net/interfaces: handle removed interfaces in State.Equal

This wasn't previously handling the case where an interface in s2 was
removed and not present in s1, and would cause the Equal method to
incorrectly return that the states were equal.

Updates tailscale/corp#19124

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I3af22bc631015d1ddd0a1d01bfdf312161b9532d

net/interfaces/interfaces.go

@@ -343,6 +343,13 @@ func (s *State) Equal(s2 *State) bool {
 		s.PAC != s2.PAC {
 		return false
 	}
+	// If s2 has more interfaces than s, it's not equal.
+	if len(s.Interface) != len(s2.Interface) || len(s.InterfaceIPs) != len(s2.InterfaceIPs) {
+		return false
+	}
+	// Now that we know that both states have the same number of
+	// interfaces, we can check each interface in s against s2. If it's not
+	// present or not exactly equal, then the states are not equal.
 	for iname, i := range s.Interface {
 		i2, ok := s2.Interface[iname]
 		if !ok {

net/interfaces/interfaces_test.go

@@ -286,6 +286,13 @@ func TestStateString(t *testing.T) {
 
 // tests (*State).Equal
 func TestEqual(t *testing.T) {
+	pfxs := func(addrs ...string) (ret []netip.Prefix) {
+		for _, addr := range addrs {
+			ret = append(ret, netip.MustParsePrefix(addr))
+		}
+		return ret
+	}
+
 	tests := []struct {
 		name   string
 		s1, s2 *State
@@ -362,6 +369,29 @@ func TestEqual(t *testing.T) {
 			},
 			want: false,
 		},
+
+		// See tailscale/corp#19124
+		{
+			name: "interface-removed",
+			s1: &State{
+				InterfaceIPs: map[string][]netip.Prefix{
+					"rmnet16":    pfxs("2607:1111:2222:3333:4444:5555:6666:7777/64"),
+					"rmnet17":    pfxs("2607:9999:8888:7777:666:5555:4444:3333/64"),
+					"tun0":       pfxs("100.64.1.2/32", "fd7a:115c:a1e0::1/128"),
+					"v4-rmnet16": pfxs("192.0.0.4/32"),
+					"wlan0":      pfxs("10.0.0.111/24"), // removed below
+				},
+			},
+			s2: &State{
+				InterfaceIPs: map[string][]netip.Prefix{
+					"rmnet16":    pfxs("2607:1111:2222:3333:4444:5555:6666:7777/64"),
+					"rmnet17":    pfxs("2607:9999:8888:7777:666:5555:4444:3333/64"),
+					"tun0":       pfxs("100.64.1.2/32", "fd7a:115c:a1e0::1/128"),
+					"v4-rmnet16": pfxs("192.0.0.4/32"),
+				},
+			},
+			want: false,
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {