Create mutable files with home-manager and Nix

example.nix

{
  home.file."test-file" = {
    text = "Hello world";
    force = true;
    mutable = true;
  };
}

mutability.nix

# This module extends home.file, xdg.configFile and xdg.dataFile with the `mutable` option.
{ config, lib, ... }:
let
  fileOptionAttrPaths =
    [ [ "home" "file" ] [ "xdg" "configFile" ] [ "xdg" "dataFile" ] ];
in {
  options = let

    mergeAttrsList = builtins.foldl' (lib.mergeAttrs) { };

    fileAttrsType = lib.types.attrsOf (lib.types.submodule ({ config, ... }: {
      options.mutable = lib.mkOption {
        type = lib.types.bool;
        default = false;
        description = ''
          Whether to copy the file without the read-only attribute instead of
          symlinking. If you set this to `true`, you must also set `force` to
          `true`. Mutable files are not removed when you remove them from your
          configuration.

          This option is useful for programs that don't have a very good
          support for read-only configurations.
        '';
      };
    }));

  in mergeAttrsList (map (attrPath:
    lib.setAttrByPath attrPath (lib.mkOption { type = fileAttrsType; }))
    fileOptionAttrPaths);

  config = {
    home.activation.mutableFileGeneration = let

      allFiles = (builtins.concatLists (map
        (attrPath: builtins.attrValues (lib.getAttrFromPath attrPath config))
        fileOptionAttrPaths));

      filterMutableFiles = builtins.filter (file:
        (file.mutable or false) && lib.assertMsg file.force
        "if you specify `mutable` to `true` on a file, you must also set `force` to `true`");

      mutableFiles = filterMutableFiles allFiles;

      toCommand = (file:
        let
          source = lib.escapeShellArg file.source;
          target = lib.escapeShellArg file.target;
        in ''
          $VERBOSE_ECHO "${source} -> ${target}"
          $DRY_RUN_CMD cp --remove-destination --no-preserve=mode ${source} ${target}
        '');

      command = ''
        echo "Copying mutable home files for $HOME"
      '' + lib.concatLines (map toCommand mutableFiles);

    in (lib.hm.dag.entryAfter [ "linkGeneration" ] command);
  };
}

vscode.nix

{ config, pkgs, lib, ... }:
let
  # Path logic from:
  # https://github.com/nix-community/home-manager/blob/3876cc613ac3983078964ffb5a0c01d00028139e/modules/programs/vscode.nix
  cfg = config.programs.vscode;

  vscodePname = cfg.package.pname;

  configDir = {
    "vscode" = "Code";
    "vscode-insiders" = "Code - Insiders";
    "vscodium" = "VSCodium";
  }.${vscodePname};

  userDir = if pkgs.stdenv.hostPlatform.isDarwin then
    "Library/Application Support/${configDir}/User"
  else
    "${config.xdg.configHome}/${configDir}/User";

  configFilePath = "${userDir}/settings.json";
  tasksFilePath = "${userDir}/tasks.json";
  keybindingsFilePath = "${userDir}/keybindings.json";

  snippetDir = "${userDir}/snippets";

  pathsToMakeWritable = lib.flatten [
    (lib.optional (cfg.userTasks != { }) tasksFilePath)
    (lib.optional (cfg.userSettings != { }) configFilePath)
    (lib.optional (cfg.keybindings != [ ]) keybindingsFilePath)
    (lib.optional (cfg.globalSnippets != { })
      "${snippetDir}/global.code-snippets")
    (lib.mapAttrsToList (language: _: "${snippetDir}/${language}.json")
      cfg.languageSnippets)
  ];
in {
  home.file = lib.genAttrs pathsToMakeWritable (_: {
    force = true;
    mutable = true;
  });
}

Thanks for your super helpful module.
Are you able to use the mutable attribute on xdg.configFile apps? I'm getting an error like the following when I try to.

       error: The option `home-manager.users.jnnk.xdg.configFile."mimeapps.list".mutable' does not exist. Definition values:
       - In `/nix/store/llwv93n27s5z4g781rf68qw8z4272w24-home-manager/desktop-env/xdg-mime.nix': true

Hello, I am not sure if this is the solution I am looking for and would like some guidance.
I initially tried symlinking my file ../configs/vscode-settings.json to /.config/Code/User/settings.json but had the unable to write to file issue. Original symlink:

".config/Code/User/settings.json".source = ../configs/vscode-settings.json;

Looking for a way for it to be writable and update the file: ../configs/vscode-settings.json I have tried using this module like this:

      ".config/Code/User/settings.json" = {
        source = ../configs/vscode-settings.json;
        force = true;
        mutable = true;
      };

The settings.json is now writable. I can see the updates appear in .config/Code/User/settings.json but not in vscode-settings.json.
Am I missing something here?

@becknik I can reproduce this issue, specifically with xdg.configFile only. Though I don't currently have time to investigate, so I'm hoping someone else will chip in

@j4t1nd3r It's the expected behavior because the file is copied, not symlinked. I believe mkOutOfStoreSymlink achieves exactly what you want

Thank you, I saw this in places but without context. I am struggling to find good documentation on commands / options.
Even for home.file in the home-manager documentation. It seems looking at nix related github issues, searching discord history in nix related channels is the way to go or reaching out directly.

Since a few days, I'm getting the following errors when trying to nixos-rebuild:

error:
       … while calling the 'head' builtin
         at /nix/store/z7j461v8da67nwrad4xz50dx2l5wk0wz-source/lib/attrsets.nix:1575:11:
         1574|         || pred here (elemAt values 1) (head values) then
         1575|           head values
             |           ^
         1576|         else

       … while evaluating the attribute 'value'
         at /nix/store/z7j461v8da67nwrad4xz50dx2l5wk0wz-source/lib/modules.nix:809:9:
          808|     in warnDeprecation opt //
          809|       { value = builtins.addErrorContext "while evaluating the option `${showOption loc}':" value;
             |         ^
          810|         inherit (res.defsFinal') highestPrio;

       (stack trace truncated; use '--show-trace' to show the full trace)

       error: getting status of '/nix/store/34ww4vpn116rriqbhzh4snayh7837k7v-mutability.nix': No such file or directory

I have no clue why this is happening, since I haven't changed anything related to this part of the system for a while. Does anybody have an idea?

To help solve the issue, I suggest sharing your nix configuration, ideally a public repository link if your configuration is saved in a repo.

@j4t1nd3r My intention was to reach people having this snippet in their config, since my problem might also be caused by something else.

If you want to take a look anyways: https://github.com/becknik/dotfiles.nix
Every help is greatly appreciated :)