GNU bug report logs - #24886
Grafting triggers a download of all the outputs of each derivation

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: ludo <at> gnu.org (Ludovic Courtès)
To: Hartmut Goebel <h.goebel <at> crazy-compilers.com>
Cc: bug-guix <at> gnu.org, help-guix <at> gnu.org
Subject: Re: Why is die "doc" output downloaded when building this package?
Date: Sat, 05 Nov 2016 22:53:57 +0100

Hi,

(Adding a bug at bug-guix <at> gnu.org.)

Hartmut Goebel <h.goebel <at> crazy-compilers.com> skribis:

> when building this package, qt-4.8.7-doc (the doc-output of qt-4.7.8)
> will be downloaded. I do not understand why.

Most likely this is due to a limitation of the current implementation of
grafts: all the outputs of packages on a “grafting path” need to be
downloaded, even if some of these outputs are unused.

This is because ‘graft-derivation’ takes a derivation, such as Qt’s, and
returns a derivation with as many outputs, but marked as
non-substitutable (locally built).  Because it’s locally built, all the
outputs of the original derivation must be fetched just to be able to
build the grafted derivation, even if only one of those outputs is
needed.

I think we could fix that by creating one graft derivation for each
output of the original derivation.

Thanks,
Ludo’.

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