feature/sdnotify: move util/systemd to a modular feature

Updates #12614

Change-Id: I08e714c83b455df7f538cc99cafe940db936b480
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

feature/sdnotify/sdnotify_linux.go

@@ -0,0 +1,83 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build linux && !android
+
+package sdnotify
+
+import (
+	"errors"
+	"log"
+	"os"
+	"sync"
+
+	"github.com/mdlayher/sdnotify"
+	"tailscale.com/feature"
+)
+
+func init() {
+	feature.HookSystemdReady.Set(ready)
+	feature.HookSystemdStatus.Set(status)
+}
+
+var getNotifyOnce struct {
+	sync.Once
+	v *sdnotify.Notifier
+}
+
+type logOnce struct {
+	sync.Once
+}
+
+func (l *logOnce) logf(format string, args ...any) {
+	l.Once.Do(func() {
+		log.Printf(format, args...)
+	})
+}
+
+var (
+	readyOnce  = &logOnce{}
+	statusOnce = &logOnce{}
+)
+
+func notifier() *sdnotify.Notifier {
+	getNotifyOnce.Do(func() {
+		var err error
+		getNotifyOnce.v, err = sdnotify.New()
+		// Not exist means probably not running under systemd, so don't log.
+		if err != nil && !errors.Is(err, os.ErrNotExist) {
+			log.Printf("systemd: systemd-notifier error: %v", err)
+		}
+	})
+	return getNotifyOnce.v
+}
+
+// ready signals readiness to systemd. This will unblock service dependents from starting.
+func ready() {
+	err := notifier().Notify(sdnotify.Ready)
+	if err != nil {
+		readyOnce.logf("systemd: error notifying: %v", err)
+	}
+}
+
+// status sends a single line status update to systemd so that information shows up
+// in systemctl output. For example:
+//
+//	$ systemctl status tailscale
+//	● tailscale.service - Tailscale client daemon
+//	Loaded: loaded (/nix/store/qc312qcy907wz80fqrgbbm8a9djafmlg-unit-tailscale.service/tailscale.service; enabled; vendor preset: enabled)
+//	Active: active (running) since Tue 2020-11-24 17:54:07 EST; 13h ago
+//	Main PID: 26741 (.tailscaled-wra)
+//	Status: "Connected; user@host.domain.tld; 100.101.102.103"
+//	IP: 0B in, 0B out
+//	Tasks: 22 (limit: 4915)
+//	Memory: 30.9M
+//	CPU: 2min 38.469s
+//	CGroup: /system.slice/tailscale.service
+//	└─26741 /nix/store/sv6cj4mw2jajm9xkbwj07k29dj30lh0n-tailscale-date.20200727/bin/tailscaled --port 41641
+func status(format string, args ...any) {
+	err := notifier().Notify(sdnotify.Statusf(format, args...))
+	if err != nil {
+		statusOnce.logf("systemd: error notifying: %v", err)
+	}
+}