sctd: add module

This adds a module for enabling the `sctd` daemon, as well as setting
the preferred base temperature.

.github/CODEOWNERS

@@ -478,3 +478,5 @@ Makefile                                              @thiagokokada
 /modules/programs/ion.nix                             @jo1gi
 
 /modules/services/plex-mpv-shim.nix                   @starcraft66
+
+/modules/services/sctd.nix                            @somasis

modules/misc/news.nix

@@ -575,6 +575,14 @@ in
           A new module is available: 'programs.pistol'.
         '';
       }
+
+      {
+        time = "2022-06-26T19:29:25+00:00";
+        condition = hostPlatform.isLinux;
+        message = ''
+          A new module is available: 'services.sctd'.
+        '';
+      }
     ];
   };
 }

modules/modules.nix

@@ -246,6 +246,7 @@ let
     ./services/redshift-gammastep/redshift.nix
     ./services/rsibreak.nix
     ./services/screen-locker.nix
+    ./services/sctd.nix
     ./services/spotifyd.nix
     ./services/stalonetray.nix
     ./services/status-notifier-watcher.nix

modules/services/sctd.nix

@@ -0,0 +1,74 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+  meta.maintainers = [ maintainers.somasis ];
+
+  options = {
+    services.sctd = {
+      enable = mkEnableOption "sctd";
+
+      baseTemperature = mkOption {
+        type = types.ints.between 2500 9000;
+        default = 4500;
+        description = ''
+          The base color temperature used by sctd, which should be between 2500 and 9000.
+          See
+          <citerefentry>
+            <refentrytitle>sctd</refentrytitle>
+            <manvolnum>1</manvolnum>
+          </citerefentry>
+          for more details.
+        '';
+      };
+    };
+  };
+
+  config = mkIf config.services.sctd.enable {
+    assertions =
+      [ (hm.assertions.assertPlatform "services.sctd" pkgs platforms.linux) ];
+
+    systemd.user.services.sctd = {
+      Unit = {
+        Description =
+          "Dynamically adjust the screen color temperature twice every minute";
+        After = [ "graphical-session-pre.target" ];
+        PartOf = [ "graphical-session.target" ];
+      };
+
+      Install.WantedBy = [ "graphical-session.target" ];
+
+      Service = {
+        ExecStart = "${pkgs.sct}/bin/sctd ${
+            toString config.services.sctd.baseTemperature
+          }";
+        ExecStopPost = "${pkgs.sct}/bin/sct";
+        Restart = "on-abnormal";
+        SuccessExitStatus = 1;
+
+        Environment = let
+          # HACK: Remove duplicate messages in the journal; `sctd` calls
+          #       both `logger -s` (which outputs the message to stderr)
+          #       *and* outputs to stderr itself. We can at least silence
+          #       `logger`'s output without hiding sctd's own stderr.
+          logger = pkgs.writeShellScriptBin "logger" ''
+            exec 2>/dev/null
+            exec ${pkgs.util-linux}/bin/logger "$@"
+          '';
+        in [
+          "PATH=${
+            lib.makeBinPath [
+              pkgs.bash
+              pkgs.coreutils
+              pkgs.gnused
+              pkgs.which
+              pkgs.sct
+              logger
+            ]
+          }"
+        ];
+      };
+    };
+  };
+}