bug#65225: [PATCH] environment: Build the profile for the requested system.

[Josselin Poiret via Bug reports for GNU Guix <bug-guix@gnu.org>]

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Hi Tobias,

Tobias Geerinckx-Rice <me@tobias.gr> writes:

> Previously, ‘--system=’ did not affect profile hooks, meaning that all
> packages would be built for both the host and requested systems.
>
> * guix/scripts/environment.scm (guix-environment*): Parameterize
> %current-system to match the requested ‘--system=’.
>
> Reported by ekaitz in #guix.
> ---
>  guix/scripts/environment.scm | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/guix/scripts/environment.scm b/guix/scripts/environment.scm
> index 9712389842..27f7e53549 100644
> --- a/guix/scripts/environment.scm
> +++ b/guix/scripts/environment.scm
> @@ -1146,7 +1146,8 @@ (define (guix-environment* opts)
>            (warning (G_ "no packages specified; creating an empty environment~%")))
>  
>          ;; Use the bootstrap Guile when requested.
> -        (parameterize ((%graft? (assoc-ref opts 'graft?))
> +        (parameterize ((%current-system system)
> +                       (%graft? (assoc-ref opts 'graft?))
>                         (%guile-for-build
>                          (and store-needed?
>                               (package-derivation
>
> base-commit: 9e71d4fd6b3893ae87cb079b57d7a8fe6e9e7914
> -- 
> 2.41.0

So, I've looked into this deeper because this fix didn't seem satisfying
to me: it suggests that the implementation of profile-derivation itself
is wrong, and I wanted to fix it instead.  Here's what I uncovered:

%current-system (applies mutatis mutandis to %current-target-system)
is a Guile parameter.  That means that it is accessed through a function
call, and its values really depends on where that function call occurs.
Now, this interacts with the store monad in a cumbersome way: monadic
values in this case are functions (lambda (store) ...) returning two
values, the actual output and the store.  These functions are run only
at the run-with-store call.

Now, there are two non-equivalent ways of getting the current system in
a monadic context.  You can either do

(mlet ((system -> (%current-system))
  ...)

or

(mlet ((system (current-system))
  ...)

The former directly evaluates (%current-system), while the latter only
evaluates (%current-system) when the monadic value is run!

What does this mean for our case here?  Well, the problem lies with how
the hooks are lowered: they use (gexp->derivation ...) without the
optional #:system keyword.  This looks up the current system at call
time with the mlet -> construct, so everything should be okay, right?
Well, the hooks are run through a mapm/accumulate-builds call, which
puts everything in a monadic value, effectively delaying the look-up
until monadic run time.

--8<---------------cut here---------------start------------->8---
(mlet* %store-monad (...
                     (extras (if (null? (manifest-entries manifest))
                                 (return '())
                                 (mapm/accumulate-builds (lambda (hook)
                                                           (hook manifest))
                                                         hooks))))
                     ...)
--8<---------------cut here---------------end--------------->8---

At this point, I thought: “Well, I could just parameterize
%current-system inside the (lambda (hook) ...), and all would be well,
right?  Well, it didn't seem to work and I was pretty confused by it.  I
tested a bit and noticed that actually, contrary to what was intended
(there even is a comment in gexp-derivation about it), gexp-derivation
looks up the system at monadic run time!  It looks like this:

--8<---------------cut here---------------start------------->8---
(mlet* %store-monad ( ;; The following binding forces '%current-system' and
                     ;; '%current-target-system' to be looked up at >>=
                     ;; time.
                     (graft?    (set-grafting graft?))

                     (system -> (or system (%current-system)))
                     (target -> (if (eq? target 'current)
                                    (%current-target-system)
                                    target))
                     ...)
  ...)
--8<---------------cut here---------------end--------------->8---

Well, the issue here is that such an mlet starts by translating the
graft? binding into a >>= call, which ends up putting the rest of the
translation into a function call that will *not* be called until the
monadic value is run.  That means that the system and target bindings
afterwards are *not* looked up at call time but at monadic run time!
And sure enough, moving the (graft? ...) binding after the (target ->
...) one does solve this!

IMO, this is way too complicated to keep in mind at all times, and there
are bugs lurking under the surface absolutely everywhere, waiting for a
corner case to be uncovered.

I'll send a new patch once I've fixed and tested this further.

Best,
-- 
Josselin Poiret

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