Hi guix-devel, 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 unprivileged users. 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.
