Re: avoid Computing Guix derivation when not necessary

[zimoun]

Hi,

On Sun, 12 Feb 2023 at 01:10, Ludovic Courtès <ludo@gnu.org> wrote:

>> The principle is simple: get commit and directory info from the profile
>> manifest, compare commits, if commits for all channels are the same, do
>> not try to update the profile.

Indeed, some improvements could be done in that direction.  For
instance,

--8<---------------cut here---------------start------------->8---
$ guix describe
Generation 89   Jan 17 2023 15:20:08    (current)
  guix 29efa27
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: master
    commit: 29efa2791dafb042ca8ace77bcf8538fb404d492

$ guix pull --commit=29efa27
Updating channel 'guix' from Git repository at 
'https://git.savannah.gnu.org/git/guix.git'...
Building from this channel:
  guix      https://git.savannah.gnu.org/git/guix.git   29efa27
Computing Guix derivation for 'x86_64-linux'... |
[...]
--8<---------------cut here---------------end--------------->8---

Going from one commit to the exact same commit should not trigger some
intensive «Computing Guix derivation».

Note that if I run “guix pull --commit=29efa27” two times in row, the
second one it displays: «nothing to be done» after «Computing Guix
derivation».

Last, note that:

    /var/guix/profiles/per-user/simon/current-guix -> current-guix-90-link

and

--8<---------------cut here---------------start------------->8---
$ /var/guix/profiles/per-user/simon/current-guix-90-link/bin/guix describe
  guix 29efa27
    repository URL: https://git.savannah.gnu.org/git/guix.git
    commit: 29efa2791dafb042ca8ace77bcf8538fb404d492

$ /var/guix/profiles/per-user/simon/current-guix-89-link/bin/guix describe
  guix 29efa27
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: master
    commit: 29efa2791dafb042ca8ace77bcf8538fb404d492

$ readlink -f /var/guix/profiles/per-user/simon/current-guix-90-link
/gnu/store/g32zjrbr40fzp1rj5i6gx7hal20myfyv-profile

$ readlink -f /var/guix/profiles/per-user/simon/current-guix-89-link
/gnu/store/pk1i1dsagdby5pqydmvfbffg0i80wwvy-profile

$ diff -r --no-dereference \
  /gnu/store/g32zjrbr40fzp1rj5i6gx7hal20myfyv-profile/manifest \
  /gnu/store/pk1i1dsagdby5pqydmvfbffg0i80wwvy-profile/manifest
12c12
<       "/gnu/store/96bibk75vy5yvmilnycd8pl0l2bydcww-guix-29efa2791"
---
>       "/gnu/store/ldp0snjsac6hp1fikk5b8413pihm77di-guix-29efa2791"
20c20
<             (branch #f)
---
>             (branch "master")
--8<---------------cut here---------------end--------------->8---

Hum, maybe I have something twisted somewhere in my configuration.  From
my understanding, it comes from (guix scripts pull):

--8<---------------cut here---------------start------------->8---
        (match (find guix-channel? channels)
          ((? channel? guix)
           ;; Apply '--url', '--commit', and '--branch' to the 'guix' channel.
           (let ((url (or url (channel-url guix))))
             (cons (match ref
                     (('commit . commit)
                      (channel (inherit guix)
                               (url url) (commit commit) (branch #f)))
--8<---------------cut here---------------end--------------->8---

where ’branch’ should not be #f but instead inherit from ’guix’.  I
remember discussing this but I do not find where. :-)


> One marginal improvement would be sharing the cache that ‘time-machine’
> uses with ‘guix pull’.  Last I looked it wasn’t as easy as one might
> hope, but I forgot the details.

It could be nice! :-)  Last time I gave a look, I had an headache. ;-)

Cheers,
simon