Ludovic Courtès <ludo@gnu.org> writes:

> Christopher Baines <mail@cbaines.net> 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

Sure, I've sent an updated patch that updates docstrings and makes the
above changes.