net/interfaces: define DefaultRouteInterface and State.DefaultRouteInterface

It was pretty ill-defined before and mostly for logging. But I wanted
to start depending on it, so define what it is and make Windows match
the other operating systems, without losing the log output we had
before. (and add tests for that)

Change-Id: I0fbbba1cfc67a265d09dd6cb738b73f0f6005247
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/interfaces/interfaces_test.go

@@ -104,3 +104,55 @@ func TestStateEqualFilteredIPFilter(t *testing.T) {
 		t.Errorf("%+v == %+v when restricting to interesting interfaces and IPs", s1, s2)
 	}
 }
+
+func TestStateString(t *testing.T) {
+	tests := []struct {
+		name string
+		s    *State
+		want string
+	}{
+		{
+			name: "typical_linux",
+			s: &State{
+				DefaultRouteInterface: "eth0",
+				Interface: map[string]Interface{
+					"eth0": {
+						Interface: &net.Interface{
+							Flags: net.FlagUp,
+						},
+					},
+					"wlan0": {
+						Interface: &net.Interface{},
+					},
+				},
+				InterfaceIPs: map[string][]netaddr.IPPrefix{
+					"eth0": []netaddr.IPPrefix{
+						netaddr.MustParseIPPrefix("10.0.0.2/8"),
+					},
+				},
+				HaveV4: true,
+			},
+			want: `interfaces.State{defaultRoute=eth0 ifs={eth0:[10.0.0.2/8]} v4=true v6=false}`,
+		},
+		{
+			name: "default_desc",
+			s: &State{
+				DefaultRouteInterface: "foo",
+				Interface: map[string]Interface{
+					"foo": {
+						Desc: "a foo thing",
+					},
+				},
+			},
+			want: `interfaces.State{defaultRoute=foo (a foo thing) ifs={} v4=false v6=false}`,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := tt.s.String()
+			if got != tt.want {
+				t.Errorf("wrong\n got: %s\nwant: %s\n", got, tt.want)
+			}
+		})
+	}
+}