bug#22017: Pinning a Guix version

[ludo@gnu.org (Ludovic Courtès)]

Hello!

In some cases, it’s useful to be able to pin a particular Guix version
and to use that version (see the RepPar paper for some examples.)

A naive way to do that is:

  git clone …/guix.git my-pinned-guix
  (cd my-pinned-guix; git checkout deadbeef)
  guix package -L my-pinned-guix --manifest=my-manifest.scm

However, this is currently broken for several reasons:

  1. ‘fold-packages’ recursively traverses ‘my-pinned-guix’ for .scm
     files, and on its way it finds build-aux/build-self.scm,
     tests/*.scm, and emacs/*.scm, which breaks it all.  These
     directories have to be explicitly removed before we can do
     something.

     Internally, ‘%package-module-path’ can have entries that are pairs
     and where the cdr restricts the search of .scm files to a
     sub-directory.  We should expose that facility to -L and
     ‘GUIX_PACKAGE_PATH’ using some special syntax.

     That way, one could do (say):

       guix package -L my-pinned-guix,gnu/packages

     meaning that only the ‘gnu/packages’ sub-directory is searched.

     OTOH, it may be advisable to use the (guix …) modules from
     ‘my-pinned-guix’ and not just the (gnu packages …) modules, because
     the former obviously contribute to the final result.

  2. The ‘gnu/packages/patches’ sub-directory is not automatically added
     to the search path, so in fact one has to run:

       guix package -L my-pinned-guix \
         -L my-pinned-guix/gnu/packages/patches …
  
     so that patches are found.  Not convenient.

  3. When doing so, we get loads of:

     ;;; note: source file /tmp/guix/guix/build/perl-build-system.scm
     ;;;       newer than compiled /home/ludo/src/guix/guix/build/perl-build-system.go

     We should silence Guile.

  4. Related to #1: since everything is evaluated, this is ~10 times
     slower than the normal thing.

I think ‘guix pull’ would be a good place to add support for tagging
Guixes and similar, but it would be good if the naive approach above
would work just as well.

Ludo’.