Permission denied when creating .devenv in git worktrees with flakes

[apt-get-coke]

Description

Devenv fails with permission errors when used in git worktrees with flakes enabled. This prevents developers from using git worktrees for parallel development workflows.

Problem

When using devenv in a git worktree with flakes, the source gets copied to the read-only Nix store, but devenv still tries to create its .devenv directory there, resulting in:

error: Permission denied (os error 13) at path "/nix/store/xxx-source/.devenv"
Error: IoError(Custom { kind: Other, error: "Failed to initialize task cache: Database error" })

Minimal Reproduction

1. Create a temporary directory and cd into it
2. Create devenv.nix:

{ pkgs, ... }:
{
  languages.python.enable = true;
  packages = [ pkgs.git ];
}

3. Create flake.nix:

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
    devenv.url = "github:cachix/devenv";
  };

  outputs = { self, nixpkgs, devenv, ... }@inputs:
    let
      systems = [ "x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin" ];
      forEachSystem = nixpkgs.lib.genAttrs systems;
    in {
      packages = forEachSystem (system: {
        devenv-up = self.devShells.${system}.default.config.procfileScript;
      });

      devShells = forEachSystem (system:
        let pkgs = nixpkgs.legacyPackages.${system};
        in {
          default = devenv.lib.mkShell {
            inherit inputs pkgs;
            modules = [ ./devenv.nix ];
          };
        });
    };
}

4. Initialize git and create worktree:

git init
git add .
git commit -m "Initial commit"
git worktree add worktree-test -b test-branch
cd worktree-test
nix develop  # or devenv shell

5. Observe the error:

error: Permission denied (os error 13) at path "/nix/store/xxx-source/.devenv"

Current Behavior

Devenv attempts to create .devenv in the source directory, which fails when the source is in the read-only Nix store.

Expected Behavior

Devenv should:

1. Detect when running from a read-only source location
2. Use an alternative location for .devenv (e.g., $XDG_CACHE_HOME/devenv/<project-hash>)
3. Respect environment variables like DEVENV_ROOT or DEVENV_DOTFILE

Proposed Solutions

Option 1: Environment Variable Override

Allow users to override the .devenv location:

export DEVENV_DOTFILE="$HOME/.cache/devenv/my-project"

Option 2: Automatic Detection

Detect read-only sources and use cache directory automatically:

if source_is_readonly() {
    use_cache_directory();
}

Option 3: Configuration Option

Add configuration in devenv.nix:

{
  devenv.stateDirectory = "~/.cache/devenv/my-project";
}

Impact

This issue prevents effective use of git worktrees with devenv, impacting:

• Parallel development workflows
• CI/CD environments
• Automated testing in isolated environments

Workaround

Currently, users must use complex wrapper scripts to work around this limitation. See our tracking issue for examples: https://github.com/exit0x/voxdia/issues/1002

Environment Information

• OS: Linux 6.1.0-37-amd64
• Architecture: x86_64
• Nix version: nix (Nix) 2.29.0
• Devenv version: devenv 1.6.1

Additional Context

This issue significantly impacts developer productivity when working on multiple features or reviewing PRs in parallel. Git worktrees are a standard workflow tool, and devenv's incompatibility with them in flake-enabled projects is a major limitation.

Related discussions:

• voxdia#965 - Original issue discovery
• voxdia#1001 - PR with initial workarounds
• voxdia#1002 - Tracking issue for this problem

Would you consider any of the proposed solutions? I'm happy to contribute to the implementation if you can provide guidance on the preferred approach.

[sandydoo]

Try using devenv from #1992

[sandydoo]

@apt-get-coke, I followed the repro instructions and:

1. I could not reproduce this with nix develop. I added --impure as required to get the shell to load.
2. I could not reproduce this with devenv shell, both v1.6.1 and v1.7.
3. The voxdia repo is private, so I can't see the context behind this.

[apt-get-coke]

Hi @sandydoo, I created a minimal reproduction: https://github.com/apt-get-coke/devenv-worktree-issue-reproduction

The issue occurs in pure mode (default) but not with --impure.

[sandydoo]

We don't officially support pure mode with Nix flakes. See https://devenv.sh/guides/using-with-flakes/.
There are hacks to provide arguments to the flake to set the devenv root in pure mode (see flake-parts integration).
Note that this is a Nix flakes limitation.

What I get (in pure mode) is an assertion that devenv cannot determine the root project directory. This is what should normally happen before devenv tries to create the .devenv directory.

I will try your repo again, but it looks the same as the steps above.