bug#19816: 'guix environment' always downloads all the outputs of dependencies

[Ludovic Courtès]

>From <http://debbugs.gnu.org/cgi/bugreport.cgi?bug=19641#16>:

There’s another problem, though.  When a dependency is a multiple-output
package, all its outputs are added to the environment, because
‘package->transitive-inputs’ discards the information of which output is
needed.

So for instance, both the ‘out’ and the ‘debug’ output of Coreutils end
up being downloaded and added to the environment, even though only ‘out’
is an input.

Now, the problem is that ‘build-derivations’ can only build *all* the
outputs of the given derivation.  This could be worked around either:

  1. by creating a “sink” derivation, for instance with
     ‘profile-derivation’, that could refer precisely to the output(s)
     needed; not ideal.

  2. by using (build-things (list "/the/output/path")) and resorting to
     ‘build-derivations’ only if the ‘build-things’ call did nothing
     (when passed a non-.drv store item, ‘build-things’ tries to
     substitute and does nothing if that fails.)

Ludo’.