Subject: Re: ‘guix pull’ and external dependencies

From: Chris Marusich <[email-masked]>
Date: Mon, 28 Nov 2016 17:58:06 -0800

Hi Ludo`,

[email-masked] (Ludovic Courtès) writes:

> So I think what we need to do is for “guix pull-ng” to build and install
> a complete ‘guix’ package, and to manage it pretty much like other
> packages is managed, 

I think that's very reasonable.  It seems more intuitive than the
current way 'guix pull' works.  I suspect that managing the installed
version of Guix via the same Guix mechanisms that we use to manage any
other package might be the best, most intuitive solution.

Would it simplify the problem if we packaged the "Guix client stuff",
the "Guix daemon stuff", and maybe the "Guix package definition stuff"
separately?  Then a user could just install the "Guix client stuff"
package if she wanted to upgrade the Guix client tools, or the "Guix
package definition stuff" package if she wanted to get the latest
package definitions.

> except not in the user’s main profile (because that could lead to
> undesirable behavior, 

If we don't store Guix in the user's main profile, where would it go?  A
system profile (like in GuixSD)?  What if another user wants to run a
different version of Guix?  It might be nice to let them do that.

It's not clear to me why it's riskier to store Guix in a profile rather
than outside the profile (but still in the store via the
$HOME/.config/guix/latest symlink), which is what 'guix pull' does now.
You seem to think it's riskier; I'm curious to know more about why.

> where upgrading Guix creates a new generation, 

Why should upgrading Guix NOT create a new generation?  I thought that a
new profile generation would be created any time you upgrade a package,
and I thought that was a good thing because it facilitates easy,
transactional roll-back.  Perhaps I'm missing something.

> or, in theory, unrecoverable problems, where you cannot roll back
> because previous generations use an old Guix that does not understand
> the new manifest format.)

Why would a change in manifest format be unrecoverable?  It looks like
each profile generation contains a manifest file.  Assuming that the new
Guix functions well enough to perform roll back, couldn't we just roll
back to the previous profile generation, where we would have both (1)
the old profile's manifest file, and (2) the previous Guix, which
understands that format?  Since rolling back a profile is basically just
a symlink flip, I think the new Guix could probably do that even if it
didn't understand the old manifest format.

> The difficulty is that ./configure && make && make install in Guix takes
> some time, and we probably wouldn’t want to do that on each ‘guix pull’
> invocation (esp. with Guile 2.2’s compilation times.)
>
> So we may have to provide substitutes of Guix itself, and arrange so
> that ‘guix pull’ pulls up to a tag for which we have substitutes.

What if we had a special package version of Guix (e.g., "v0.11.0") which
we kept up to date via some mechanism?  Maybe something as simple as a
Git hook could help increase the likelihood of that version being
substitutable.  For example, we could have a Git hook that prevents
someone from checking in a change if the latest Git tag does not
correspond to a Guix package version.  Maybe we can do better.

I actually think it would be a good thing if we can run "guix pull"
without substitutes available.  But it should use a substitute by
default, and "build from source" should be a fallback mechanism that the
user has to explicitly request, just like when installing new packages.
That would help avoid unexpectedly long "guix pull" invocations.

-- 
Chris