bug#70339: Constructing hg-fetch fixed-output derivation requires Mercurial

[Ludovic Courtès]

Simon Tournier <zimon.toutoune@gmail.com> skribis:

> Hi Ludo,
>
> On Fri, 12 Apr 2024 at 11:30, Ludovic Courtès <ludovic.courtes@inria.fr> 
> wrote:
>
>> > $ guix build -S -d hg-commitsigs
>> > substitute: updating substitutes from 'https://ci.guix.gnu.org'... 100.0%
>> > 3,7 MB will be downloaded:
>> >   /gnu/store/6fya762sz5hjdj04vdn5g3v6zii6f11d-mercurial-6.2.2
>> > substituting /gnu/store/6fya762sz5hjdj04vdn5g3v6zii6f11d-mercurial-6.2.2...
>> > downloading from 
>> > https://ci.guix.gnu.org/nar/lzip/6fya762sz5hjdj04vdn5g3v6zii6f11d-mercurial-6.2.2
>> >  ...
>> >  mercurial-6.2.2  3.5MiB                                                   
>> > 529KiB/s 00:07 ▕██████████████████▏ 100.0%
>> >
>> > /gnu/store/pkb6zd9xfmxx6rsh4p7w3glh7xqg5sqy-hg-commitsigs-0.1.0-0.b53eb68-checkout.drv
>> >
>> > and it is unexpected.
>>
>> That running ‘hg clone’ requires Mercurial isn’t totally unexpected to
>> me.  :-)
>
> There is a misunderstanding, I guess.
>
> Running 'hg clone' requires to have a local copy of Mercurial, yes for sure. 
> :-)
>
> However, just ask what it will run (please note the dash d in guix
> build -S -d hg-commitsigs) must not require to have a local copy of
> Mercurial (binary).  If you still think yes, why is it not the case
> for fixed-output derivations relying on the old Git builder?

Oh sorry, I had missed the ‘-d’ bit.

In this case, what’s happening is grafts: Guix downloads (or builds)
Mercurial so it can compute its grafted derivation.

>> Now, the ‘guix recover’ tool (or whatever you call it) you’re working on
>> could create a different fixed-output derivation producing the same
>> result but without using Mercurial; typically, the builder of that
>> derivation would download from SWH.
>>
>> Does that make sense?
>
> Yes, it makes sense; see my very first attempt in [1] :-).

[...]

> 1: https://gitlab.com/zimoun/guix-drv

Nice!

Thanks,
Ludo’.