`nix fmt` does not work `Error: path . not inside the tree root`

[malikwirin]

Describe the bug

nix fmt suddenly causes the following response: Error: path . not inside the tree root /path/to/datalad-nix Last time formatting worked was this commit.

To Reproduce

After adding these changes the nix fmt command suddenly does not terminate succesfull

Expected behavior

It working like in https://github.com/malikwirin/datalad-nix/commit/dc9630acdda236cbea64c4f52967aceeec8f5604

System information

"treefmt-nix": {
      "inputs": {
        "nixpkgs": [
          "nixpkgs-unstable"
        ]
      },
      "locked": {
        "lastModified": 1744707583,
        "narHash": "sha256-IPFcShGro/UUp8BmwMBkq+6KscPlWQevZi9qqIwVUWg=",
        "owner": "numtide",
        "repo": "treefmt-nix",
        "rev": "49d05555ccdd2592300099d6a657cc33571f4fe0",
        "type": "github"
      },
      "original": {
        "owner": "numtide",
        "repo": "treefmt-nix",
        "type": "github"
      }
    }

• system: "x86_64-linux"
• host os: Linux 6.6.86, NixOS, 24.11 (Vicuna), 24.11.20250409.60e405b
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.24.14
• nixpkgs: /nix/store/97hykhxj3miqqhfnf55b1rphiwcpxlcr-source

Using zsh

[brianmcgee]

Is this the branch where you're having issues? https://github.com/malikwirin/datalad-nix/tree/forgejo-aneksajo

I've checked it out and nix fmt is working for me:

❯ nix fmt
traversed 38 files
emitted 20 files for processing
formatted 20 files (1 changed) in 25ms

[brianmcgee]

I am using a later nix version, 2.28.3. There was an this issue with older versions of nix https://github.com/NixOS/nix/pull/11438, I'm not sure at what version number this PR was included though.

When I downgrade to nix 2.24.14 I still don't see the issue:

❯ nix fmt
traversed 38 files
emitted 20 files for processing
formatted 1 files (1 changed) in 16ms
❯ nix --version
nix (Nix) 2.24.14

[jfly]

It looks like https://github.com/NixOS/nix/commit/c60e1be62cc28494cc969f2d84151367c3db2220 first landed in nix 2.25.0, so I believe @malikwirin is using a version of nix fmt that invokes treefmt ..

I am able to reproduce this issue with if I clone the project, create a symlink to it, and try to run nix fmt inside the symlink-ed project root:

./ $ git clone [email protected]:malikwirin/datalad-nix.git
Cloning into 'datalad-nix'...
remote: Enumerating objects: 601, done.
remote: Counting objects: 100% (191/191), done.
remote: Compressing objects: 100% (80/80), done.
remote: Total 601 (delta 140), reused 142 (delta 105), pack-reused 410 (from 1)
Receiving objects: 100% (601/601), 106.95 KiB | 793.00 KiB/s, done.
Resolving deltas: 100% (326/326), done.

./ $ ln -s datalad-nix datalad-nix-link

./ $ cd datalad-nix-link

datalad-nix-link/ $ nix run nixpkgs#nixVersions.nix_2_24 fmt
Error: path . not inside the tree root /tmp/tmp.F51y6Yqv6x/datalad-nix-link

It works with a newer version of nix with https://github.com/NixOS/nix/commit/c60e1be62cc28494cc969f2d84151367c3db2220:

datalad-nix-link/ $ nix run nixpkgs#nixVersions.nix_2_28 fmt
traversed 34 files
emitted 18 files for processing
formatted 18 files (0 changed) in 29ms

But fails again if I explicitly specify .:

datalad-nix-link/ $ nix run nixpkgs#nixVersions.nix_2_28 fmt .
Error: path . not inside the tree root /tmp/tmp.F51y6Yqv6x/datalad-nix-link

This is all with treefmt v2.2.1. I was curious if this is fixed in the latest version of treefmt (I know we made some changes recently with symlinks), and I ran into a segfault (!), which I reported in https://github.com/numtide/treefmt/issues/594. I'll work on fixing that one right now.

I don't think there's anything actionable here, especially not in the treefmt-nix repo. @malikwirin, you should upgrade nix or treefmt and this specific problem will go away.

@brianmcgee, I suggest that we close this issue (I would, but it looks like I don't have the power to).