mpd: systemd socket activation

modules/services/mpd.nix

@@ -97,6 +97,13 @@ in {
       };
 
       network = {
+        startWhenNeeded = mkOption {
+          type = types.bool;
+          default = false;
+          description = ''
+           Enable systemd socket activation.
+          '';
+        };
 
         listenAddress = mkOption {
           type = types.str;
@@ -143,7 +150,7 @@ in {
         Description = "Music Player Daemon";
       };
 
-      Install = {
+      Install = mkIf (!cfg.network.startWhenNeeded) {
         WantedBy = [ "default.target" ];
       };
 
@@ -154,6 +161,23 @@ in {
         ExecStartPre = ''${pkgs.bash}/bin/bash -c "${pkgs.coreutils}/bin/mkdir -p '${cfg.dataDir}' '${cfg.playlistDirectory}'"'';
       };
     };
+    systemd.user.sockets.mpd = mkIf cfg.network.startWhenNeeded {
+      Socket = {
+        ListenStream = let
+          listen =
+            if cfg.network.listenAddress == "any"
+            then toString cfg.network.port
+            else "${cfg.network.listenAddress}:${toString cfg.network.port}";
+        in [ listen "%t/mpd/socket" ];
+
+        Backlog = 5;
+        KeepAlive = true;
+      };
+
+      Install = {
+        WantedBy = [ "sockets.target" ];
+      };
+    };
   };
 
 }