emacs: add service module

(cherry picked from commit 0ca1bf3cfd)

modules/misc/news.nix

@@ -951,6 +951,21 @@ in
           A new module is available: 'programs.irssi'.
         '';
       }
+
+      {
+        time = "2019-02-09T14:09:58+00:00";
+        condition = hostPlatform.isLinux;
+        message = ''
+          A new module is available: 'services.emacs'.
+
+          This module provides a user service that runs the Emacs
+          configured in
+
+              programs.emacs
+
+          as an Emacs daemon.
+        '';
+      }
     ];
   };
 }

modules/modules.nix

@@ -87,6 +87,7 @@ let
     (loadModule ./services/blueman-applet.nix { })
     (loadModule ./services/compton.nix { })
     (loadModule ./services/dunst.nix { })
+    (loadModule ./services/emacs.nix { condition = hostPlatform.isLinux; })
     (loadModule ./services/flameshot.nix { })
     (loadModule ./services/gnome-keyring.nix { })
     (loadModule ./services/gpg-agent.nix { })

modules/programs/emacs.nix

@@ -66,7 +66,6 @@ in
 
   config = mkIf cfg.enable {
     home.packages = [ cfg.finalPackage ];
-
     programs.emacs.finalPackage = emacsWithPackages cfg.extraPackages;
   };
 }

modules/services/emacs.nix

@@ -0,0 +1,44 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+
+  cfg = config.services.emacs;
+  emacsCfg = config.programs.emacs;
+  emacsBinPath = "${emacsCfg.finalPackage}/bin";
+
+in
+
+{
+  options.services.emacs = {
+    enable = mkEnableOption "the Emacs daemon";
+  };
+
+  config = mkIf cfg.enable {
+    assertions = [
+      {
+        assertion = emacsCfg.enable;
+        message = "The Emacs service module requires"
+          + " 'programs.emacs.enable = true'.";
+      }
+    ];
+
+    systemd.user.services.emacs = {
+      Unit = {
+        Description = "Emacs: the extensible, self-documenting text editor";
+        Documentation = "info:emacs man:emacs(1) https://gnu.org/software/emacs/";
+      };
+
+      Service = {
+        ExecStart = "${pkgs.stdenv.shell} -l -c 'exec ${emacsBinPath}/emacs --fg-daemon'";
+        ExecStop = "${emacsBinPath}/emacsclient --eval '(kill-emacs)'";
+        Restart = "on-failure";
+      };
+
+      Install = {
+        WantedBy = [ "default.target" ];
+      };
+    };
+  };
+}