htop: fix htoprc when fields is not explicitly set

When the 'fields' setting is not set in htoprc, the htop program won't read any
of the settings. Provide a default value for fields in case it's not explicitly
set by the user.

modules/programs/htop.nix

@@ -145,9 +145,27 @@ in {
 
     home.packages = [ pkgs.htop ];
 
-    xdg.configFile."htop/htoprc" = mkIf (cfg.settings != { }) {
-      text = concatStringsSep "\n" (mapAttrsToList formatOption cfg.settings)
-        + "\n";
+    xdg.configFile."htop/htoprc" = let
+      defaults = {
+        fields = with fields; [
+          PID
+          USER
+          PRIORITY
+          NICE
+          M_SIZE
+          M_RESIDENT
+          M_SHARE
+          STATE
+          PERCENT_CPU
+          PERCENT_MEM
+          TIME
+          COMM
+        ];
+      };
+
+    in mkIf (cfg.settings != { }) {
+      text = concatStringsSep "\n"
+        (mapAttrsToList formatOption (defaults // cfg.settings)) + "\n";
     };
   };
 }

tests/modules/programs/htop/default.nix

@@ -1,4 +1,5 @@
 {
   htop-empty-settings = ./empty-settings.nix;
   htop-example-settings = ./example-settings.nix;
+  settings-without-fields = ./settings-without-fields.nix;
 }

tests/modules/programs/htop/settings-without-fields.nix

@@ -0,0 +1,25 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+  config = {
+    programs.htop.enable = true;
+    programs.htop.settings = { color_scheme = 6; };
+
+    # Test that the 'fields' key is written in addition to the customized
+    # settings or htop won't read the options.
+    nmt.script = ''
+      htoprc=home-files/.config/htop/htoprc
+      assertFileExists $htoprc
+      assertFileContent $htoprc \
+        ${
+          builtins.toFile "htoprc-expected" ''
+            color_scheme=6
+            fields=0 48 17 18 38 39 40 2 46 47 49 1
+          ''
+        }
+    '';
+  };
+
+}