nix will pointlessly re-fetch registry entries if they are defined as store paths

[lf-]

Describe the bug

On my NixOS and nix-darwin machines I pin the system nixpkgs as nixpkgs so building stuff is faster, using nix.registry.nixpkgs.flake = nixpkgs; in my configuration.

/etc/nix/registry.json looks like so:

{"flakes":[{"exact":true,"from":{"id":"nixpkgs","type":"indirect"},"to":{"path":"/nix/store/cn1ag0zf4388i4s9ymx80hrqvma0p9qy-nixpkgs-src","rev":"36cc29d837e7232e3176e4651e8e117a6f231793","type":"path"}}],"version":2}

 » nix registry list
system flake:nixpkgs path:/nix/store/cn1ag0zf4388i4s9ymx80hrqvma0p9qy-nixpkgs-src?rev=36cc29d837e7232e3176e4651e8e117a6f231793
........

and every time I use nix-command with flakes it pointlessly copies from the nix store to the nix store:

$ nix shell nixpkgs#hello
copying '/nix/store/cn1ag0zf4388i4s9ymx80hrqvma0p9qy-nixpkgs-src'

Steps To Reproduce

1. Put the result of:

» nix eval --expr '<nixpkgs>' --impure
/nix/store/cn1ag0zf4388i4s9ymx80hrqvma0p9qy-nixpkgs-src

into /etc/nix/registry.json replacing my path above.
2. nix run nixpkgs#hello

Expected behavior

Should execute instantly because it avoids gratuitously copying a store path to the store.

nix-env --version output

 » nix --version
nix (Nix) 2.10.3

however also reproduced on master:

co/nix - [master] » result/bin/nix --version
nix (Nix) 2.12.0pre20220921_f704c27
co/nix - [master] » result/bin/nix build nixpkgs#hello
copying '/nix/store/cn1ag0zf4388i4s9ymx80hrqvma0p9qy-nixpkgs-src

Additional context

Add any other context about the problem here.

[lf-]

Hmmm. So I've gone bughunting and I think I have found a part answer:

I think based on reading src/libfetchers/path.cc that it will possibly do this silly copying business if it is not a "store path", for instance, it will do the bad thing if you put /nix/store/6hmwry071zcdf5jphlrxgv3amgwwqc7n-nixpkgs/nixpkgs as you would get if it's a channel.

Not fully confident about this though: there's also checks that the name of the store path is "source". I wonder if that's related.

[thufschmitt]

Are you sure that it's actually copying it, or do you just see the copying message?

Looking at the code, Nix prints the copying message, but then short-circuits if the path is a valid store path (which is still a problem because then we print a misleading message, but a slightly different one)

EDIT: nevermind, didn't see your previous message. Indeed the check for the source name is probably the cause of the copy.

[lf-]

I see the copying message, and it also takes a long time. The taking a long time leads me to believe it is being copied.

[lf-]

A fun thing is that /nix/store/xxx-blah/somesubdir is not considered a "store path" to Nix in this context, due to the subdirectory.

[thufschmitt]

A fun thing is that /nix/store/xxx-blah/somesubdir is not considered a "store path" to Nix in this context, due to the subdirectory.

Yes. That's a gross approximation, but also required for purity (until/if ever #6530 lands) because relative paths (to the parent) will have a different meaning if you keep it as a subdir.

I think you can do /nix/store/xxx-blah?dir=somesubdir though, although I'm not 100% what the semantics are

[lf-]

Update from later: if you name your thing source, Nix will indeed not do the buggy behaviour. Rather odd.

[Aleksanaa]

Update from later: if you name your thing source, Nix will indeed not do the buggy behaviour. Rather odd.

But how did you rename it with nix.registry.nixpkgs.flake = nixpkgs;?