feat: full NFSv4.2 server and client in pure Go

Assisted-by: GLM 5.3 Flash

cmd/nfsd/notify.go

@@ -0,0 +1,41 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: MIT
+
+package main
+
+import (
+	"errors"
+	"log"
+	"net"
+	"os"
+)
+
+// notifyReady announces readiness to the service manager over
+// $NOTIFY_SOCKET, the raw unix datagram socket of the sd_notify
+// protocol. The message is the one READY=1 line; nothing else is sent
+// and no dependency is pulled in. Without the variable, the process is
+// not running under a Type=notify unit and the step is skipped quietly.
+func notifyReady() error {
+	addr := os.Getenv("NOTIFY_SOCKET")
+	if addr == "" {
+		return nil
+	}
+	conn, err := net.DialUnix("unixgram", nil, &net.UnixAddr{Name: addr, Net: "unixgram"})
+	if err != nil {
+		return err
+	}
+	defer conn.Close()
+	if _, err := conn.Write([]byte("READY=1")); err != nil {
+		return err
+	}
+	return nil
+}
+
+// notifyReadyOrLog announces readiness and reports a failure on the log
+// without ending the service: a manager that expects the notification
+// times the unit out, but the server itself is ready to serve either way.
+func notifyReadyOrLog() {
+	if err := notifyReady(); err != nil && !errors.Is(err, os.ErrNotExist) {
+		log.Printf("nfsd: the readiness notification failed: %v", err)
+	}
+}