[bug#32634] RFC: Process build output

[Ricardo Wurmus]

Hi Guix,

this patch set is a first draft to stylize (potentially confusing) build
output when using “guix package” and “guix build”.

This is done by adding a soft port that matches on lines in the build
output and colorizes them (unless INSIDE_EMACS or NO_COLOR are set, or
when output is redirected).  For “guix package” the default behaviour is
to also hide all build output that does not announce progress (unless
“--verbose” is passed) and to let a spinner show progress instead.  For
“guix build” all build output is still printed.

Honestly, I’m not really happy with the results, but I think it’s enough
to start a discussion about where this should lead.

One thing I don’t like is that I had to set the “print-build-trace?”
default option to be able to display what build is currently happening.
Unfortunately, for small derivations this leads to output like this:

--8<---------------cut here---------------start------------->8---
Building /gnu/store/2x5xmvimja0pbkvvr8rym91q0249ajiv-fonts-dir.drv - 
x86_64-linux
Built /gnu/store/2x5xmvimja0pbkvvr8rym91q0249ajiv-fonts-dir.drv
Building /gnu/store/diz3pmgrqibvp2pyvgh4wyr4nx5vlx0y-glib-schemas.drv - 
x86_64-linux
Built /gnu/store/diz3pmgrqibvp2pyvgh4wyr4nx5vlx0y-glib-schemas.drv
Building /gnu/store/ss70j6lf8xxiiykdys92iw92khx68ix9-info-dir.drv - x86_64-linux
Built /gnu/store/ss70j6lf8xxiiykdys92iw92khx68ix9-info-dir.drv
Building /gnu/store/rh92rslbj4x9abyna6lc11jqifbavx13-librsvg-2.40.20.drv - 
x86_64-linux
Built /gnu/store/rh92rslbj4x9abyna6lc11jqifbavx13-librsvg-2.40.20.drv
--8<---------------cut here---------------end--------------->8---

I would prefer:

    Building /gnu/store/rh92rslbj4x9abyna6lc11jqifbavx13-librsvg-2.40.20.drv … 
DONE

or similar.

I don’t know about whether the colours are any good; I think the bold
green is hard to read on a bright terminal, while the black is hard to
read on a dark terminal.

Lastly: the spinner.  It’s a bit boring, I think.

What do you think?  Is this a step in the right direction?

--
Ricardo