ipn: implement Prefs.Equals efficiently.

Signed-off-by: David Anderson <dave@natulte.net>

control/controlclient/direct.go

@@ -39,6 +39,21 @@ type Persist struct {
 	LoginName         string
 }
 
+func (p *Persist) Equals(p2 *Persist) bool {
+	if p == nil && p2 == nil {
+		return true
+	}
+	if p == nil || p2 == nil {
+		return false
+	}
+
+	return p.PrivateMachineKey.Equal(p2.PrivateMachineKey) &&
+		p.PrivateNodeKey.Equal(p2.PrivateNodeKey) &&
+		p.OldPrivateNodeKey.Equal(p2.OldPrivateNodeKey) &&
+		p.Provider == p2.Provider &&
+		p.LoginName == p2.LoginName
+}
+
 func (p *Persist) Pretty() string {
 	var mk, ok, nk wgcfg.Key
 	if !p.PrivateMachineKey.IsZero() {

control/controlclient/persist_test.go

@@ -0,0 +1,98 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package controlclient
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/tailscale/wireguard-go/wgcfg"
+)
+
+func TestPersistEqual(t *testing.T) {
+	persistHandles := []string{"PrivateMachineKey", "PrivateNodeKey", "OldPrivateNodeKey", "Provider", "LoginName"}
+	if have := fieldsOf(reflect.TypeOf(Persist{})); !reflect.DeepEqual(have, persistHandles) {
+		t.Errorf("Persist.Equal check might be out of sync\nfields: %q\nhandled: %q\n",
+			have, persistHandles)
+	}
+
+	newPrivate := func() wgcfg.PrivateKey {
+		k, err := wgcfg.NewPrivateKey()
+		if err != nil {
+			panic(err)
+		}
+		return k
+	}
+	k1 := newPrivate()
+	tests := []struct {
+		a, b *Persist
+		want bool
+	}{
+		{nil, nil, true},
+		{nil, &Persist{}, false},
+		{&Persist{}, nil, false},
+		{&Persist{}, &Persist{}, true},
+
+		{
+			&Persist{PrivateMachineKey: k1},
+			&Persist{PrivateMachineKey: newPrivate()},
+			false,
+		},
+		{
+			&Persist{PrivateMachineKey: k1},
+			&Persist{PrivateMachineKey: k1},
+			true,
+		},
+
+		{
+			&Persist{PrivateNodeKey: k1},
+			&Persist{PrivateNodeKey: newPrivate()},
+			false,
+		},
+		{
+			&Persist{PrivateNodeKey: k1},
+			&Persist{PrivateNodeKey: k1},
+			true,
+		},
+
+		{
+			&Persist{OldPrivateNodeKey: k1},
+			&Persist{OldPrivateNodeKey: newPrivate()},
+			false,
+		},
+		{
+			&Persist{OldPrivateNodeKey: k1},
+			&Persist{OldPrivateNodeKey: k1},
+			true,
+		},
+
+		{
+			&Persist{Provider: "google"},
+			&Persist{Provider: "o365"},
+			false,
+		},
+		{
+			&Persist{Provider: "google"},
+			&Persist{Provider: "google"},
+			true,
+		},
+
+		{
+			&Persist{LoginName: "foo@tailscale.com"},
+			&Persist{LoginName: "bar@tailscale.com"},
+			false,
+		},
+		{
+			&Persist{LoginName: "foo@tailscale.com"},
+			&Persist{LoginName: "foo@tailscale.com"},
+			true,
+		},
+	}
+	for i, test := range tests {
+		if got := test.a.Equals(test.b); got != test.want {
+			t.Errorf("%d. Equals = %v; want %v", i, got, test.want)
+		}
+	}
+}