Re: Adding Substitute Mirrors page to installer

[raid5atemyhomework <raid5atemyhomework@protonmail.com>]

>     The ability to also use the same mirror during install rather than after it would be very nice. After all, the guix daemon has to be restarted during installation in the meantime anyway, so on restart it should be possible to switch the `substitute-urls`. However the complications are:
>
> -   The `(gnu installer service)` module inherently assumes that services are completely orthogonal to everything else being configured in the installation. I'm not sure what the best way to extract the substitute mirror selection would be.
> -   The installation image has to do a local `guix system reconfigure` of itself so that its shepherd points the guix daemon to a new mirror, so that the guix daemon restart in `install-system` of `(gnu installer final)` will refer to a new mirror.

Another way to do this would be to add another argument to the `start` action of `guix-daemon`, in much the same way the installer passes in its pid so that the guix daemon can access the copy-on-write store on the destination.  This argument would override the `--substitute-urls`.

So in `(gnu services base)` the `guix-shepherd-service` procedure would have something like:

    (define substitute-urls
            (match args
              ((_ substitute-urls . __) substitute-urls)
              (else                     #$(string-join substitute-urls))))

    #; ....

    (fork-exec-command/container
      #; ...
      #:pid
      (match args
        ((pid . _) pid)
        (else      (getpid)))
      #; ...)


The question now is how does the `install-system` procedure in `(gnu installer final)` determine what substitute URL to pass into `(start-service 'guix-daemon (list (number->string (getpid)) <>))`?

* Change the architecture of the installer somehow so that a single page can both manipulate the `services` field and also manipulate how `install-system` is invoked, and add a new argument to `install-system`.
* Or have `install-system` `read` in the `(%installer-configuration-file)`, look for the `operating-system` form, then delve in its `services` field for a `substitute-urls` field.  This feels fairly brittle but does take advantage of the homoiconicity of Scheme.

Thanks
raid5atemyhomework