ipn/ipnlocal: don't warn about serve listener failing on IPv6-less machines

Fixes #6303

Change-Id: Ie1ce12938f68dfa0533246bbe3b9d7f3e749a243
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/serve.go

@@ -93,7 +93,9 @@ func (s *serveListener) Run() {
 	for {
 		ln, err := net.Listen("tcp", s.ap.String())
 		if err != nil {
+			if s.shouldWarnAboutListenError(err) {
 				s.logf("serve failed to listen on %v, backing off: %v", s.ap, err)
+			}
 			s.bo.BackOff(s.ctx, err)
 			continue
 		}
@@ -111,6 +113,17 @@ func (s *serveListener) Run() {
 	}
 }
 
+func (s *serveListener) shouldWarnAboutListenError(err error) bool {
+	if !s.b.e.GetLinkMonitor().InterfaceState().HasIP(s.ap.Addr()) {
+		// Machine likely doesn't have IPv6 enabled (or the IP is still being
+		// assigned). No need to warn. Notably, WSL2 (Issue 6303).
+		return false
+	}
+	// TODO(bradfitz): check errors.Is(err, syscall.EADDRNOTAVAIL) etc? Let's
+	// see what happens in practice.
+	return true
+}
+
 // handleServeListenersAccept accepts connections for the Listener.
 // Calls incoming handler in a new goroutine for each accepted connection.
 func (s *serveListener) handleServeListenersAccept(ln net.Listener) error {

net/interfaces/interfaces.go

@@ -400,6 +400,22 @@ func (s *State) EqualFiltered(s2 *State, useInterface InterfaceFilter, useIP IPF
 	return true
 }
 
+// HasIP reports whether any interface has the provided IP address.
+func (s *State) HasIP(ip netip.Addr) bool {
+	if s == nil {
+		return false
+	}
+	want := netip.PrefixFrom(ip, ip.BitLen())
+	for _, pv := range s.InterfaceIPs {
+		for _, p := range pv {
+			if p == want {
+				return true
+			}
+		}
+	}
+	return false
+}
+
 func interfacesEqual(a, b Interface) bool {
 	return a.Index == b.Index &&
 		a.MTU == b.MTU &&