net/interfaces: add IsExpensive and up state to State

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

net/interfaces/interfaces.go

@@ -174,6 +174,12 @@ func ForeachInterfaceAddress(fn func(Interface, net.IP)) error {
 // For now it's pretty basic.
 type State struct {
 	InterfaceIPs map[string][]net.IP
+	InterfaceUp  map[string]bool
+
+	// IsExpensive is whether the current network interface is
+	// considered "expensive", which currently means LTE/etc
+	// instead of Wifi. This field is not populated by GetState.
+	IsExpensive bool
 }
 
 func (s *State) Equal(s2 *State) bool {
@@ -188,14 +194,22 @@ func (s *State) RemoveTailscaleInterfaces() {
 	for name := range s.InterfaceIPs {
 		if strings.HasPrefix(name, "tailscale") { // TODO: use --tun flag value, etc; see TODO in method doc
 			delete(s.InterfaceIPs, name)
+			delete(s.InterfaceUp, name)
 		}
 	}
 }
 
+// GetState returns the state of all the current machine's network interfaces.
+//
+// It does not set the returned State.IsExpensive. The caller can populate that.
 func GetState() (*State, error) {
-	s := &State{InterfaceIPs: make(map[string][]net.IP)}
+	s := &State{
+		InterfaceIPs: make(map[string][]net.IP),
+		InterfaceUp:  make(map[string]bool),
+	}
 	if err := ForeachInterfaceAddress(func(ni Interface, ip net.IP) {
 		s.InterfaceIPs[ni.Name] = append(s.InterfaceIPs[ni.Name], ip)
+		s.InterfaceUp[ni.Name] = ni.IsUp()
 	}); err != nil {
 		return nil, err
 	}

net/interfaces/interfaces_test.go

@@ -27,5 +27,23 @@ func TestIsTailscaleIP(t *testing.T) {
 			t.Errorf("F(%q) = %v; want %v", tt.ip, got, tt.want)
 		}
 	}
+}
+
+func TestGetState(t *testing.T) {
+	st, err := GetState()
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("Got: %#v", st)
 
+	st2, err := GetState()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if !st.Equal(st2) {
+		// let's assume nobody was changing the system network interfaces between
+		// the two GetState calls.
+		t.Fatal("two States back-to-back were not equal")
+	}
 }

wgengine/userspace.go

@@ -644,6 +644,7 @@ func (e *userspaceEngine) LinkChange(isExpensive bool) {
 		e.logf("LinkChange: interfaces.GetState: %v", err)
 		return
 	}
+	cur.IsExpensive = isExpensive
 	needRebind := e.setLinkState(cur)
 
 	e.logf("LinkChange(isExpensive=%v); needsRebind=%v", isExpensive, needRebind)