Re: Adding Substitute Mirrors page to installer

[raid5atemyhomework]

Hello Mathieu,

> Hello,
>
> Thanks for this patch, it seems to work fine!
>
> > -           ;; Extract the substitute URLs of the user configuration.
> >
> >
> > -           (os              (read-operating-system 
> > (%installer-configuration-file)))
> >
> >
> > -           (substitute-urls (and=> (find
> >
> >
> > -                                     (lambda (service)
> >
> >
> > -                                       (eq? guix-service-type
> >
> >
> > -                                            (service-kind service)))
> >
> >
> > -                                     (operating-system-services os))
> >
> >
> > -                                   (compose 
> > guix-configuration-substitute-urls
> >
> >
> > -                                            service-value)))
> >
> >
>
> We could make the mirror selection a proper step, adding it to (gnu
> installer record). Then in (gnu installer), you could add:
>
> --8<---------------cut here---------------start------------->8---
> (installer-step
> (id 'mirrors)
> (description (G_ "Mirror substitute server"))
> (compute (lambda _
> ((installer-mirrors-page current-installer)))))
> --8<---------------cut here---------------end--------------->8---
>
> This way, you should be able to select the step result in
> "run-final-page" this way:
>
> --8<---------------cut here---------------start------------->8---
> (let* ((configuration (format-configuration prev-steps result))
> (user-partitions (result-step result 'partition))
> (locale (result-step result 'locale))
> (users (result-step result 'user))
> (mirrors (result-step result 'mirrors))
> (install-ok?
> (with-mounted-partitions
> user-partitions
> (configuration->file configuration)
>
>          (run-config-display-page #:locale locale)
>          (run-install-shell locale #:users users #:mirrors mirrors))))
>
>
> ...)
> --8<---------------cut here---------------end--------------->8---
>
> That would avoid the need to parse the resulting configuration file.

I don't think that will work.

The intent is that mirror selection affects ***two*** things:

* The actual installed system, i.e. `/etc/config.scm`.
* The installer's Guix Daemon.

If my understanding of the above is correct, it will only affect the 
installer's Guix Daemon (via `run-install-shell` which will call into 
`install-system`, presumably with the added argument), but not affect the 
***installed*** system, because the `configuration` cannot be modified by the 
`'mirrors` page in the above construction.
Currently what the installer assumes is that a page will at most include a 
unique entry into the `operating-system`, but it cannot handle a page which 
modifies the entry created by a previous page.

It would be a breakage of expectations to ask the user to select some mirror, 
then the installed system, when `guix system reconfigure`d, uses the default 
`ci.guix.gnu.org`.

My patch affects both, because that is logically what a user would expect, 
after all, they selected a particular server to install from.
Thus, they expect, that both the installed system, and the installer, will use 
the selected mirror.
Of course, it feels somewhat dirty, as there is a need to read the 
configuration file.
It would be cleaner if a page could affect the field created by a previous page.

For example, perhaps the result of a page is not an AST that is a field of an 
`operating-system` form, but instead a procedure that accepts a 
`(operating-system ...)` form and returns it.
Most existing pages would just append their keyed sub-form.
However the `mirrors` page would modify an existing `services` field in the 
input `operating-system` form instead.

> > +(define (run-substitute-mirror-page)
> >
> > -   (let ((title (G_ "Substitute mirror")))
> > -   (run-listbox-selection-page
> > -        #:title title
> >
> >
> > -        #:info-text (G_ "Choose a server to get substitutes from.
> >
> >
> > -
> >
> > +Depending on your location, the official substitutes server can be slow; \
> > +in that case, using a mirror may be faster.")
>
> I wonder if it would make sense to select multiple mirrors, as fallback
> if the preferred mirror is not up to date. We could also add the
> possibility for the user to add a mirror manually.
>
> In that case, the mirror page could look like the "User creation" page,
> with an "Add" button opening a popup proposing to type a mirror URL or
> to select one from an existing list.
>
> WDYT?

I think Less is More, and deploying an installer with a simple "select one 
mirror" page *now* will cover 90% of use-cases, and we can add the more 
complicated page later when there is more time.

In particular, there is really only one public mirror of Guix that I know of, 
the SJTU mirror, so all the flexibility here is not very useful, at least not 
to me.
Since "multiple mirrors" aren't even available yet anyway, why add the 
complication now?

Maybe you can encourage more people to actually run mirrors if the installer 
has a visible "select mirrors" page, so that you can actually get more than 
just the SJTU mirror and "select multiple mirrors" is now a (good) problem to 
have to solve.

For now I think a simple "select one mirror, we'll add ci.guix.gnu.org as a 
fallback" would be better.
You can add a "select multiple mirrors in an order I want to specify" later, 
when there *are* multiple mirrors existing.

Thanks
raid5atemyhomework