wgengine: fix link monitor / magicsock Start race

Fixes #2733

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

wgengine/userspace.go

@@ -84,6 +84,7 @@ type userspaceEngine struct {
 	wgLogger          *wglog.Logger //a wireguard-go logging wrapper
 	reqCh             chan struct{}
 	waitCh            chan struct{} // chan is closed when first Close call completes; contrast with closing bool
+	magicConnStarted  chan struct{} // chan is closed after magicConn.Start
 	timeNow           func() mono.Time
 	tundev            *tstun.Wrapper
 	wgdev             *device.Device
@@ -256,6 +257,7 @@ func NewUserspaceEngine(logf logger.Logf, conf Config) (_ Engine, reterr error)
 		tundev:           tsTUNDev,
 		router:           conf.Router,
 		confListenPort:   conf.ListenPort,
+		magicConnStarted: make(chan struct{}),
 	}
 	e.isLocalAddr.Store(tsaddr.NewContainsIPFunc(nil))
 	e.isDNSIPOverTailscale.Store(tsaddr.NewContainsIPFunc(nil))
@@ -371,7 +373,7 @@ func NewUserspaceEngine(logf logger.Logf, conf Config) (_ Engine, reterr error)
 
 	// It's a little pointless to apply no-op settings here (they
 	// should already be empty?), but it at least exercises the
-	// router implementation early on the machine.
+	// router implementation early on.
 	e.logf("Clearing router settings...")
 	if err := e.router.Set(nil); err != nil {
 		return nil, err
@@ -380,6 +382,7 @@ func NewUserspaceEngine(logf logger.Logf, conf Config) (_ Engine, reterr error)
 	e.linkMon.Start()
 	e.logf("Starting magicsock...")
 	e.magicConn.Start()
+	close(e.magicConnStarted)
 
 	go e.pollResolver()
 
@@ -1092,6 +1095,10 @@ func (e *userspaceEngine) LinkChange(_ bool) {
 }
 
 func (e *userspaceEngine) linkChange(changed bool, cur *interfaces.State) {
+	// Issue 2733: wait for e.magicConn to be started; there's two tiny
+	// windows at startup where this callback can be run before Start
+	<-e.magicConnStarted
+
 	up := cur.AnyInterfaceUp()
 	if !up {
 		e.logf("LinkChange: all links down; pausing: %v", cur)