wgengine/monitor: make Darwin monitor shut down cleanly, add test

Don't use os.NewFile or (*os.File).Close on the AF_ROUTE socket. It
apparently does weird things to the fd and at least doesn't seem to
close it. Just use the unix package.

The test doesn't actually fail reliably before the fix, though. It
was an attempt. But this fixes the integration tests.

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

wgengine/monitor/monitor_darwin.go

@@ -7,7 +7,7 @@ package monitor
 import (
 	"fmt"
 	"log"
-	"os"
+	"sync"
 
 	"golang.org/x/net/route"
 	"golang.org/x/sys/unix"
@@ -31,22 +31,27 @@ func newOSMon(logf logger.Logf) (osMon, error) {
 	}
 	return &darwinRouteMon{
 		logf: logf,
-		f:    os.NewFile(uintptr(fd), "AF_ROUTE"),
+		fd:   fd,
 	}, nil
 }
 
 type darwinRouteMon struct {
-	logf logger.Logf
-	f    *os.File // AF_ROUTE socket
-	buf  [2 << 10]byte
+	logf      logger.Logf
+	fd        int // AF_ROUTE socket
+	buf       [2 << 10]byte
+	closeOnce sync.Once
 }
 
 func (m *darwinRouteMon) Close() error {
-	return m.f.Close()
+	var err error
+	m.closeOnce.Do(func() {
+		err = unix.Close(m.fd)
+	})
+	return err
 }
 
 func (m *darwinRouteMon) Receive() (message, error) {
-	n, err := m.f.Read(m.buf[:])
+	n, err := unix.Read(m.fd, m.buf[:])
 	if err != nil {
 		return nil, err
 	}