Update: Implementing guix system rollback / switch-generation

[Chris Marusich]

Hi Guix,

I've attached a preliminary patch which adds rudimentary roll-back and
switch-generation commands to "guix system".  Currently, the commands
just flip symlinks, which is part of the first milestone that Ludo
suggested in the following thread:

https://lists.gnu.org/archive/html/guix-devel/2016-06/msg00178.html

Please let me know what you think.  After getting feedback, I intend to
follow up a more complete patch that contains the following additional
changes:

* Regenerate grub.cfg, to complete the first milestone.  I'm not sure
  exactly how this will play out, but I imagine I'll need to use the
  store somehow to get it done.

* Use a properly formatted ChangeLog-style Git commit message.

* Mention these new commands in the manual.

* Add tests.  I'm not new to automated testing, but I haven't done it
  using autoconf/make or in Guile, so tips here would be welcome!

* Add these new commands to the emacs interface.

I've tested the changes on my bare metal GuixSD installation.  I would
have used a VM, but my system has trouble launching VMs.  I first
verified manually that the roll-back command switched the profile's
symlink to the previous generation, by examining the output of
list-generations and ls.  I then verified, in the same way, that the
switch-generation command switched the profile's symlink to the
specified generation.  I verified that switch-generation works for
arbitrary numbers and relative numbers (note that a negative relative
number must be preceded by the special "--" argument to prevent it from
being interpreted as an option).  I also verified that if the user
attempts to switch to a nonexistent generation (or to roll back from
generation 1 in the case of "guix system"), an error is raised without
modifying anything.  I repeated this manual verification process for
both the "guix system" and "guix package" commands, since my changes
involve some refactoring in "guix package".

Thank you,

-- 
Chris

0001-Implement-system-roll-back-and-switch-generation-com.patch
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