The way Linux updates are done in practice is significantly different
from ClamAV virus signature updates.
With ClamAV, freshclam is automatically run periodically, sees (by
some low-cost means) that a new file version is *supposed* to be
available and tries to download it. If either it can't, or worse yet,
it's the wrong one, it tries the next mirror. This all takes time and
bandwidth.
With Linux updates, I explicitly ask (via aptitude) what new updates
are available: It takes some time to retrieve the list. Then I select
the ones I want and ask to install them. I have *never*, *ever* seen
this mechanism deliver the wrong version and thus fail to install it.
This is due to the fact that the same Debian mirror machine provides
the new versions of a group of files as provides the list of new
versions. Thus there is an almost zero chance of a race condition
(unless some idiot adds a version to the list before uploading the
actual deb file). Even if set to auto update, I think the *lists*
always come from the same servers as the files.
It's not a matter of using DNS TXT records, it's a matter of sourcing
them on a *different* computer than the actual files. This separation
virtually begs for synchronization problems.