bug#22629: Towards a new 'guix pull'

[Leo Famulari <leo@famulari.name>]

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On Wed, Feb 22, 2017 at 07:57:06AM +0000, Pjotr Prins wrote:
> To keep things simple I propose a channel simply reflects a fixed
> version of a git tree of a Guix package list. I.e., the exact *state*
> of the combined list of packages is what we support in a channel. The
> git checkout can be stored in something like

You shared *a lot* of ideas following this paragraph, but I think that
from here we can start thinking about how to incrementally evolve from
the current `guix pull` to something better.

Currently, you can `guix pull` any commit of GNU Guix with `guix pull
--url` [0]. This feature depends on the Git server implementation's HTTP
interface, but I bet it's a common feature, and it can already be used
with other Git repositories besides our Savannah repo. This achieves the
goal of channels, "easy specialization of deployments". Well, "easy" is
a matter of degrees ;)

I was thinking we could start by keeping `guix pull` the same from the
user's point of view, but change it to support Git in addition to
downloading a tarball. We already have code for this in (guix
git-download) which uses the Git command-line tool. [1]

The next step would be to keep some metadata about which channel the
last pull came from, so that subsequent pulls would point to it.

What do people think?

I haven't wrapped my head around all the proposed changes. I think it
will be best to keep the goal of "easy specialization of deployments" in
mind and work towards it incrementally.

[0]
https://git.savannah.gnu.org/cgit/guix.git/commit/?id=8a9cffb202414b20081910115ba76402924bdcdd

[1] In the long run, we should ditch the Git CLI and use something like
guile-git:
https://gitlab.com/amirouche/guile-git

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