allow only one copy of saturn to run at a time

saturn/default.nix

@@ -15,6 +15,10 @@ let fennel = fetchurl {
       url = "https://fennel-lang.org/downloads/fennel-1.0.0";
       hash = "sha256:1nha32yilzagfwrs44hc763jgwxd700kaik1is7x7lsjjvkgapw7";
     };
+    dbusProxy = callPackage ../beehive/dbus-proxy.nix {
+      inherit (lua53Packages) lgi buildLuaPackage;
+      lua = lua5_3;
+    };
     inifile = let lua = lua5_3; in lua53Packages.buildLuaPackage rec {
       pname  = "inifile";
       name = "${pname}-${version}";
@@ -32,6 +36,7 @@ let fennel = fetchurl {
       '';
     };
     lua = lua5_3.withPackages (ps: with ps; [
+      dbusProxy
       inifile
       inspect
       lgi

saturn/main.fnl

@@ -1,3 +1,23 @@
+(local dbus (require :dbus_proxy))
+
+(local bus (dbus.Proxy:new
+            {
+             :bus dbus.Bus.SESSION
+             :name "org.freedesktop.DBus"
+             :interface "org.freedesktop.DBus"
+             :path "/org/freedesktop/DBus"
+             }))
+
+(local DBUS_NAME_FLAG_DO_NOT_QUEUE 4)
+(let [ret (bus:RequestName "net.telent.saturn" DBUS_NAME_FLAG_DO_NOT_QUEUE)]
+  (if (or (= ret 1) (= ret 4))
+      true
+      (= ret 2)
+      (error "unexpected DBUS_REQUEST_NAME_REPLY_IN_QUEUE")
+      (= ret 3)
+      (do
+        (print "already running")
+        (os.exit 0))))
 
 (local lfs (require :lfs))
 (local inifile (require :inifile))