ipn/ipnserver: enable systemd-notify support

Addresses #964

Still to be done:
- Figure out the correct logging lines in util/systemd
- Figure out if we need to slip the systemd.Status function anywhere
  else
- Log util/systemd errors? (most of the errors are of the "you cannot do
  anything about this, but it might be a bad idea to crash the program if
  it errors" kind)

Assistance in getting this over the finish line would help a lot.

Signed-off-by: Christine Dodrill <me@christine.website>

util/systemd: rename the nonlinux file to appease the magic

Signed-off-by: Christine Dodrill <me@christine.website>

util/systemd: fix package name

Signed-off-by: Christine Dodrill <me@christine.website>

util/systemd: fix review feedback from @mdlayher

Signed-off-by: Christine Dodrill <me@christine.website>

cmd/tailscale{,d}: update depaware manifests

Signed-off-by: Christine Dodrill <me@christine.website>

util/systemd: use sync.Once instead of func init

Signed-off-by: Christine Dodrill <me@christine.website>

control/controlclient: minor review feedback fixes

Signed-off-by: Christine Dodrill <me@christine.website>

{control,ipn,systemd}: fix review feedback

Signed-off-by: Christine Dodrill <me@christine.website>

review feedback fixes

Signed-off-by: Christine Dodrill <me@christine.website>

ipn: fix sprintf call

Signed-off-by: Christine Dodrill <me@christine.website>

ipn: make staticcheck less sad

Signed-off-by: Christine Dodrill <me@christine.website>

ipn: print IP address in connected status

Signed-off-by: Christine Dodrill <me@christine.website>

ipn: review feedback

Signed-off-by: Christine Dodrill <me@christine.website>

final fixups

Signed-off-by: Christine Dodrill <me@christine.website>

util/systemd/doc.go

@@ -0,0 +1,14 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+/*
+Package systemd contains a minimal wrapper around systemd-notify to enable
+applications to signal readiness and status to systemd.
+
+This package will only have effect on Linux systems running Tailscale in a
+systemd unit with the Type=notify flag set. On other operating systems (or
+when running in a Linux distro without being run from inside systemd) this
+package will become a no-op.
+*/
+package systemd

util/systemd/systemd_linux.go

@@ -0,0 +1,75 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build linux
+
+package systemd
+
+import (
+	"log"
+	"sync"
+
+	"github.com/mdlayher/sdnotify"
+)
+
+var getNotifyOnce struct {
+	sync.Once
+	v *sdnotify.Notifier
+}
+
+type logOnce struct {
+	sync.Once
+}
+
+func (l *logOnce) logf(format string, args ...interface{}) {
+	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()
+		if err != nil {
+			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 ...interface{}) {
+	err := notifier().Notify(sdnotify.Statusf(format, args...))
+	if err != nil {
+		statusOnce.logf("systemd: error notifying: %v", err)
+	}
+}

util/systemd/systemd_nonlinux.go

@@ -0,0 +1,10 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !linux
+
+package systemd
+
+func Ready()                        {}
+func Status(string, ...interface{}) {}