RFC: add maximum debootstrap storing time, or check for updates and invalidate

[Henning Schild <henning.schild@siemens.com>]

Hi,

Isar does have the problem that a debootstrap does the only real
"apt-get update". That is in fact intentional but can lead to the
situation where an outdated chrootfs can break a build. I.e. apt-get
wants to install a package in a version that is not available upstream
anymore.

Now when this happens is impossible to say. It depends on the amount of
packages one has, maybe the distro one is using (sid moving faster than
oldstable), and frozen custom mirror never moving.

With local builds people usually clean all, and for sstate people will
also have to find their own way of dealing with it. But one could say
... it is becoming more of a problem. Not something to not merge the
sstate changes for, but something to think about for the future. 

My current idea would be to introduce
DISTRO_MAX_CHROOT_AGE?="3d"

anon python function
  if /var/..apt/ older than DISTRO_MAX_AGE
    cleanall bootstrap recipe, or somehow enforce re-run

An alternative (maybe better) would be to say

check_for_updates after rootfs_install
  copy rootfs to rootfs_tmp
  chroot rootfs_tmp
  apt-get update
  if "apt-get upgrade is not empty"
    bbwarn(image lacking some updates that meanwhile have been
    published)
    cleanall bootstrap recipe
    if ISAR_BE_STRICT_ABOUT_THAT=true
      fail

Henning