Hi guix,

These two patches should speed up profile generation
by optimising the 'fonts-dir-file' hook.  The first patch
is the most important; from the commit message:

‘Only let the build G-exp refer to inputs that might actually
have fonts.  That way, if the list of fonts in the manifest
didn't change and the new manifest is built, the font files
will not be rebuilt.’

The second patch doesn't bring much.

To test, you can test whether things like ...

$ ./pre-inst-env guix environment --ad-hoc --pure lagrange -- lagrange

still work (lagrange is a graphical application using fonts).
Not sure what a good ‘benchmark’ would be.

Unfortunately, this does not help with the "guix package -i" case,
as in that case, the code doesn't have access to all the package objects,
and will have to satisfy itself with the store paths, in which case the
profile code pessimistically assumes the store item has fonts ...

I suppose this restriction could be lifted if/when the gs-fonts package
is renamed to font-ghostscript or something like that ...

However, the optimisation should work in the "guix environment ... --ad-hoc ...",
"guix system reconfigure ..." and (I'd presume) the guix home-manager cases,
though I only tested the first.

Greetings,
Maxime.