[bug#45104] pull: Add a "with-substitutes" option.

[Christopher Baines]

Mathieu Othacehe <othacehe@gnu.org> writes:

> When "guix pull" is run before the CI server is done baking substitutes,
> the user may need to build some derivations, which can be long on
> low-end hardware.
>
> This patch adds an option to "guix pull" so that it updates to the
> latest commit with available substitutes.
>
> This work is still in progress, but I'd like to gather some impressions
> on that before going further.

Hey!

I think it's definitely a nice idea, although I think there are some
things in the naming.

My first thought is that the option doesn't really do what it says it
will do. The option is named --with-substitutes, but that actually means
something like "with successful build on ci.guix.gnu.org". This could
cause confusion if you're not fetching substitutes from ci.guix.gnu.org,
and it starts building things, or perhaps if you're fetching substitutes
from two servers, one of which is ci.guix.gnu.org which doesn't have a
substitute, but the other server does, this option would fetch an older
revision than what's truly available.

Maybe part of what I've said above is incorrect if a users substitute
URLs are used, but there also seems to be an assuption that something
which provides substitutes provides a Cuirass compatible API, which
isn't always the case.

One more thought on this is that it's probably important to convey that
this doesn't pick a revision where substitutes will be available. I
think this is something some users would be eager to assume, and then be
confused when it doesn't work.

Thanks,

Chris

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