bug#22990: Grafts leads to inefficient substitute info retrieval

[ludo@gnu.org (Ludovic Courtès)]

ludo@gnu.org (Ludovic Courtès) skribis:

> ludo@gnu.org (Ludovic Courtès) skribis:
>
>> ludo@gnu.org (Ludovic Courtès) skribis:
>
> [...]
>
>>> To achieve this, I’m thinking of extending gexp code such that gexp
>>> compilers can return a list of applicable grafts.  The ‘package’
>>> compiler would do #:graft? #f and instead let ‘gexp->derivation’ call
>>> ‘graft-derivation’.
>>
>> The ‘wip-gexp-grafts’ branch does that.  Namely, it’s possible to know
>> what grafts would apply to a gexp derivation build, so that one can
>> first build ungrafted, and then apply the grafts to the results.
>> So for a profile, we’d first build the profile as is, and only then
>> would we graft it.
>>
>> Right now the tip of this branch is a hack such that ‘guix package’:
>>
>>   1. Builds the original (ungrafted) derivation of the profile;
>>
>>   2. Manually calls ‘graft-derivation’ on that, passing it the list of
>>      applicable grafts.
>>
>> Conceptually it’s what we want to do, but the drawback is that the
>> caller (here ‘guix package’) goes through a lot of hops to get the list
>> of grafts and to apply it.
>>
>> I think we should instead have a way to annotate a derivation with a
>> list of grafts, as well as a procedure to build that derivations in two
>> phases (first the original derivation, then the grafts).
>
> The current iteration introduces “build continuation”: a derivation can
> be annotated with a continuation, and the ‘build-things’ procedure will
> loop over continuations and return the final results (that only works
> for derivations directly passed as an argument to ‘build-things’.)

On second thought, the whole idea of applying grafts on the final result
(instead of applying grafts at each step like we do now) doesn’t fly.
It works well for things like a profile or the system derivation, but
breaks for less trivial things.

For example, if you’re building a VM image or a binary tarball, you
really need to graft packages early on; trying to graft the VM image or
binary tarball wouldn’t have the desired effect.

I’ve pushed ‘wip-gexp-grafts’ again but it seems to be a dead end.

Ludo’.