bug#22990: Grafts leads to inefficient substitute info retrieval

[Ludovic Courtès]

Hi younger self!

address@hidden (Ludovic Courtès) skribis:

> As of right now (v0.9.0-2007-g66a30a3), ‘graft-derivation’ works either by:
>
>   1. Fetching substitute info about the things being built so that it
>      can determine its references, which in turns allows it to determine
>      whether they need to be grafted.
>
>   2. Building stuff, as a last resort, so that it can determine its
>      references.
>
> Case #1 is hopefully going to be the most common.
>
> The problem with #1 is that when building a profile, we do one
> ‘package-derivation’ call for each package in the profile, which
> translates in one ‘graft-derivation’ call for each relevant package¹,
> which translates into one ‘references/substitutes’ call for each.
>
> Concretely, what this means is this:
>
>   $ guix package -u
>   substitute: updating list of substitutes from 'http://mirror.guixsd.org'... 
> 100.0%
>   substitute: updating list of substitutes from 'http://mirror.guixsd.org'... 
> 100.0%
>   substitute: updating list of substitutes from 'http://mirror.guixsd.org'... 
> 100.0%
>   substitute: updating list of substitutes from 'http://mirror.guixsd.org'... 
> 100.0%
>   substitute: updating list of substitutes from 'http://mirror.guixsd.org'... 
> 100.0%
>   substitute: updating list of substitutes from 'http://mirror.guixsd.org'... 
> 100.0%
>   substitute: updating list of substitutes from 'http://mirror.guixsd.org'... 
> 100.0%
>   […]
>   The following files would be downloaded:
>
> Each of the initial “updating list” message corresponds to an HTTP
> request for a single narinfo file, which can take around 1 second.

This is finally fixed with commit
710854304b1ab29332edcb76f3de532e0724c197, as discussed earlier this
week¹.  \o/

The solution is simpler and more orthogonal than what was envisioned
earlier in this thread, which is nice.

I also potentially addresses other uses of “dynamic dependencies”
elsewhere in the code base, such as ‘remote-eval’ calls in ‘guix
deploy’.

Feedback welcome!

Ludo’.

¹ https://lists.gnu.org/archive/html/guix-devel/2020-03/msg00337.html