ipn/ipnlocal: fix peerapi6 port being report as 0 in netstack mode

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

ipn/ipnlocal/local.go

@@ -1327,7 +1327,7 @@ func (b *LocalBackend) getPeerAPIPortForTSMPPing(ip netaddr.IP) (port uint16, ok
 	defer b.mu.Unlock()
 	for _, pln := range b.peerAPIListeners {
 		if pln.ip.BitLen() == ip.BitLen() {
-			return uint16(pln.Port()), true
+			return uint16(pln.port), true
 		}
 	}
 	return 0, false
@@ -1341,7 +1341,7 @@ func (b *LocalBackend) peerAPIServicesLocked() (ret []tailcfg.Service) {
 		}
 		ret = append(ret, tailcfg.Service{
 			Proto: proto,
-			Port:  uint16(pln.Port()),
+			Port:  uint16(pln.port),
 		})
 	}
 	return ret
@@ -1556,13 +1556,12 @@ func (b *LocalBackend) initPeerAPIListener() {
 			ln: ln, // nil for 2nd+ on netstack
 			lb: b,
 		}
-		var port int
 		if skipListen {
-			port = b.peerAPIListeners[0].Port()
+			pln.port = b.peerAPIListeners[0].port
 		} else {
-			port = pln.Port()
+			pln.port = ln.Addr().(*net.TCPAddr).Port
 		}
-		pln.urlStr = "http://" + net.JoinHostPort(a.IP.String(), strconv.Itoa(port))
+		pln.urlStr = "http://" + net.JoinHostPort(a.IP.String(), strconv.Itoa(pln.port))
 		b.logf("peerapi: serving on %s", pln.urlStr)
 		go pln.serve()
 		b.peerAPIListeners = append(b.peerAPIListeners, pln)

ipn/ipnlocal/peerapi.go

@@ -220,9 +220,17 @@ func (s *peerAPIServer) listen(ip netaddr.IP, ifState *interfaces.State) (ln net
 type peerAPIListener struct {
 	ps *peerAPIServer
 	ip netaddr.IP
-	ln     net.Listener // or nil for 2nd+ address family in netstack mdoe
 	lb *LocalBackend
+
+	// ln is the Listener. It can be nil in netstack mode if there are more than
+	// 1 local addresses (e.g. both an IPv4 and IPv6). When it's nil, port
+	// and urlStr are still populated.
+	ln net.Listener
+
+	// urlStr is the base URL to access the peer API (http://ip:port/).
 	urlStr string
+	// port is just the port of urlStr.
+	port int
 }
 
 func (pln *peerAPIListener) Close() error {
@@ -232,17 +240,6 @@ func (pln *peerAPIListener) Close() error {
 	return nil
 }
 
-func (pln *peerAPIListener) Port() int {
-	if pln.ln == nil {
-		return 0
-	}
-	ta, ok := pln.ln.Addr().(*net.TCPAddr)
-	if !ok {
-		return 0
-	}
-	return ta.Port
-}
-
 func (pln *peerAPIListener) serve() {
 	if pln.ln == nil {
 		return