nixos-option no longer works after collect garbage

[liaowang11]

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Expected Behavior

nixos-option from profile/core/default.nix should still works after nix-collect-garbage -d
profile/core/default.nix includes an alias to fix nixos-option:

nixos-option = "nixos-option -I nixpkgs=${toString ../../compat}";

Current Behavior

nixos-option :

terminate called after throwing an instance of 'nix::EvalError'
  what():  cannot import '/nix/store/slf28rxb23rmx2qgid4bwfifwbkyb5ji-dxl4kb7jclicnk2mignizz53bk8wl9z4-source/flake.nix', since path
 '/nix/store/slf28rxb23rmx2qgid4bwfifwbkyb5ji-dxl4kb7jclicnk2mignizz53bk8wl9z4-source' is not valid, at /nix/store/2mpgi4bvn8py4liv9
w3mjxd2c5r7bvv8-source/default.nix:131:19
fish: Job 1, “command nixos-option -I nixpkgs…” terminated by signal SIGABRT (Abort)

Possible Solution

The nixpkgs used by flake-compat in compat/default.nix has no referrers, which is why it’s got garbage collect.
I don’t know enough of Nix to add some kind of references to the nixpkgs source to fix this.

Steps to Reproduce

1. nixos-rebuild switch
2. nix-collect-garbage -d
3. nixos-option errors

Context

nixos-option use /nix/store/slf28rxb23rmx2qgid4bwfifwbkyb5ji-dxl4kb7jclicnk2mignizz53bk8wl9z4-source/
which is included in gc print-dead.

> nixos-option services.earlyoom                                   
unpacking 'https://github.com/edolstra/flake-compat/archive/99f1c2157fba4bfe6211a321fd0ee43199025dbf.tar.gz'...
terminate called after throwing an instance of 'nix::EvalError'
  what():  cannot import '/nix/store/slf28rxb23rmx2qgid4bwfifwbkyb5ji-dxl4kb7jclicnk2mignizz53bk8wl9z4-source/flake.nix', since path
 '/nix/store/slf28rxb23rmx2qgid4bwfifwbkyb5ji-dxl4kb7jclicnk2mignizz53bk8wl9z4-source' is not valid, at /nix/store/2mpgi4bvn8py4liv9
w3mjxd2c5r7bvv8-source/default.nix:131:19
fish: Job 1, “command nixos-option -I nixpkgs…” terminated by signal SIGABRT (Abort)

> nix-store -q --references /nix/store/slf28rxb23rmx2qgid4bwfifwbkyb5ji-dxl4kb7jclicnk2mignizz53bk8wl9z4-source

> nix-store --gc --print-dead | rg /nix/store/slf28rxb23rmx2qgid4bwfifwbkyb5ji-dxl4kb7jclicnk2mignizz53bk8wl9z4-source
finding garbage collector roots...
removing stale link from '/nix/var/nix/gcroots/auto/xmssbbj2d1pfm51cqci88ggxnmi3j9dj' to '/tmp/nixos-rebuild.CRuJ2V/result'
determining live/dead paths...
/nix/store/slf28rxb23rmx2qgid4bwfifwbkyb5ji-dxl4kb7jclicnk2mignizz53bk8wl9z4-source

Your Environment

[nrdxp]

I can't reproduce this. IIRC, I found this issue myself, and I believe the toString fixed it. Here's what happens on my machine:

[liaowang11]

@nrdxp So what is currently referring that nixpkgs which nixos-option is using?
On my two machines, all no referrers(the store path may varies).
❯ nix-store -q --referrers /nix/store/slf28rxb23rmx2qgid4bwfifwbkyb5ji-dxl4kb7jclicnk2mignizz53bk8wl9z4-source

[nrdxp]

❯ alias nixos-option
nixos-option='nixos-option -I nixpkgs=/nix/store/1ivyv4zwqzqjpqz5qb0yp9xbax331m6l-source/compat'

[liaowang11]

Ok, i will dive into this further, close for now.

[nrdxp]

I will also take a closer look later as well by clearing all non declarative gcroots and see if maybe the issue will arise then.