bug#64858: ~guix shell -C -f guix.scm …~ should not always need ~--rebuild-cache~ option to build the expected environment.

[Ludovic Courtès]

Hi Pierre-Henry,

Pierre-Henry Fröhring <phfrohring@deeplinks.com> skribis:

> $ guix shell -C -f guix.scm ripgrep fd coreutils emacs

[...]

> This very file (~pkgex.el.org~) is updated with this content then the
> package is
> built again.
>
> #+begin_example
> $ make build # equivalent to: guix build -f guix.scm
> …
> /gnu/store/8k18bghzcijbps8kix3wqp34x4smfc5l-pkgex-1
> #+end_example
>
>
> ** pkgex-1 -> /gnu/store/0yk3xz85…
>
> Unexpectedly, the package linked from within the container using the same
> command as above is not updated, we observe:

I don’t fully understand the setup, but I can at least explain what you
can expect.

When using ‘-f guix.scm’, ‘guix shell’ caches based on the mtime of
‘guix.scm’: if ‘guix.scm’ is modified, then the cache is invalidated,
otherwise the cache is considered up-to-date and used.

IIUC, you’re modifying a different file, ‘pkgex.el.org’.  ‘guix shell’
does not know about it and thus goes ahead and reuses the previous
value.

I guess the current behavior is good when you’re doing:

  guix shell -D -f guix.scm

which is the primary use case that comes to mind, but it’s inappropriate
when doing:

  guix shell -f guix.scm

in cases where ‘guix.scm’ defines a package with $PWD as its source.

I guess we could maybe try to special-case that in
‘profile-cached-gc-root’.

Ludo’.