bug#70339: Constructing hg-fetch fixed-output derivation requires Mercurial

[Simon Tournier]

Hi,

For instance,

--8<---------------cut here---------------start------------->8---
$ guix build -S -d hg-commitsigs
substitute: updating substitutes from 'https://ci.guix.gnu.org'... 100.0%
3,7 MB will be downloaded:
  /gnu/store/6fya762sz5hjdj04vdn5g3v6zii6f11d-mercurial-6.2.2
substituting /gnu/store/6fya762sz5hjdj04vdn5g3v6zii6f11d-mercurial-6.2.2...
downloading from 
https://ci.guix.gnu.org/nar/lzip/6fya762sz5hjdj04vdn5g3v6zii6f11d-mercurial-6.2.2
 ...
 mercurial-6.2.2  3.5MiB                                                   
529KiB/s 00:07 ▕██████████████████▏ 100.0%

/gnu/store/pkb6zd9xfmxx6rsh4p7w3glh7xqg5sqy-hg-commitsigs-0.1.0-0.b53eb68-checkout.drv
--8<---------------cut here---------------end--------------->8---

and it is unexpected.  The construction of the fixed-output derivation
does not need to download stuff; it only needs to compose stuff
detailing how to do.  Any download (or build) must happen when running
the derivation itself.

The issue: later – say 1 or 2 years from now – the command:

   guix time-machone --commit=929ddec -- build -S -d hg-commitsigs

will start to build all what Mercurial needs (python etc.).  If for some
reasons*, only one of Mercurial dependencies fails then we are doomed.

    ( Context: I am working on a fixed-output translator; rely on
      current strategies for downloading and do not require all the past
      stack just for downloading source code. )

I think it comes from this part:

--8<---------------cut here---------------start------------->8---
                     (hg-fetch '#$(hg-reference-url ref)
                               '#$(hg-reference-changeset ref)
                               #$output
                               #:hg-command (string-append #+hg "/bin/hg")))
--8<---------------cut here---------------end--------------->8---

from ’hg-fetch’ in (guix hg-download).  Here the #+hg is not required
because just before there is:

            (set-path-environment-variable "PATH" '("bin")
                                           (match '#+inputs
                                             (((names dirs outputs ...) ...)
                                              dirs)))

So relying on string "hg" should be enough; as it is done in
’git-fetch/in-band*’ for one example.

Do I miss something?


Cheers,
simon

*reasons of failures: See
 
https://guix.gnu.org/en/blog/2024/adventures-on-the-quest-for-long-term-reproducible-deployment