"Infinite recursion encountered" by making module configurable

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1

I’ve been changing my NixOS config to parameterize my config modules using options declarations. This has been working fine, except for my XServer config. The below is a minimal version of the offending module:

{ config, pkgs, lib, ... }:

with lib;
{
  options.os.xorg = {
    enable = pkgs.lib.mkOption {
      default = false;
      type = types.bool;
    };
  };

  config = mkIf config.os.xorg.enable {
    services.xserver = {
      enable = true;

      displayManager.lightdm = {
        enable = true;
        greeters.enso = {
          enable = true;
          blur = true;
        };
      };

      displayManager.defaultSession = "xsession";
      displayManager.session = [
        {
          manage = "desktop";
          name = "xsession";
          start = ''exec $HOME/.xsession'';
        }
      ];
    };
  };
}

When I include that module from my NixOS config, and set os.xorg.enable = true; from another module, I get the following error when I sudo nixos-rebuild test --flake '.':

error: infinite recursion encountered

       at /nix/store/kmy3yfc596gvfm5lkxzbq37kxizdj78r-source/lib/types.nix:461:58:

          460|         # Push down position info.
          461|         (map (def: mapAttrs (n: v: { inherit (def) file; value = v; }) def.value) defs);
             |                                                          ^
          462|       emptyValue = { value = {}; };

--show-trace does not seem to produce any useful results; I can attach those if wanted.

If I comment out enable = true and the LightDM config block, it seems to move forwards. Similarly, if I comment out the options block, the config = mkIf ... line, and the corresponding ending curly brace, it works fine.

Expected outcome

I’d expect to be able to parameterize this module, like I’ve done with others.

Other information

  • NixOS 22.05
  • Nix v2.8.1
1 Like
2

I’ve tried narrowing down my flake output to literally just my xserver config and my machine’s settings with no other software; no dice.

I’ve also tried disabling nonfree software as per this older, probably unrelated Github issue: Infinite recursion when using allowUnfree · Issue #24251 · NixOS/nixpkgs · GitHub – that still doesn’t help (unsurprisingly).

The machine in question is an AMD A10-7850K; it seems to be using the radeon driver for graphics. I haven’t tried it on other machines yet; that’s next I suppose.

Here’s what I get when I run sudo nixos-rebuild test --flake '.' --show-trace: NixOS infinite recursion error - Pastebin.com It’s too large to paste into Discourse it seems.

Any help, including “here’s some references on how to read that stacktrace” or “here’s a better way to debug your Nix modules” are welcome :stuck_out_tongue:

3

I can’t really test my theory, though config = mkIf is always dangerous, therefore:

--- parts/foo.nix       2022-11-27 10:37:02.863466398 +0100
+++ parts/foo.nix2      2022-11-27 10:37:45.428234978 +0100
@@ -4,8 +4,7 @@
     };
   };
 
-  config = mkIf config.os.xorg.enable {
-    services.xserver = {
+  config.services.xserver = mkIf config.os.xorg.enable {
       enable = true;
 
       displayManager.lightdm = {
2 Likes
4

Interesting, I didn’t know config = mkIf was dangerous; it’s threaded throughout my config, haha.

I’ll have to give it a try later and report back, thanks!

5

Unfortunately, I’m still getting the same error. My module now looks like

{ config, pkgs, lib, ... }:

with lib;
{
  options.os.xorg = {
    enable = pkgs.lib.mkOption {
      default = false;
      type = types.bool;
    };
  };

  config.services.xserver = mkIf config.os.xorg.enable {
    // elided everything inside mkIf
  };
}

Playing around a bit more, I find if I comment out the two lines pointed out below, the module gets past the error message. I’ve always killed the process at that point – not interested in leaving my system without X or a graphical login screen…

  config.services.xserver = mkIf config.os.xorg.enable {
    #enable = true; # <-- commented this line

    displayManager.lightdm = {
      #enable = true; # <-- and this line
      greeters.enso = {
        enable = true;
        blur = true;
      };
    };
  };
6

I’m still seeing this issue under NixOS 22.11.

I made a minimal replication of the issue, which I’ve uploaded here:

All I need to replicate the problem is to check this out somewhere and run

nixos-rebuild build --flake '.#infrectest'

to see the problem.

Any thoughts are welcome – I’m still not counting out that I’ve missed something/done something wrong/have a bad mental model!

(Edit: Changed command to do nixos-rebuild build instead of nixos-rebuild switch, which is correctly pointed out as dangerous on an arbitrary machine – thanks NobbZ)

1 Like
7 
diff --git a/modules/nixos/desktop/default.nix b/modules/nixos/desktop/default.nix
index 2f0ff42..bd302e3 100644
--- a/modules/nixos/desktop/default.nix
+++ b/modules/nixos/desktop/default.nix
@@ -3,7 +3,7 @@
 with lib;
 {
   options.os.xorg = {
-    enable = pkgs.lib.mkOption {
+    enable = lib.mkOption {
       default = false;
       type = types.bool;
     };

This fails with a message about having no root file system and boot manager configured, so it’s probably fine.

PS: never ask people to run nixos-rebuild switch but nixos-rebuild build instead!

8

Thanks, I think that did it!

Good catch on the build vs . switch.

9

Thank you, I had a similar issue and this fixed it for me. I had no idea config = mkIf is an anti-pattern, and the official docs even explicitly use it in the Example section: NixOS modules - NixOS Wiki

Should it be noted there? And do you know why it causes problems?

10

Wait what? No, config = mkIf definitely is not an antipattern. It should work basically 100% of the time. It’s used all the time in nixpkgs/nixos/modules.

1 Like