Re: Seeking guidance regarding system roll-back and switch-generation

[Chris Marusich]

Hi Ludo,

Thank you for the concrete suggestions.  I've looked into using the
parameters file.  It contains almost all the info I need, but I think
some is still missing.

> The output of ‘guix system build’ contains the ‘parameters’ file, which
> is enough to generate grub.cfg (see ‘previous-grub-entries’ in (guix
> scripts system)).

What if someone specifies extra "menu-entries" in their operating system
configuration file?  (as described in "(guix) GRUB Configuration")?
Those extra entries don't appear to be stored in the parameters file.

It would be very convenient if we could just store the entire
<grub-configuration> in the built system output (e.g., the parameters
file).  Is that possible?

> However, the activation script is indeed missing.  We can add it to the
> output of ‘guix system build’ by extending ‘system-service-type’:
>
>
> diff --git a/gnu/services.scm b/gnu/services.scm
> index 5479bfa..fc3e17e 100644
> --- a/gnu/services.scm
> +++ b/gnu/services.scm
> @@ -352,11 +352,18 @@ ACTIVATION-SCRIPT-TYPE."
>  
>  (define (second-argument a b) b)
>  
> +(define (gexps->activation-system-entry gexps)
> +  "Return a directory entry to add to the result of the 'system' derivation."
> +  (mlet %store-monad ((script (activation-script gexps)))
> +    (return `(("activate" ,script)))))
> +
>  (define activation-service-type
>    (service-type (name 'activate)
>                  (extensions
>                   (list (service-extension boot-service-type
> -                                          gexps->activation-gexp)))
> +                                          gexps->activation-gexp)
> +                       (service-extension system-service-type
> +                                          gexps->activation-system-entry)))
>                  (compose append)
>                  (extend second-argument)))
>  
>
>
> This way we have direct access to each generation’s activation script
> and we should be fine with (3).
>
> WDYT?

I'm afraid I don't yet know enough about gexps and the activation
process to give an informed opinion on that suggestion.  After I finish
the first milestone (switch symlinks and rebuild grub.cfg), I'll study
those topics in more detail and revisit your proposal.

Thank you,

-- 
Chris

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