Re: hard dependency on Git? (was bug#65866: [PATCH 0/8] Add built-in builder for Git checkouts)

[Simon Tournier]

Re,

On Mon, 11 Sep 2023 at 17:17, Simon Tournier <zimon.toutoune@gmail.com> wrote:
> On Mon, 11 Sep 2023 at 16:23, Ludovic Courtès <ludo@gnu.org> wrote:
>
>> Note that the patch series adds a hard dependency on Git.
>> This is because the existing ‘git-fetch’ code depends on Git,
>> which is itself motivated by the fact that Git supports
>> shallow clones and libgit2/Guile-Git doesn’t.

[...]

> Personally, I do not have a strong opinion about the Big Plan™.  I note
> that the introduction of Git as a hard dependency is a slippery slope
> considering the current state of libgit2.  Here, it starts with “git
> clone”, then “git gc” (unsupported by libgit2) is also in the pipes
> (#65720 [1]).  And after timing, I am almost sure that many operations
> using Guile-Git will be slower than their plain Git counter-parts.  And
> we will start to parse the output of ’git’ plumbing commands.  Slippery
> slope for pushing Guile-Git out, no?

For example, having plain Git command with shallow clone would allow to
save resource when cloning the first time the Guix checkout cache.

Compare,

--8<---------------cut here---------------start------------->8---
$ git clone https://git.savannah.gnu.org/git/guix.git guix-full
Cloning into 'guix-full'...                                                     
                                                                                
                           
remote: Counting objects: 696917, done.                                         
                                                                                
                           
remote: Compressing objects: 100% (143179/143179), done.                        
                                                                                
                           
remote: Total 696917 (delta 552872), reused 696909 (delta 552867)               
                                                                                
                           
Receiving objects: 100% (696917/696917), 347.14 MiB | 29.31 MiB/s, done.        
                                                                                
                           
Resolving deltas: 100% (552872/552872), done.
--8<---------------cut here---------------end--------------->8---

and,

--8<---------------cut here---------------start------------->8---
$ git clone --shallow-since=2019-04-30 
https://git.savannah.gnu.org/git/guix.git guix-oldest
Cloning into 'guix-oldest'...
remote: Counting objects: 426879, done.
remote: Compressing objects: 100% (87246/87246), done.
remote: Total 426879 (delta 342111), reused 423970 (delta 339518)
Receiving objects: 100% (426879/426879), 259.75 MiB | 11.26 MiB/s, done.
Resolving deltas: 100% (342111/342111), done.
Checking connectivity: 426863, done.
--8<---------------cut here---------------end--------------->8---

(Here, 2019-04-30 is the date that contains the %oldest-possible-commit
"6298c3ffd9654d3231a6f25390b056483e8f407c" v1.0.)

Well, ’shallow’ probably implies an overload on server side.  But that
is far less than the bits it saves: 87.39 MiB (= 347.14 - 259.75).  It
saves something like 25% to download, if I read correctly.  I let you do
some maths for the improvement you will get. :-)

And there is no integration with Guile-Git, if I read correctly.  The
job is done by the procedure ’clone*’ from the module (guix git).  This
procedure call the Guile-Git procedure ’clone’ which could drop-in
replaced by (invoke git-command "clone --shallow-since=2019-04-30" …).


Cheers,
simon