Substitutes and multiple servers

[Ludovic Courtès]

Hi!

Starting from commit 895d1ed, one can fearlessly use multiple substitute
servers (specified via --substitute-urls), but currently only one at a
time.

That is, one can do:

  guix build --substitute-urls=http://hydra.gnu.org foo

and later:

  guix build --substitute-urls=http://hydra.example.org bar

and then switch back and so on, and ‘guix substitute’ will not be
confused.

The next step is obviously to process the whole list of servers when
more than one URL is given.

Comments welcome.

Ludo’.