fedup for F23 and beyond

[Will Woods]

[tl;dr: fedup is going away and should be re-implemented by the system
packaging tools.]

Hey all,

F22 is the fifth release we've handled with fedup. A lot has changed
since F17, and we've learned some valuable lessons about how upgrades
work (and how they fail).

We've come to the conclusion that the current design is unsupportable,
mostly due to upgrade.img, which turns out to cause more problems than
it solves[1].

So! For F23, fedup needs to be redesigned. Here's how it should work:

1) Download packages for the new system
2) Use the systemd Offline Updates[2] facility to install packages

This is really simple - simple enough that it should probably be
provided by the system packaging tools themselves.

As a proof-of-concept, I've implemented it as a DNF plugin, which you
can see here[3]: https://github.com/wgwoods/dnf-plugin-fedup

This behavior could also be implemented by PackageKit, which would make
it easy to write a proper GUI.

So that's the plan: drop upgrade.img, move upgrade support into the
system packaging tools. Sometimes simpler is better.

-w

[1] For example, here are three F22 release-blockers, all caused by
upgrade.img:
  https://bugzilla.redhat.com/show_bug.cgi?id=1185604
  https://bugzilla.redhat.com/show_bug.cgi?id=1209941
  https://bugzilla.redhat.com/show_bug.cgi?id=1207251
That first one is a nasty crash inside systemd, which led to a
mailing-list discussion[1a] where Lennart concludes that the
double-switchroot thing we're doing with upgrade.img is just not
supportable[1b]. And I totally agree.
[1a] http://lists.freedesktop.org/archives/systemd-devel/2015-March/029030.html
[1b] http://lists.freedesktop.org/archives/systemd-devel/2015-April/031013.html
[2] http://www.freedesktop.org/wiki/Software/systemd/SystemUpdates/
[3] It works just fine in F21, if you're feeling brave..