Subject: [bug#47989] [PATCH] channels: Add a #:system argument to channel-instances->manifest.

From: Ludovic Courtès <[email-masked]>
Date: Tue, 04 May 2021 15:52:37 +0200

Christopher Baines <[email-masked]> skribis:

> This allows computing a manifest for a specific system. Previously this was
> possible, but only through changing %current-system, which caused the
> derivation to be computed using that system as well (so computing a derivation
> for aarch64-linux on x86_64-linux would require running aarch64-linux code).
>
> This new argument adds the possibility of computing derivations for non-native
> systems, without having to run non-native code.
>
> I'm looking at this as it will enable the Guix Data Service to compute channel
> instance derivations without relying on QEMU emulation for non-native
> systems (it should be faster as well).
>
> * guix/channels.scm (build-from-source): Add #:system argument and pass to
> build.
> (build-channel-instance): Add system argument and pass to build-from-source.
> (channel-instance-derivations): Add #:system argument and pass to
> build-channel-instance, also rename system to current-system-value.
> (channel-instances->manifest): Add #:system argument and pass to
> channel-instance-derivations.

[...]

>  (define* (build-from-source instance
> -                            #:key core verbose? (dependencies '()))
> +                            #:key core verbose? (dependencies '()) system)
>    "Return a derivation to build Guix from INSTANCE, using the self-build
>  script contained therein.  When CORE is true, build package modules under
>  SOURCE using CORE, an instance of Guix."

Please mention SYSTEM in the docstring.

> +(define* (channel-instance-derivations instances #:key system)
>    "Return the list of derivations to build INSTANCES, in the same order as
>  INSTANCES."
>    (define core-instance
> @@ -757,14 +759,15 @@ INSTANCES."
>      (resolve-dependencies instances))
>  
>    (define (instance->derivation instance)
> -    (mlet %store-monad ((system (current-system)))
> +    (mlet %store-monad ((current-system-value (current-system)))
>        (mcached (if (eq? instance core-instance)
> -                   (build-channel-instance instance)
> +                   (build-channel-instance instance system)
>                     (mlet %store-monad ((core (instance->derivation core-instance))
>                                         (deps (mapm %store-monad instance->derivation
>                                                     (edges instance))))
> -                     (build-channel-instance instance core deps)))
> +                     (build-channel-instance instance system core deps)))
>                 instance
> +               current-system-value
>                 system)))

Here, there should not be any additional key to ‘mcached’ since there’s
only one system being targeted.

Instead, it should look something like this:

  (define (instance->derivation core-instance)
    (mlet %store-monad ((system (if system (return system) (current-system))))
      (mcached …         ;pass ‘system’ to callees
               instance
               system))) ;unchanged

HTH,
Ludo’.