Subject: [bug#50878] [PATCH] union: Resolve collisions by stable-sort'ing them.

From: Attila Lendvai <[email-masked]>
Date: Thu, 30 Sep 2021 08:10:52 +0000

> > -   (let* ((original-files (list->vector files))
> > -           (count (vector-length original-files))
> > -           (stripped-files (vector-map (lambda (_ el)
> > -                                         (strip-store-file-name el))
> > -                                       original-files))
> > -           (indices (vector-unfold values count)))
> >
> > -   (stable-sort! indices
> > -                    (lambda (a b)
> > -                      (string> (vector-ref stripped-files a)
> > -                               (vector-ref stripped-files b))))
> > -   (vector-ref original-files (vector-ref indices 0))))
>
> Instead of stable-sort!-ing the indices of a vector, what about stable-
> sort!-ing (map strip-store-file-name original-files) in more or less
> one go?

the hash also needs to be dropped from the path for sorting to be
useful, but the return value must be the full path, hence the
complexity with sorting the indices, pointing both to the full paths
and the cut parts.

> I don't think the default collision resolver ought to sort the files.
>
> The rationale behind ignoring certain collisions, e.g. icon caches
> relies on the fact that Guix will use the correct files because they
> are put first in the manifest. The hooks themselves have no special
> names that could put them "always first" and profiles are themselves
> union-built.
>
> I do however support the addition of sorting methods as collision
> resolvers in general and would welcome a way of doing so for profiles
> pre-hook.

please note that i almost completely lack the knowledge of the
relevant internals.

with that in mind, do i read you right? this should add a new exported
resolver funtion, and leave the default one as it was?

and use the new resolver somewhere (where?) that will affect only the
union of profiles, i.e. user visible effecs when installing software?
(because DIRECTORY-UNION is called in other contexts also?)

thanks for the insights! i'd appreciate a bit more higher level
guidance, and then i'll resend the patch accordingly.

- attila
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