bug#68797: Channel dependencies should be propagated

["Ludovic Courtès" <ludovic.courtes@inria.fr>]

Hello!

With channels flourishing, we found a bug that’s always been there.

When channel C depends on B, which depends on A, in general you need
both A and B to be present in the build environment of C (the derivation
that compiles C when you run ‘pull’ or ‘time-machine’) because
potentially B is going to load modules from A.

However, currently only B is present in C’s environment.

To illustrate that, the script below creates three such channels.
Right now it fails like this:

--8<---------------cut here---------------start------------->8---
building /gnu/store/2pnfkl9l6lsndsgk4qg7rq4wjh2iisj0-my-channel-c.drv...
|builder for `/gnu/store/2pnfkl9l6lsndsgk4qg7rq4wjh2iisj0-my-channel-c.drv' failed to produce output path `/gnu/store/lix9sz9g2x2ixr3dsyrbnxhxfcqn41l7-my-channel-c'
build of /gnu/store/2pnfkl9l6lsndsgk4qg7rq4wjh2iisj0-my-channel-c.drv failed
View build log at '/var/log/guix/drvs/2p/nfkl9l6lsndsgk4qg7rq4wjh2iisj0-my-channel-c.drv.gz'.
cannot build derivation `/gnu/store/alh2yy1258206zki24z9nzi1zwrggd4d-profile.drv': 1 dependencies couldn't be built
guix time-machine: error: build of `/gnu/store/alh2yy1258206zki24z9nzi1zwrggd4d-profile.drv' failed
$ zcat /var/log/guix/drvs/2p/nfkl9l6lsndsgk4qg7rq4wjh2iisj0-my-channel-c.drv.gz
(repl-version 0 1 1)
(exception misc-error (value #f) (value "no code for module ~S") (value ((my-channel-a))) (value #f))
--8<---------------cut here---------------end--------------->8---

Here’s the script:

--8<---------------cut here---------------start------------->8---
#!/bin/sh

set -ex

mkdir a
echo '(define-module (my-channel-a))' > a/my-channel-a.scm
(cd a; git init; git add .; git commit -m init)

mkdir b
cat > b/my-channel-b.scm <<EOF
(define-module (my-channel-b)
  #:use-module (my-channel-a))
EOF
cat > b/.guix-channel <<EOF
(channel
  (version 0)
  (dependencies
   (channel (name my-channel-a) (url "$PWD/a"))))
EOF
(cd b; git init; git add .; git commit -m init)

mkdir c
cat > c/my-channel-c.scm <<EOF
(define-module (my-channel-c)
  #:use-module (my-channel-b))
EOF
cat > c/.guix-channel <<EOF
(channel
  (version 0)
  (dependencies
   (channel (name my-channel-b) (url "$PWD/b"))))
EOF
(cd c; git init; git add .; git commit -m init)

cat > "channels.scm" <<EOF
(list (channel
        (name 'guix)
        (url "https://git.savannah.gnu.org/git/guix.git")
        (commit
          "65dc2d40cb113382fb98796f1d04099f28cab355")
        (introduction
          (make-channel-introduction
            "9edb3f66fd807b096b48283debdcddccfea34bad"
            (openpgp-fingerprint
              "BBB0 2DDF 2CEA F6A8 0D1D  E643 A2A0 6DF2 A33A 54FA"))))
      (channel
        (name 'my-channel-c)
        (url "$PWD/c")))
EOF

exec guix time-machine -C channels.scm -- describe
--8<---------------cut here---------------end--------------->8---

Ludo’.