nix copies inputs already in the nix store

[yshui]

Describe the bug

I have this in the nix registry.json

    {
      "exact": true,
      "from": {
        "id": "nixpkgs",
        "type": "indirect"
      },
      "to": {
        "path": "/nix/store/<some path>",
        "type": "path"
      }
    },

Running nix eval 'nixpkgs#hello.meta produces output "copying /nix/store/<some path>" every time.

I notice this does not always happen. It happens if the "path" points to something that is built. In this case, my nixpkgs source is built by calling applyPatches. It does not happen if the "path" points to something that is fetched directly. e.g. if I use inputs.nixpkgs.url = github.com:nixos/nixpkgs, then points "path" to inputs.nixpkgs, then nix eval does not copy.

Expected behavior

nix should not copy if the input is already in the nix store.

Priorities

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[yshui]

oh wait,

nix/src/libfetchers/path.cc

Line 153 in ed0934b

if (!storePath || storePath->name() != "source" || !store->isValidPath(*storePath)) {

nix just special cases store paths whose names contain "source"??? 🤦

why? why not just do the same for all store paths?

[yshui]

yep, just need to name my derivation "source"...

this is kinda dumb

[misuzu]

The 2.18 version was removed from nixpkgs and I had to upgrade to 2.24 and I already hate it:

% nix run nixpkgs#mcabber
copying '/nix/store/vqhkmj60457j8nrw9vb94ln40rkic1p6-source/' to the store

It's literally already in the store! Why is it doing that? It took like a minute or two on my really slow VPS.

And the derivation name is source!

[avnik]

Upvote this. Iit very annoying

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/nix-copying-a-store-path-into-the-store/60409/1

[Frontear]

+1 to this, its still relevant on bleeding-edge 2.25 and it's quite a hassle because it constantly causes upwards to 15-20s just "copying" who knows what to the store, despite the stuff already being part of the store. This is extremely noticeable when using nix shell nixpkgs#any-drv, where it will attempt to copy the nixpkgs flake to the store, despite the fact that it already exists since my system is built off that exact revision.