When I am hacking on some library Z, I continuously want to test the
effects that my changes to Z have on packages A/B/C which depend on
Z. The same applies, in general, when hacking on any package Z which
other packages A/B/C depend on: While developing, I want to be able to
rapidly mutate Z and see how this affects A/B/C.
I am not sure how to best achieve this. Here are some solutions:
- When you change Z, rebuild A/B/C.
This is much too slow at present.
- Use grafts to update A/B/C through binary patching.
This is also too slow, and AFAIK it can't really be sped up.
- Use LD_LIBRARY_PATH to cause A/B/C to search for Z in a mutable place.
This works for C libraries, but not generically; there are equivalent
variables for some other languages but it's not a full solution.
- Before starting to hack on Z, build a new version of Z which includes
a random hash and which A/B/C depend on; then bind-mount a mutable
directory on top of that. (suggested by mark_weaver on IRC)
This is the most palatable hack, but still a hack. The inclusion of a
random hash prevents collision with other packages and the use of
bind-mounting means we aren't actually mutating the store. This
unfortunately also requires privileges: it's not usable by
Here are some not currently available possibilities:
- Create some kind of GUIX_LIBRARY_PATH in which packages are looked up
first before looking at the compiled-in hash.
This would be an attempt to make a generic equivalent of
LD_LIBRARY_PATH. This could theoretically be implemented with variant
symlinks, if they were available: https://lwn.net/Articles/680705/
- Currently every dependency is located at a well known globally unique
and globally meaningful path; add some kind of "variant package"
construct which specifies a package which is "passed in" to the
environment (maybe by bind-mounting it in a filesystem namespace;
implementation specifics aren't important). To put this in a
programming language sense, one of these packages being present would
turn a Guix distribution from a value into a function.
- Massively speed up rebuilding A/B/C by performing incremental
builds. Not sure how exactly this could work, it's just a thought.
What do you think, guix-devel?
By the way, I think this is a pretty important feature overall (though
it's described in fairly abstract terms in this email). I think this is
one of the last missing features to make the Guix approach a clear and
uncontroversial improvement over existing widespread practices.