apt and do-release-upgrade question - what's sources.list.distUpgrade for?

[Ralf Mardorf]

On Mon, 20 Jul 2020 10:36:41 +0100, Chris Green wrote:
>Ignoring the stuff in the sources.list.d (though that follows the same
>pattern), what is sources.list.distUpgrade?  I can see that
>sources.list.save is just my old/previous sources.list renamed which
>is handy to see if anything needs re-enabling after the upgrade.

"sources.list.distUpgrade" is a backup of the old sources list.
"sources.list" and "sources.list.save" are _not_ a backup of the old
source list.

The way config files are handled when updating a package, let alone
when updating from one distro release to another distro release has got
two pitfalls. 1. The inability of mind reading, the package management
does not know what config options are wanted, if an old config was
edited and a new config has got default values and maybe does introduce 
new settings. 2. Even if replacing a config is wanted, it somehow
should be kind of an atomic operation to replace the file. So a new
config will be saved with some extension marking it as new, or the old
is moved to a backup file and if editing is involved, there even might
be temporary files stored. After all is done automatic cleaning is
possible, but disputable.

How many undo steps should be provided by an email editor? Should the
undo history be based on chars or on words? A mail not necessarily
contains words only, OTOH most emails are manly based on words and
undoing char by char could be a PITA. How often should log files
rotate? How many backups of config files should be kept?