#60847 [PATCH] Enable cross-compilation for the pyproject-build-system.

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From: Christopher Baines <mail@cbaines.net>
To: Maxim Cournoyer <maxim.cournoyer@gmail.com>
Subject: Re: bug#60847: [PATCH] Enable cross-compilation for the
 pyproject-build-system.
Date: Tue, 07 Mar 2023 19:26:47 +0000
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[Message part 1 (text/plain, inline)]

Maxim Cournoyer <maxim.cournoyer@gmail.com> writes:

> Hi Christopher,
>
> Christopher Baines <mail@cbaines.net> writes:
>
>> Maxim Cournoyer <maxim.cournoyer@gmail.com> writes:
>>
>>> Hi Ludo!
>>>
>>> Ludovic Courtès <ludo@gnu.org> writes:
>>>
>>>> Hello,
>>>>
>>>> Maxim Cournoyer <maxim.cournoyer@gmail.com> skribis:
>>>>
>>>>> +++ b/guix/packages.scm
>>>>> @@ -1864,28 +1864,30 @@ (define* (bag->derivation bag #:optional context)
>>>>
>>>> […]
>>>>
>>>>> +  (let ((builder-name (procedure-name (bag-build bag))))
>>>>> +    (if (or (bag-target bag)
>>>>> +            (eq? 'pyproject-build builder-name))
>>>>> +        (bag->cross-derivation bag)
>>>>
>>>> This one part is a showstopper to me, for two reasons:
>>>>
>>>>   1. We cannot rely on ‘procedure-name’ (it’s a debugging aid and it’s
>>>>      not guaranteed to return something useful).
>>>>
>>>>   2. Special-casing build systems here is not okay: the bag and build
>>>>      system abstractions exist to maintain separation of concerns.
>>>>
>>>> I understand there’s an actual bug to fix and the desire to fix a more
>>>> common issue, but I think this one approach is not the way forward.
>>>>
>>>> I hope that makes sense!
>>>
>>> I agree this is not "pretty", but it would be a "temporary" kludge until
>>> all the build systems can be migrated (and the package adjusted for) the
>>> "new" way, which is: native-inputs and inputs always co-exist, whether
>>> the build is a native one or a cross one.
>>>
>>> In light of this, it seems OK to test the water with a not so
>>> significant build system (only a handful of package relies on
>>> pyproject-build-system thus far).  When all the build systems will have
>>> been migrated to the new way (a too big undertaking to be done in one
>>> shot), this kludge can be removed.
>>>
>>> Otherwise, could you offer a concrete suggestion as the way forward?  I
>>> appreciate the "that's not the way", but stopping short of suggesting a
>>> better alternative leaves me wanting more :-).
>>
>> I think it would be clearer to see other potential ways forward if the
>> end goal was more clearly articulated.
>>
>> Guessing at that here from the changes proposed to guix/packages.scm,
>> once all build systems are adjusted, cross derivations will be produced
>> for all bags, regardless of whether there's a target.
>>
>> That doesn't make much sense to me. One explaination is that the current
>> naming is confusing when thinking about this goal, so maybe
>> bag->cross-derivation happens to do what you want it to do in all
>> circumstances, even when target is #f?
>
> Thanks for tipping in.  The end goal is to avoid loosing the information
> of which inputs are native (build inputs) vs regular in the bag, and yes
> bag->cross-derivation allows that.  It appears to me the distinction in
> the bag representations (native vs cross) was originally perceived
> useful as some kind of optimization (there's less variables to worry
> about, and we can squash the inputs/search-paths together, since they're
> all native anyway), but this information (currently discarded) ends up
> being very useful even on the build side (to wrap only the target
> inputs, say, and not all the native/build inputs).
>
> So yes, the change long term would be to integrate the
> bag->cross-derivation logic into bag->derivation, at which point it
> would be unified for any type of build (the bag representation would be
> shared between native and cross builds).

Thanks for the explanation, so maybe an alternative to trying to get
bag->derivation to function differently for different build systems
would be to push combining the inputs down in to each build system.

Take the gnu-build-system as an example, gnu-build in (guix build-system
gnu) would be changed to take multiple lists of inputs, rather than a
single list. It can then combine the lists of inputs as is done in
bag->derivation, to avoid affecting any packages.

While this does require changing all the build systems, I think it's a
bit more forward thinking compared to trying to add a kludge in to
bag->derivation, since hopefully the change there can be the longer term
one.

Does that make sense?

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