net/interfaces, wgengine/monitor: fix false positives link changes

interfaces.State.String tries to print a concise summary of the
network state, removing any interfaces that don't have any or any
interesting IP addresses. On macOS and iOS, for instance, there are a
ton of misc things.

But the link monitor based its are-there-changes decision on
interfaces.State.Equal, which just used reflect.DeepEqual, including
comparing all the boring interfaces. On macOS, when turning wifi on or off, there
are a ton of misc boring interface changes, resulting in hitting an earlier
check I'd added on suspicion this was happening:

    [unexpected] network state changed, but stringification didn't

This fixes that by instead adding a new
interfaces.State.RemoveUninterestingInterfacesAndAddresses method that
does, uh, that. Then use that in the monitor. So then when Equal is
used later, it's DeepEqualing the already-cleaned version with only
interesting interfaces.

This makes cmd/tailscaled debug --monitor much less noisy.

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

net/interfaces/interfaces.go

@@ -297,6 +297,29 @@ func (s *State) AnyInterfaceUp() bool {
 	return s != nil && (s.HaveV4 || s.HaveV6Global)
 }
 
+// RemoveUninterestingInterfacesAndAddresses removes uninteresting IPs
+// from InterfaceIPs, also removing from both the InterfaceIPs and
+// InterfaceUp map any interfaces that don't have any interesting IPs.
+func (s *State) RemoveUninterestingInterfacesAndAddresses() {
+	for ifName := range s.InterfaceUp {
+		ips := s.InterfaceIPs[ifName]
+		keep := ips[:0]
+		for _, pfx := range ips {
+			if isInterestingIP(pfx.IP) {
+				keep = append(keep, pfx)
+			}
+		}
+		if len(keep) == 0 {
+			delete(s.InterfaceUp, ifName)
+			delete(s.InterfaceIPs, ifName)
+			continue
+		}
+		if len(keep) < len(ips) {
+			s.InterfaceIPs[ifName] = keep
+		}
+	}
+}
+
 // RemoveTailscaleInterfaces modifes s to remove any interfaces that
 // are owned by this process. (TODO: make this true; currently it
 // uses some heuristics)
@@ -360,6 +383,7 @@ func GetState() (*State, error) {
 	}); err != nil {
 		return nil, err
 	}
+
 	s.DefaultRouteInterface, _ = DefaultRouteInterface()
 
 	if s.AnyInterfaceUp() {

wgengine/monitor/monitor.go

@@ -100,6 +100,7 @@ func (m *Mon) interfaceStateUncached() (*interfaces.State, error) {
 	s, err := interfaces.GetState()
 	if s != nil {
 		s.RemoveTailscaleInterfaces()
+		s.RemoveUninterestingInterfacesAndAddresses()
 	}
 	return s, err
 }