The roots of the garbage collector are all store paths to which there
are symlinks in the directory
instance, the following command makes the path
/nix/store/d718ef...-foo a root of the collector:
$ ln -s /nix/store/d718ef...-foo /nix/var/nix/gcroots/bar
That is, after this command, the garbage collector will not remove
/nix/store/d718ef...-foo or any of its dependencies.
prefix/nix/var/nix/gcroots are also searched for
symlinks. Symlinks to non-store paths are followed and searched for
roots, but symlinks to non-store paths inside the paths reached in
that way are not followed to prevent infinite recursion.