services/emacs: add option to set emacsclient as the default editor (#2545)

* services/emacs: add `defaultEditor` option

Sets `emacsclient` as the default editor using the `EDITOR` environment variable.

modules/services/emacs.nix

@@ -80,6 +80,16 @@ in {
     socketActivation = {
       enable = mkEnableOption "systemd socket activation for the Emacs service";
     };
+
+    defaultEditor = mkOption rec {
+      type = types.bool;
+      default = false;
+      example = !default;
+      description = ''
+        Whether to configure <command>emacsclient</command> as the default
+        editor using the <envar>EDITOR</envar> environment variable.
+      '';
+    };
   };
 
   config = mkIf cfg.enable (mkMerge [
@@ -139,7 +149,16 @@ in {
         Install = { WantedBy = [ "default.target" ]; };
       };
 
-      home.packages = optional cfg.client.enable (hiPrio clientDesktopItem);
+      home = {
+        packages = optional cfg.client.enable (hiPrio clientDesktopItem);
+
+        sessionVariables = mkIf cfg.defaultEditor {
+          EDITOR = getBin (pkgs.writeShellScript "editor" ''
+            exec ${
+              getBin cfg.package
+            }/bin/emacsclient "''${@:---create-frame}"'');
+        };
+      };
     }
 
     (mkIf cfg.socketActivation.enable {

tests/modules/services/emacs/default.nix

@@ -3,4 +3,5 @@
   emacs-service-28 = ./emacs-service-28.nix;
   emacs-socket-27 = ./emacs-socket-27.nix;
   emacs-socket-28 = ./emacs-socket-28.nix;
+  emacs-default-editor = ./emacs-default-editor.nix;
 }

tests/modules/services/emacs/emacs-default-editor.nix

@@ -0,0 +1,24 @@
+{ config, lib, pkgs, ... }:
+
+{
+  nixpkgs.overlays = [
+    (self: super: {
+      # Use `cat` instead of `echo` to prevent arguments from being
+      # interpreted as an option.
+      emacs = pkgs.writeShellScriptBin "emacsclient"
+        ''${pkgs.coreutils}/bin/cat <<< "$*"'';
+    })
+  ];
+
+  services.emacs = {
+    defaultEditor = true;
+    enable = true;
+  };
+
+  nmt.script = "source ${
+      pkgs.substituteAll {
+        inherit (pkgs) coreutils;
+        src = ./emacs-default-editor.sh;
+      }
+    }";
+}

tests/modules/services/emacs/emacs-default-editor.sh

@@ -0,0 +1,18 @@
+set +u
+source $TESTED/home-path/etc/profile.d/hm-session-vars.sh
+set -u
+
+check_arguments () {
+    if [ "$1" != "$2" ]; then
+	@coreutils@/bin/cat <<- EOF
+	Expected arguments:
+	$1
+	but got:
+	$2
+	EOF
+	exit 1
+    fi
+}
+
+check_arguments "--create-frame" "$($EDITOR)"
+check_arguments "foo bar baz" "$($EDITOR foo bar baz)"