home: tmux: add tmux service

The service definition is pretty awful, but it deals with the secure
socket correctly...

This avoids having a small lapse when starting the very first shell of a
session, which must first launch the server.

home/tmux/default.nix

@@ -5,6 +5,10 @@ in
 {
   options.my.home.tmux = with lib.my; {
     enable = mkDisableOption "tmux terminal multiplexer";
+
+    service = {
+      enable = mkDisableOption "tmux server service";
+    };
   };
 
   config.programs.tmux = lib.mkIf cfg.enable {
@@ -44,4 +48,30 @@ in
       bind-key -T copy-mode-vi 'y' send -X copy-selection-and-cancel
     '';
   };
+
+  config.systemd.user.services.tmux = lib.mkIf cfg.service.enable {
+    Unit = {
+      Description = "tmux server";
+    };
+
+    Install = {
+      WantedBy = [ "default.target" ];
+    };
+
+    Service =
+      let
+        # Wrap `tmux` in a login shell and set the socket path
+        tmuxCmd = "${config.programs.tmux.package}/bin/tmux";
+        socketExport = lib.optionalString
+          config.programs.tmux.secureSocket
+          ''export TMUX_TMPDIR=''${XDG_RUNTIME_DIR:-"/run/user/\$(id -u)"};'';
+        mkTmuxCommand =
+          c: "${pkgs.runtimeShell} -l -c '${socketExport} ${tmuxCmd} ${c}'";
+      in
+      {
+        Type = "forking";
+        ExecStart = mkTmuxCommand "new -d -s ambroisie";
+        ExecStop = mkTmuxCommand "kill-server";
+      };
+  };
 }