wgengine/monitor: add a darwin implementation for tailscaled mode

Tangentially related to #987, #177, #594, #925, #505

Motivated by rebooting a launchd-controlled tailscaled and it going
into SetNetworkUp(false) mode immediately because there really is no
network up at system boot, but then it got stuck in that paused state
forever, without a monitor implementation.

wgengine/monitor/monitor_darwin_tailscaled.go

@@ -0,0 +1,65 @@
+// Copyright (c) 2021 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 darwin,!redo
+
+package monitor
+
+import (
+	"bufio"
+	"os/exec"
+
+	"tailscale.com/types/logger"
+)
+
+// unspecifiedMessage is a minimal message implementation that should not
+// be ignored. In general, OS-specific implementations should use better
+// types and avoid this if they can.
+type unspecifiedMessage struct{}
+
+func (unspecifiedMessage) ignore() bool { return false }
+
+func newOSMon(logf logger.Logf) (osMon, error) {
+	return new(routeMonitorSubProcMon), nil
+}
+
+// routeMonitorSubProcMon is a very simple (temporary? but I know
+// better) monitor implementation for darwin in tailscaled-mode where
+// we can just shell out to "route -n monitor". It waits for any input
+// but doesn't parse it. Then we poll to see if something is different.
+type routeMonitorSubProcMon struct {
+	cmd *exec.Cmd // of "/sbin/route -n monitor"
+	br  *bufio.Reader
+	buf []byte
+}
+
+func (m *routeMonitorSubProcMon) Close() error {
+	if m.cmd != nil {
+		m.cmd.Process.Kill()
+		m.cmd = nil
+	}
+	return nil
+}
+
+func (m *routeMonitorSubProcMon) Receive() (message, error) {
+	if m.cmd == nil {
+		cmd := exec.Command("/sbin/route", "-n", "monitor")
+		outPipe, err := cmd.StdoutPipe()
+		if err != nil {
+			return nil, err
+		}
+		if err := cmd.Start(); err != nil {
+			return nil, err
+		}
+		m.br = bufio.NewReader(outPipe)
+		m.cmd = cmd
+		m.buf = make([]byte, 16<<10)
+	}
+	_, err := m.br.Read(m.buf)
+	if err != nil {
+		m.Close()
+		return nil, err
+	}
+	return unspecifiedMessage{}, nil
+}

wgengine/monitor/monitor_unsupported.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !linux,!freebsd,!windows android
+// +build !linux,!freebsd,!windows,!darwin android darwin,redo
 
 package monitor