Mechanism for helping in multi-channels configuration

[Simon Tournier <zimon.toutoune@gmail.com>]

Hi,

Well, using Guix bdab356 from a little bit more than one month old, then
associating the channel guix-science 0b3d4a2f last week, I get the
failure:

--8<---------------cut here---------------start------------->8---
$ guix build /gnu/store/g3aa5rh7bs5pyxd3q1gvhwz1s9z1vh3z-guix-science.drv
The following derivation will be built:
  /gnu/store/g3aa5rh7bs5pyxd3q1gvhwz1s9z1vh3z-guix-science.drv
building /gnu/store/g3aa5rh7bs5pyxd3q1gvhwz1s9z1vh3z-guix-science.drv...
(repl-version 0 1 1)
WARNING: (guix-science build bazel-build-system): imported module (guix build utils) overrides core binding `delete'
(exception unbound-variable (value #f) (value "Unbound variable: ~S") (value (python-nr-stream)) (value #f))
builder for `/gnu/store/g3aa5rh7bs5pyxd3q1gvhwz1s9z1vh3z-guix-science.drv' failed to produce output path `/gnu/store/qzgj4vig3vklbznz1i0pgy11nr3z4rv9-guix-science'
build of /gnu/store/g3aa5rh7bs5pyxd3q1gvhwz1s9z1vh3z-guix-science.drv failed
View build log at '/var/log/guix/drvs/g3/aa5rh7bs5pyxd3q1gvhwz1s9z1vh3z-guix-science.drv.gz'.
guix build: error: build of `/gnu/store/g3aa5rh7bs5pyxd3q1gvhwz1s9z1vh3z-guix-science.drv' failed
--8<---------------cut here---------------end--------------->8---

Well, that’s expected!  Guix bdab356 does not contain python-nr-stream
introduced by commit 7dfe41aa71a4a4a9d6065a44e9c6271717215b3e.

The wishlist is: provide a machine-readable description on guix-science
channel side in order to help in finding the good overlap between
commits of different channels.

It could be nice if instead of an hard error, “guix pull” could say:
« the channel ’guix’ needs to be at least at commit 1234abc ».

WDYT?

Cheers,
simon