Comment 5 for bug 431993

On Tue, Oct 06, 2009 at 11:07:05PM -0000, dhlii wrote:
> And honestly I am not sure that I accept that you have to delete
> anything. I grasp that you must replace numerous files, that to ensure a
> working and robust system you must replace libraries that are found with
> those that match this particular install - it is even useful to be able
> to go backwards - re-install intrepid over karmic as an example.
>
> But off the top of my head I can think of extremely few instances
> where if standards are being followed the mere presence of a file from a
> previous install should destabalize a system. There are a few instances
> but they seem to be unique to things like udev that act on any file
> present in a given directory.

On the contrary, there are lots and lots of situations where we check
for the presence or absence of a file. For instance, it's standard for
init scripts to check for the presence of a binary before proceeding.
Many programs have cascading conditionals which check for one
alternative after another (gnome-wm comes quickly to mind as a fairly
dramatic example, but it's just one example). The result of not cleaning
up old system files is, in very many cases, quite likely to be a broken
system.

My greatest worry (which I think is very well-founded) about leaving old
system files around is that they will never be upgraded and yet there's
a good chance that they'll be used anyway. In this day and age where
security vulnerabilities might lurk nearly anywhere, and where people
expect that in general bug fixes will be applied automatically on
upgrade, I just don't think that's a tenable policy.

> Anyway with respect to principles and policy I think the best
> approach is for the installer to complain and make note of the presence
> of files in system directories.
> I do not care if you tell the user this is unsupported.

Frankly, I'd rather that the feature of installing over the top of an
existing system without reformatting weren't supported at all, but it
was added due to overwhelming demand. Going back and telling people that
it's unsupported (since there will be mismatching files in system
directories in nearly all cases) doesn't seem likely to help.

> The second is that there appears to be an actual intent to get
> much more aggressive about file deletion.

No more so than at present; indeed your bug suggests that we need to be
more surgical about it, and I think that's our planned response to this
bug.