wgengine/magicsock: don't update control if only endpoint order changes

Updates #1559

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

wgengine/magicsock/magicsock.go

@@ -630,7 +630,7 @@ func (c *Conn) setEndpoints(endpoints []string, reasons map[string]string) (chan
 		delete(c.onEndpointRefreshed, de)
 	}
 
-	if stringsEqual(endpoints, c.lastEndpoints) {
+	if stringSetsEqual(endpoints, c.lastEndpoints) {
 		return false
 	}
 	c.lastEndpoints = endpoints
@@ -1111,12 +1111,32 @@ func (c *Conn) determineEndpoints(ctx context.Context) (ipPorts []string, reason
 	return eps, already, nil
 }
 
-func stringsEqual(x, y []string) bool {
+// stringSetsEqual reports whether x and y represent the same set of
-	if len(x) != len(y) {
+// strings. The order doesn't matter.
-		return false
+//
+// It does not mutate the slices.
+func stringSetsEqual(x, y []string) bool {
+	if len(x) == len(y) {
+		orderMatches := true
+		for i := range x {
+			if x[i] != y[i] {
+				orderMatches = false
+				break
+			}
+		}
+		if orderMatches {
+			return true
+		}
 	}
-	for i := range x {
+	m := map[string]int{}
-		if x[i] != y[i] {
+	for _, v := range x {
+		m[v] |= 1
+	}
+	for _, v := range y {
+		m[v] |= 2
+	}
+	for _, n := range m {
+		if n != 3 {
 			return false
 		}
 	}

wgengine/magicsock/magicsock_test.go

@@ -1793,3 +1793,61 @@ func TestRebindStress(t *testing.T) {
 		t.Fatalf("Got ReceiveIPv4 error: %v (is closed = %v). Log:\n%s", err, errors.Is(err, net.ErrClosed), logBuf.Bytes())
 	}
 }
+
+func TestStringSetsEqual(t *testing.T) {
+	s := func(nn ...int) (ret []string) {
+		for _, n := range nn {
+			ret = append(ret, strconv.Itoa(n))
+		}
+		return
+	}
+	tests := []struct {
+		a, b []string
+		want bool
+	}{
+		{
+			want: true,
+		},
+		{
+			a:    s(1, 2, 3),
+			b:    s(1, 2, 3),
+			want: true,
+		},
+		{
+			a:    s(1, 2),
+			b:    s(2, 1),
+			want: true,
+		},
+		{
+			a:    s(1, 2),
+			b:    s(2, 1, 1),
+			want: true,
+		},
+		{
+			a:    s(1, 2, 2),
+			b:    s(2, 1),
+			want: true,
+		},
+		{
+			a:    s(1, 2, 2),
+			b:    s(2, 1, 1),
+			want: true,
+		},
+		{
+			a:    s(1, 2, 2, 3),
+			b:    s(2, 1, 1),
+			want: false,
+		},
+		{
+			a:    s(1, 2, 2),
+			b:    s(2, 1, 1, 3),
+			want: false,
+		},
+	}
+	for _, tt := range tests {
+		if got := stringSetsEqual(tt.a, tt.b); got != tt.want {
+			t.Errorf("%q vs %q = %v; want %v", tt.a, tt.b, got, tt.want)
+		}
+	}
+
+}