devenv.nix with systemd-nspawn container backed shell

nspawn-devenv.nix

{ pkgs, lib, config, ... }: {
  # Basic devenv configuration - add your project packages here
  packages = [
    pkgs.systemd
    pkgs.debootstrap
    # Add other packages your project needs
  ];

  # Define container settings with automatic unique naming
  env = {
    CONTAINER_NAME = let
      # Get the base directory name for unique container naming
      dirName = builtins.baseNameOf (builtins.toString ./.);
    in "devenv-${dirName}";
    CONTAINER_PATH = "/var/lib/machines/${config.env.CONTAINER_NAME}";
  };

  # Tasks to setup and manage the container
  tasks = {
    # Task to create the container if it doesn't exist
    "container:setup" = {
      exec = ''
        if [ ! -d "${config.env.CONTAINER_PATH}" ]; then
          echo "Setting up container filesystem at ${config.env.CONTAINER_PATH}..."
          sudo mkdir -p "${config.env.CONTAINER_PATH}"
          sudo debootstrap --include=systemd,dbus bullseye "${config.env.CONTAINER_PATH}"
          
          # Create a .nspawn file to configure the container
          cat << EOF | sudo tee /etc/systemd/nspawn/${config.env.CONTAINER_NAME}.nspawn
[Exec]
Environment=DEVENV_ROOT=${config.env.DEVENV_ROOT}
Environment=DEVENV_PROFILE=${config.env.DEVENV_PROFILE}
Environment=PATH=${config.env.DEVENV_PROFILE}/bin:/bin:/usr/bin

[Network]
VirtualEthernet=yes

[Files]
Bind=/nix/store:/nix/store
EOF
        else
          echo "Container already exists at ${config.env.CONTAINER_PATH}"
        fi
      '';
      onEnter = true;
    };

    # Task to propagate environment variables file into the container
    "container:env-update" = {
      exec = ''
        # Create a script that will source all environment variables in the container
        ENV_SCRIPT="${config.env.CONTAINER_PATH}/etc/profile.d/devenv-vars.sh"
        echo "#!/bin/bash" | sudo tee $ENV_SCRIPT
        env | grep -v "^_" | grep -v "SHELL=" | grep -v "SHLVL=" | grep -v "OLDPWD=" | while read -r line; do
          echo "export $line" | sudo tee -a $ENV_SCRIPT
        done
        sudo chmod +x $ENV_SCRIPT
      '';
      after = ["container:setup"];
      onEnter = true;
    };
  };

  # Define the enter shell hook that will launch the container
  enterShell = ''
    if [ -z "$INSIDE_CONTAINER" ]; then
      # Run the container setup tasks
      devenv tasks run container:setup
      devenv tasks run container:env-update
      
      # Launch the container with our project directory mounted
      export INSIDE_CONTAINER=1
      echo "Launching development container..."
      
      # Important - save original directory to mount inside container
      PROJECT_DIR="$(pwd)"
      
      # Use exec to replace the current shell with the container shell
      exec sudo systemd-nspawn \
        --directory="${config.env.CONTAINER_PATH}" \
        --bind="$PROJECT_DIR:/project" \
        --bind="/nix/store:/nix/store" \
        --setenv=INSIDE_CONTAINER=1 \
        --setenv=DEVENV_ROOT=${config.env.DEVENV_ROOT} \
        --setenv=DEVENV_PROFILE=${config.env.DEVENV_PROFILE} \
        --setenv=PATH=${config.env.DEVENV_PROFILE}/bin:/bin:/usr/bin \
        --chdir=/project \
        /bin/bash -l

      # This won't be reached due to exec
      exit 0
    else
      echo "Already inside container environment"
    fi
  '';

  # Define a custom prompt to indicate when we're inside the container
  scripts.container-prompt.exec = ''
    if [ ! -z "$INSIDE_CONTAINER" ]; then
      export PS1="\[\e[0;92m\][container]\[\e[0m\] $PS1"
    fi
  '';

  # Source the custom prompt script when entering the shell
  enterShell = lib.mkBefore ''
    . ${config.scripts.container-prompt.path}
  '';

  # Add any other configuration you need for your project
  # These will be available in the parent devenv and can be accessed in the container
  # through the propagated environment variables and mounted directories
  
  # Example language configuration
  languages.rust.enable = true;
}