ipn: wait for initial portpoll result before starting controlclient

We were creating the controlclient and starting the portpoll concurrently,
which frequently resulted in the first controlclient connection being canceled
by the firsdt portpoll result ~milliseconds later, resulting in another
HTTP request.

Instead, wait a bit for the first portpoll result so it's much less likely to
interrupt our controlclient connection.

Updates tailscale/corp#557

ipn/local.go

@@ -56,7 +56,8 @@ type LocalBackend struct {
 	store           StateStore
 	backendLogID    string
 	portpoll        *portlist.Poller // may be nil
-	portpollOnce    sync.Once
+	portpollOnce    sync.Once        // guards starting readPoller
+	gotPortPollRes  chan struct{}    // closed upon first readPoller result
 	serverURL       string           // tailcontrol URL
 	newDecompressor func() (controlclient.Decompressor, error)
 
@@ -114,6 +115,7 @@ func NewLocalBackend(logf logger.Logf, logid string, store StateStore, e wgengin
 		backendLogID:   logid,
 		state:          NoState,
 		portpoll:       portpoll,
+		gotPortPollRes: make(chan struct{}),
 	}
 	e.SetLinkChangeCallback(b.linkChange)
 	b.statusChanged = sync.NewCond(&b.statusLock)
@@ -407,6 +409,27 @@ func (b *LocalBackend) Start(opts Options) error {
 
 	b.updateFilter(nil, nil)
 
+	if b.portpoll != nil {
+		b.portpollOnce.Do(func() {
+			go b.portpoll.Run(b.ctx)
+			go b.readPoller()
+
+			// Give the poller a second to get results to
+			// prevent it from restarting our map poll
+			// HTTP request (via doSetHostinfoFilterServices >
+			// cli.SetHostinfo). In practice this is very quick.
+			t0 := time.Now()
+			timer := time.NewTimer(time.Second)
+			select {
+			case <-b.gotPortPollRes:
+				b.logf("got initial portlist info in %v", time.Since(t0).Round(time.Millisecond))
+				timer.Stop()
+			case <-timer.C:
+				b.logf("timeout waiting for initial portlist")
+			}
+		})
+	}
+
 	var discoPublic tailcfg.DiscoKey
 	if controlclient.Debug.Disco {
 		discoPublic = b.e.DiscoPublicKey()
@@ -433,15 +456,6 @@ func (b *LocalBackend) Start(opts Options) error {
 		return err
 	}
 
-	// At this point, we have finished using hostinfo without synchronization,
-	// so it is safe to start readPoller which concurrently writes to it.
-	if b.portpoll != nil {
-		b.portpollOnce.Do(func() {
-			go b.portpoll.Run(b.ctx)
-			go b.readPoller()
-		})
-	}
-
 	b.mu.Lock()
 	b.c = cli
 	endpoints := b.endpoints
@@ -590,6 +604,7 @@ func (b *LocalBackend) updateDNSMap(netMap *controlclient.NetworkMap) {
 // readPoller is a goroutine that receives service lists from
 // b.portpoll and propagates them into the controlclient's HostInfo.
 func (b *LocalBackend) readPoller() {
+	n := 0
 	for {
 		ports, ok := <-b.portpoll.C
 		if !ok {
@@ -616,6 +631,11 @@ func (b *LocalBackend) readPoller() {
 		b.mu.Unlock()
 
 		b.doSetHostinfoFilterServices(hi)
+
+		n++
+		if n == 1 {
+			close(b.gotPortPollRes)
+		}
 	}
 }