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.

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)"