Re: Guix, Hydra & Nix

[Ludovic Courtès]

Hello!

Sree Harsha Totakura <address@hidden> skribis:

> On 03/28/2014 10:26 PM, Ludovic Courtès wrote:
>> Sree Harsha Totakura <address@hidden> 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.

Yes.

>> 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.

Exactly (strictly speaking, it uses Guile, Guix being just a Guile
library.)

>>> 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.

Right.  Try cat $(guix build -d emacs) to see what it looks like.  Quoth
the manual (info "(guix) Programming Interface"):

     Lower-level APIs are available to interact with the daemon and the
  store.  To instruct the daemon to perform a build action, users actually
  provide it with a "derivation".  A derivation is a low-level
  representation of the build actions to be taken, and the environment in
  which they should occur—derivations are to package definitions what
  assembly is to C programs.

> 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'.

Yes.

> Does this mean that {guix,nix}-daemon are interchangeable?

Yes and no.  The core functionality is the same, but the command-line
interface is different, and the default settings is different (for
instance, guix-daemon uses chroot builds by default, with nothing in
/bin, whereas nix-daemon uses non-chroot builds by default.)

> Can Hydra run with the setup: guix clients, nix clients, guix-daemon,
> Hydra?  Or does it need a nix-daemon?

No, no need for nix-daemon.

On hydra.gnu.org, we have guix-daemon running, and Guix and Nix clients,
the latter being there for Hydra.

HTH!

Ludo’.