nix-pin-deps: GC-anchor a Nix build’s dependencies

When a Nix build fails for lack of disk space and a nix-collect-garbage is needed to make disk space available, nix-collect-garbage will unhelpfully delete the successfully-built dependencies of the derivation that failed because they are are not yet gc-anchored, because the build failed.

This tool solves this problem:

nix-pin-deps /nix/store/...-foo.drv foo-deps

will create gc-anchors in foo-deps/ for all of the dependencies of foo.drv that exist in the nix store, keeping partial build progress safe from nix-collect-garbage.