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:

[...]

>> 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’.)

Example (where Guile 2.0.12 replaced by 2.0.13):

--8<---------------cut here---------------start------------->8---
$ ./pre-inst-env  guix package -p foo -i guile-json guile-ssh gdb
The following packages will be installed:
   guile-json	0.5.0	/gnu/store/sd8jm2rw7cp3bnrk421kr97ki6sqxnhz-guile-json-0.5.0
   guile-ssh	0.10.2	/gnu/store/w90isin9pnm9ri8w9njxby8h98lfnkzq-guile-ssh-0.10.2
   gdb	7.12	/gnu/store/d2a4lmycc13ssdf47a9h410knlfqqq41-gdb-7.12

applying 6 grafts to /gnu/store/s7w6r9ih52kdzcl7kprf2rq48s69zh98-profile
3 packages in profile
The following environment variable definitions may be needed:
   export PATH="foo/bin${PATH:+:}$PATH"
$ guix gc -R foo | grep guile
/gnu/store/7fisf4frrgsjzmknjbab1dal23wxrp8d-guile-2.0.13
/gnu/store/w90isin9pnm9ri8w9njxby8h98lfnkzq-guile-ssh-0.10.2
/gnu/store/sd8jm2rw7cp3bnrk421kr97ki6sqxnhz-guile-json-0.5.0
--8<---------------cut here---------------end--------------->8---

That seems to be a good model.

Now we must make sure that it works also for ‘guix build foo’ and ‘guix
system build’.

Ludo’.