nix develop nixpkgs#hello --unpack is broken when nixpkgs is a path ("Read-only filesystem")

[cole-h]

Describe the bug

Running nix develop nixpkgs#hello --unpack to get an unpacked hello source directory gives me "Read-only filesystem" errors:

vin@scadrial /t/tmp.yXMNb8uiOD> nix develop nixpkgs#hello --unpack
unpacking source archive /nix/store/pa10z4ngm0g83kx9mssrqzz30s84vq7k-hello-2.12.1.tar.gz
tar: hello-2.12.1: Cannot mkdir: Read-only file system
[...]

Steps To Reproduce

1. Run nix develop nixpkgs#hello --unpack
2. See the pages of "Read-only filesystem" warnings, despite being in a writeable directory and filesystem.

Expected behavior

I'd expect to not see the "Read-only filesystem" errors and be dropped into a shell with access to the unpacked source.

nix-env --version output

nix-env (Nix) 2.12.0pre20221116_561440b (lazy trees branch, but also validated with the 2.11.0 release)

Additional context

N/A.

[natto1784]

cannot reproduce with 2.12.0pre20221130_f91dc02 and 2.11.0

[cole-h]

Interestingly, it works when I use github:nixos/nixpkgs/nixos-unstable#hello. The only difference is that I have a local registry reference for nixpkgs (https://github.com/cole-h/nixos-config/blob/37eef14c1f595001a124128ae3aa7700654d0c11/modules/config/nix.nix) (so that my system and nix shell nixpkgs#.... invocations use the same nixpkgs).

Here are the -vvvvv logs for both: https://gist.github.com/cole-h/761a3e55417ee8343a4b16d4597a3dc4

[natto1784]

that is weird because I use separate nixpkgs registries for my system and user, and it still worked

[thufschmitt]

I can indeed reproduce it when nixpkgs is set to a local path.

That's because there's a logic to automatically cd to the right place for local flakes (#4176):

nix/src/nix/develop.cc

Lines 563 to 566 in eece14d

	if (sourcePath) {
	if (chdir(sourcePath->c_str()) == -1) {
	throw SysError("chdir to '%s' failed", *sourcePath);
	}

That behaviour makes sense in combination with #5720 for running nix develop --build somewhere inside the source tree of my flake, but in hindsight it doesn't so much when running it from outside it.

I think we could disable that at the very least when the flake is in the store.

[cole-h]

Not stale, still happens as of nix (Nix) 2.17.0pre20230605_9c6ede8.