Re: Guix, Hydra & Nix

[Sree Harsha Totakura <sreeharsha@totakura.in>]

Hi Ludo,

Thank you very much.  Please ACK my understanding.

On 03/28/2014 10:26 PM, Ludovic Courtès wrote:
> Sree Harsha Totakura <sreeharsha@totakura.in> skribis:
> 
>> From the sources, I found that build-aux/hydra/*.scm convert packages into a
>> format hydra understands for building them.
> 
> Yes.  Specifically, build-aux/hydra/*.scm return a list of “jobs”, where
> each job is represented as an alist with a number of compulsory items,
> such as ‘derivation’ (see ‘package->alist’.)
> 

OK, so the list of jobs is what is termed as a jobset in Hydra.  Jobset is a
collection of jobs + configuration for when to rebuild the jobset and where to
get the jobset's inputs (sources).

The file build-aux/hydra/gnu-system.scm prepares the jobset which contains all
packages currently in Guix.  This jobset is configured in hydra.gnu.org to
initiate a build whenever there is a new commit in Guix source repository.

The file build-aux/hydra/guix.scm is a jobset to build Guix iteself.

> The result of those scripts is then consumed by Hydra’s
> hydra-eval-guile-jobs:
> 
>   https://github.com/NixOS/hydra/blob/master/src/script/hydra-eval-guile-jobs.in
> 
> hydra-eval-guile-jobs is itself called by hydra-evaluator, from here:
> 
>   https://github.com/NixOS/hydra/blob/master/src/lib/Hydra/Helper/AddBuilds.pm
> 

Aye, this is where the magic happens.  So it looks at the file extension of the
jobset's recipe and if it is .scm, then it uses Guix.

>> Hydra refuses to install without nix-store.  So Nix has to be installed as well.
>> I'm guessing Hydra uses nix-store command to build and retrieve store paths for
>> packages given to it.  Is it true?
> 
> Yes.  ‘nix-store’ is essentially a client of {guix,nix}-daemon, just
> like ‘guix build’.  Hydra passes ‘nix-store’ a .drv file name, a
> low-level representation of the build process; ‘nix-store’ doesn’t have
> to care whether it was produced by Guix or Nix, and can’t tell the
> difference.  (Similarly, ‘guix build’ can be passed a .drv produced by
> Nix.)
> 
>> If so, how can hydra nix-store compile Guix packages?
> 
> Just like this.  :-)

Ah, so the derivations, when sent to {guix,nix}-daemon are expressed in the same
format?  `guix build' converts our guile code into this format.

So, when Hydra sees a .scm file as input, it asks Guix to convert the jobset
into derivations which are then sent to the {guix,nix}-daemon via 'nix-store'.

Does this mean that {guix,nix}-daemon are interchangeable?  Can Hydra run with
the setup: guix clients, nix clients, guix-daemon, Hydra?  Or does it need a
nix-daemon?

Sree