[bug#66793] [PATCH 2/3] time-machine: Make target commit check cheaper.

[Maxim Cournoyer]

Hi,

Ludovic Courtès <ludo@gnu.org> writes:

> Commit 79ec651a286c71a3d4c72be33a1f80e76a560031 introduced a check to
> error out when attempting to use ‘time-machine’ to travel to a commit
> before ‘v1.0.0’.
>
> This commit fixes a performance issue with the strategy used in
> 79ec651a286c71a3d4c72be33a1f80e76a560031 (the repository was opened,
> updated, and traversed a second time by ‘validate-guix-channel’) as well
> as a user interface issue (“Updating channel” messages would be printed
> too late).
>
> This patch reimplements the check in terms of the existing #:validate-pull
> mechanism, which is designed to avoid extra repository operations.
>
> Fixes <https://issues.guix.gnu.org/65788>.
>
> * guix/inferior.scm (cached-channel-instance): Change default value
> of #:validate-channels.  Remove call to VALIDATE-CHANNELS; pass it
> as #:validate-pull to ‘latest-channel-instances’.
> * guix/scripts/time-machine.scm (%reference-channels): New variable.
> (validate-guix-channel): New procedure.
> (guix-time-machine)[validate-guix-channel]: Remove.

Nitpick: I'd say the proc was moved and simplified to ease traceability
for the reader; same for %oldest-possible-commit (not mentioned in
changelog).

Otherwise LGTM!

-- 
Thanks,
Maxim