Update

vendor/tailscale.com/feature/sdnotify.go

@@ -0,0 +1,39 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package feature
+
+import (
+	"runtime"
+
+	"tailscale.com/feature/buildfeatures"
+)
+
+// HookSystemdReady sends a readiness to systemd. This will unblock service
+// dependents from starting.
+var HookSystemdReady Hook[func()]
+
+// HookSystemdStatus holds a func that will send a single line status update to
+// systemd so that information shows up in systemctl output.
+var HookSystemdStatus Hook[func(format string, args ...any)]
+
+// SystemdStatus sends a single line status update to systemd so that
+// information shows up in systemctl output.
+//
+// It does nothing on non-Linux systems or if the binary was built without
+// the sdnotify feature.
+func SystemdStatus(format string, args ...any) {
+	if !CanSystemdStatus { // mid-stack inlining DCE
+		return
+	}
+	if f, ok := HookSystemdStatus.GetOk(); ok {
+		f(format, args...)
+	}
+}
+
+// CanSystemdStatus reports whether the current build has systemd notifications
+// linked in.
+//
+// It's effectively the same as HookSystemdStatus.IsSet(), but a constant for
+// dead code elimination reasons.
+const CanSystemdStatus = runtime.GOOS == "linux" && buildfeatures.HasSDNotify