Scans directories containing manually compiled programs and registers them with the Nix garbage collector.
One can install fancy libs using
nix-env and compile programs against them.
But, after a system update, the Nix garbage collector comes and pulls dynamic
link and other runtime dependencies out from under our manually compiled files.
This problem exists for some extent on all distros, but in NixOS it's amplified by the fact that even smallest changes somewhere down the dependency chain will change checksums from which Nix store paths are constructed.
This tool allows to scan arbitrary directories and to register all Nix dependencies found as GC roots so that they won't be taken away by the garbage collector. Since plain string search is used, it works for both dynamic linkage and other references, like config file paths.
Consider a Python virtualenv:
$ nix-env --install python3 $ pyvenv myvenv
Now let's see if there are Nix store references present (hint: there are):
$ fc-userscan -l myvenv myvenv/bin/python3.5 /nix/store/a5zbx856hyfgz2isz0j60i8w44i6av09-python3-3.5.2 myvenv/pyvenv.cfg /nix/store/a5zbx856hyfgz2isz0j60i8w44i6av09-python3-3.5.2
fc-userscan scans and registers Nix store references found either as symlinks
(like python3.5) or in files (pyvenv.cfg). The
-l flag causes found references
to be dumped to stdout. At the same time, found references are registered with
the Nix garbage collector:
$ ls -lR /nix/var/nix/gcroots/profiles/per-user/ckauhaus/home/ckauhaus/myvenv /nix/var/nix/gcroots/profiles/per-user/ckauhaus/home/ckauhaus/myvenv lrwxrwxrwx 1 ckauhaus users 57 Aug 11 13:29 a5zbx856hyfgz2isz0j60i8w44i6av09 -> /nix/store/a5zbx856hyfgz2isz0j60i8w44i6av09-python3-3.5.2 drwxr-xr-x 2 ckauhaus users 46 Aug 11 13:29 bin /nix/var/nix/gcroots/profiles/per-user/ckauhaus/home/ckauhaus/myvenv/bin lrwxrwxrwx 1 ckauhaus users 57 Aug 11 13:29 a5zbx856hyfgz2isz0j60i8w44i6av09 -> /nix/store/a5zbx856hyfgz2isz0j60i8w44i6av09-python3-3.5.2
All GC root registrations for a given dir $DIR go into
/nix/var/nix/gcroots/profiles/per-user/$USER/$DIR so this can easily be
inspected by the administrator. Should a reference vanish at the original
location, the registration will be cleaned up by the next run.
fc-userscan compiles on Rust 1.19.0 or higher. I'll happily accect pull
requests at the GitHub repository.
Please write tests if feasible.
The primary author of
fc-userscan is Christian
Kauhaus or @ckauhaus on various online services.
The software is licensed under a 3-clause BSD license.