuntu Core Developer |
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