dyn-dns: when DNS changes, restart immediately instead of blocking on another dyn-dns.service query

the new behavior though causes dyn-dns consumers to be started even before we've learned the IP. that sort of matches the semantics of the module though. not sure the best design yet

modules/services/dyn-dns.nix

@@ -56,8 +56,6 @@ in
       restartTriggers = [(builtins.toJSON cfg)];
 
       after = [ "network.target" ];
-      wantedBy = cfg.restartOnChange;
-      before = cfg.restartOnChange;
 
       script = let
         jq = lib.getExe pkgs.jq;
@@ -90,28 +88,27 @@ in
     };
 
     systemd.timers.dyn-dns = {
-      # if anything wants dyn-dns.service, they surely want the timer too.
-      wantedBy = [ "dyn-dns.service" ];
-      timerConfig = {
-        OnUnitActiveSec = cfg.interval;
-      };
+      wants = [ "dyn-dns.service" ];  #< start the service immediately
+      timerConfig.OnUnitActiveSec = cfg.interval;
     };
 
     systemd.paths.dyn-dns-watcher = {
-      before = [ "dyn-dns.timer" ];
-      wantedBy = [ "dyn-dns.timer" ];
-      pathConfig = {
-        Unit = "dyn-dns-reactor.service";
-        PathChanged = [ cfg.ipPath ];
-      };
+      wantedBy = cfg.restartOnChange;
+      before = cfg.restartOnChange;
+      wants = [ "dyn-dns.timer" ];
+      pathConfig.Unit = "dyn-dns-reactor.target";
+      pathConfig.PathChanged = [ cfg.ipPath ];
     };
 
-    systemd.services.dyn-dns-reactor = {
+    systemd.targets.dyn-dns-reactor = {
       description = "react to the system's WAN IP changing";
-      serviceConfig.Type = "oneshot";
-      script = if cfg.restartOnChange != [] then ''
-        ${lib.getExe' pkgs.systemd "systemctl"} restart ${toString cfg.restartOnChange}
-      '' else lib.getExe' pkgs.coreutils "true";
+      # when started, have systemd `stop` the services we want to restart:
+      unitConfig.Conflicts = cfg.restartOnChange;
+      after = cfg.restartOnChange;
+      # then, have systemd stop *this* unit
+      unitConfig.StopWhenUnneeded = true;
+      # and as this unit enters the inactive state, it can start the services:
+      unitConfig.OnSuccess = cfg.restartOnChange;
     };
   };
 }